Ethernet基础知识之五

bruce.chen

                ＊＊＊节选自cisco，仅供参考学习＊＊＊
   
     
Chapter Goals
•

Understand the required and optional MAC frame formats, their purposes, and their compatibility requirements.
•

List the various Ethernet physical layers, signaling procedures, and link media requirements/limitations.
•

Describe
the trade-offs associated with implementing or upgrading Ethernet
LANs—choosing data rates, operational modes, and network equipment.
Ethernet Technologies

Background
The term Ethernet
refers to the family of local-area network (LAN) products covered by
the IEEE 802.3 standard that defines what is commonly known as the
CSMA/CD protocol. Three data rates are currently defined for operation
over optical fiber and twisted-pair cables:
•

10 Mbps—10Base-T Ethernet
•

100 Mbps—Fast Ethernet
•

1000 Mbps—Gigabit Ethernet
10-Gigabit Ethernet is
under development and will likely be published as the IEEE 802.3ae
supplement to the IEEE 802.3 base standard in late 2001 or early 2002.
Other technologies and protocols have been touted as likely
replacements, but the market has spoken. Ethernet has survived as the
major LAN technology (it is currently used for approximately 85 percent
of the world's LAN-connected PCs and workstations) because its protocol
has the following characteristics:
•

Is easy to understand, implement, manage, and maintain
•

Allows low-cost network implementations
•

Provides extensive topological flexibility for network installation
•

Guarantees successful interconnection and operation of standards-compliant products, regardless of manufacturer

Ethernet—A Brief History
The original Ethernet was
developed as an experimental coaxial cable network in the 1970s by
Xerox Corporation to operate with a data rate of 3 Mbps using a carrier
sense multiple access collision detect (CSMA/CD) protocol for LANs with
sporadic but occasionally heavy traffic requirements. Success with that
project attracted early attention and led to the 1980 joint development
of the 10-Mbps Ethernet Version 1.0 specification by the three-company
consortium: Digital Equipment Corporation, Intel Corporation, and Xerox
Corporation.
The original IEEE 802.3 standard was based on, and was very similar to,
the Ethernet Version 1.0 specification. The draft standard was approved
by the 802.3 working group in 1983 and was subsequently published as an
official standard in 1985 (ANSI/IEEE Std. 802.3-1985). Since then, a
number of supplements to the standard have been defined to take
advantage of improvements in the technologies and to support additional
network media and higher data rate capabilities, plus several new
optional network access control features.
Throughout the rest of this chapter, the terms Ethernet and 802.3 will refer exclusively to network implementations compatible with the IEEE 802.3 standard.

Ethernet Network Elements
Ethernet LANs consist of network nodes and interconnecting media. The network nodes fall into two major classes:
•

Data terminal equipment (DTE)—Devices
that are either the source or the destination of data frames. DTEs are
typically devices such as PCs, workstations, file servers, or print
servers that, as a group, are all often referred to as end stations.
•

Data communication equipment (DCE)—Intermediate
network devices that receive and forward frames across the network.
DCEs may be either standalone devices such as repeaters, network
switches, and routers, or communications interface units such as
interface cards and modems.
Throughout this chapter, standalone intermediate network devices will be referred to as either intermediate nodes or DCEs. Network interface cards will be referred to as NICs.
The current Ethernet media options include two general types of copper
cable: unshielded twisted-pair (UTP) and shielded twisted-pair (STP),
plus several types of optical fiber cable.

Ethernet Network Topologies and Structures
LANs take on many topological configurations, but regardless of their
size or complexity, all will be a combination of only three basic
interconnection structures or network building blocks.
The simplest structure is the point-to-point interconnection, shown in
Figure 7-1. Only two network units are involved, and the connection may
be DTE-to-DTE, DTE-to-DCE, or DCE-to-DCE. The cable in point-to-point
interconnections is known as a network link. The maximum allowable
length of the link depends on the type of cable and the transmission
method that is used.
Figure 7-1        Example Point-to-Point Interconnection

The original Ethernet networks were implemented with a coaxial bus
structure, as shown in Figure 7-2. Segment lengths were limited to 500
meters, and up to 100 stations could be connected to a single segment.
Individual segments could be interconnected with repeaters, as long as
multiple paths did not exist between any two stations on the network
and the number of DTEs did not exceed 1024. The total path distance
between the most-distant pair of stations was also not allowed to
exceed a maximum prescribed value.
Figure 7-2        Example Coaxial Bus Topology

Although new networks are
no longer connected in a bus configuration, some older bus-connected
networks do still exist and are still useful.
Since the early 1990s, the network configuration of choice has been the
star-connected topology, shown in Figure 7-3. The central network unit
is either a multiport repeater (also known as a hub) or a network
switch. All connections in a star network are point-to-point links
implemented with either twisted-pair or optical fiber cable.
Figure 7-3        Example Star-Connected Topology


The IEEE 802.3 Logical Relationship to the ISO Reference Model
Figure 7-4 shows the IEEE
802.3 logical layers and their relationship to the OSI reference model.
As with all IEEE 802 protocols, the ISO data link layer is divided into
two IEEE 802 sublayers, the Media Access Control (MAC) sublayer and the
MAC-client sublayer. The IEEE 802.3 physical layer corresponds to the
ISO physical layer.
Figure 7-4        Ethernet's Logical Relationship to the ISO Reference Model                                                                                                                                               

The MAC-client sublayer may be one of the following:
•

Logical
Link Control (LLC), if the unit is a DTE. This sublayer provides the
interface between the Ethernet MAC and the upper layers in the protocol
stack of the end station. The LLC sublayer is defined by IEEE 802.2
standards.
•

Bridge
entity, if the unit is a DCE. Bridge entities provide LAN-to-LAN
interfaces between LANs that use the same protocol (for example,
Ethernet to Ethernet) and also between different protocols (for
example, Ethernet to Token Ring). Bridge entities are defined by IEEE
802.1 standards.
Because specifications for
LLC and bridge entities are common for all IEEE 802 LAN protocols,
network compatibility becomes the primary responsibility of the
particular network protocol. Figure 7-5 shows different compatibility
requirements imposed by the MAC and physical levels for basic data
communication over an Ethernet link.
Figure 7-5        MAC and Physical Layer Compatibility Requirements for Basic Data Communication

The MAC layer controls the
node's access to the network media and is specific to the individual
protocol. All IEEE 802.3 MACs must meet the same basic set of logical
requirements, regardless of whether they include one or more of the
defined optional protocol extensions. The only requirement for basic
communication (communication that does not require optional protocol
extensions) between two network nodes is that both MACs must support
the same transmission rate.
The 802.3 physical layer is specific to the transmission data rate, the
signal encoding, and the type of media interconnecting the two nodes.
Gigabit Ethernet, for example, is defined to operate over either
twisted-pair or optical fiber cable, but each specific type of cable or
signal-encoding procedure requires a different physical layer
implementation.

The Ethernet MAC Sublayer
The MAC sublayer has two primary responsibilities:
•

Data encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception
•

Media access control, including initiation of frame transmission and recovery from transmission failure
The Basic Ethernet Frame Format
The IEEE 802.3 standard defines a basic data frame format that is
required for all MAC implementations, plus several additional optional
formats that are used to extend the protocol's basic capability. The
basic data frame format contains the seven fields shown in Figure 7-6.
•

Preamble (PRE)—Consists
of 7 bytes. The PRE is an alternating pattern of ones and zeros that
tells receiving stations that a frame is coming, and that provides a
means to synchronize the frame-reception portions of receiving physical
layers with the incoming bit stream.
•

Start-of-frame delimiter (SOF)—Consists
of 1 byte. The SOF is an alternating pattern of ones and zeros, ending
with two consecutive 1-bits indicating that the next bit is the
left-most bit in the left-most byte of the destination address.
•

Destination address (DA)—Consists
of 6 bytes. The DA field identifies which station(s) should receive the
frame. The left-most bit in the DA field indicates whether the address
is an individual address (indicated by a 0) or a group address
(indicated by a 1). The second bit from the left indicates whether the
DA is globally administered (indicated by a 0) or locally administered
(indicated by a 1). The remaining 46 bits are a uniquely assigned value
that identifies a single station, a defined group of stations, or all
stations on the network.
•

Source addresses (SA)—Consists
of 6 bytes. The SA field identifies the sending station. The SA is
always an individual address and the left-most bit in the SA field is
always 0.
•

Length/Type—Consists
of 2 bytes. This field indicates either the number of MAC-client data
bytes that are contained in the data field of the frame, or the frame
type ID if the frame is assembled using an optional format. If the
Length/Type field value is less than or equal to 1500, the number of
LLC bytes in the Data field is equal to the Length/Type field value. If
the Length/Type field value is greater than 1536, the frame is an
optional type frame, and the Length/Type field value identifies the
particular type of frame being sent or received.
•

Data—Is a sequence of n bytes of any value, where n
is less than or equal to 1500. If the length of the Data field is less
than 46, the Data field must be extended by adding a filler (a pad)
sufficient to bring the Data field length to 46 bytes.
•

Frame check sequence (FCS)—Consists
of 4 bytes. This sequence contains a 32-bit cyclic redundancy check
(CRC) value, which is created by the sending MAC and is recalculated by
the receiving MAC to check for damaged frames. The FCS is generated
over the DA, SA, Length/Type, and Data fields.
Figure 7-6        The Basic IEEE 802.3 MAC Data Frame Format


Note       

Individual
addresses are also known as unicast addresses because they refer to a
single MAC and are assigned by the NIC manufacturer from a block of
addresses allocated by the IEEE. Group addresses (a.k.a. multicast
addresses) identify the end stations in a workgroup and are assigned by
the network manager. A special group address (all 1s—the broadcast
address) indicates all stations on the network.
Frame Transmission
Whenever an end station
MAC receives a transmit-frame request with the accompanying address and
data information from the LLC sublayer, the MAC begins the transmission
sequence by transferring the LLC information into the MAC frame buffer.
•

The preamble and start-of-frame delimiter are inserted in the PRE and SOF fields.
•

The destination and source addresses are inserted into the address fields.
•

The LLC data bytes are counted, and the number of bytes is inserted into the Length/Type field.
•

The
LLC data bytes are inserted into the Data field. If the number of LLC
data bytes is less than 46, a pad is added to bring the Data field
length up to 46.
•

An FCS value is generated over the DA, SA, Length/Type, and Data fields and is appended to the end of the Data field.
After the frame is
assembled, actual frame transmission will depend on whether the MAC is
operating in half-duplex or full-duplex mode.
The IEEE 802.3 standard currently requires that all Ethernet MACs
support half-duplex operation, in which the MAC can be either
transmitting or receiving a frame, but it cannot be doing both
simultaneously. Full-duplex operation is an optional MAC capability
that allows the MAC to transmit and receive frames simultaneously.
Half-Duplex Transmission—The CSMA/CD Access Method
The CSMA/CD protocol was originally developed as a means by which two
or more stations could share a common media in a switch-less
environment when the protocol does not require central arbitration,
access tokens, or assigned time slots to indicate when a station will
be allowed to transmit. Each Ethernet MAC determines for itself when it
will be allowed to send a frame.
The CSMA/CD access rules are summarized by the protocol's acronym:
•

Carrier sense—Each station continuously listens for traffic on the medium to determine when gaps between frame transmissions occur.
•

Multiple access—Stations may begin transmitting any time they detect that the network is quiet (there is no traffic).
•

Collision detect—If
two or more stations in the same CSMA/CD network (collision domain)
begin transmitting at approximately the same time, the bit streams from
the transmitting stations will interfere (collide) with each other, and
both transmissions will be unreadable. If that happens, each
transmitting station must be capable of detecting that a collision has
occurred before it has finished sending its frame.
Each must stop
transmitting as soon as it has detected the collision and then must
wait a quasirandom length of time (determined by a back-off algorithm)
before attempting to retransmit the frame.
The worst-case situation occurs when the two most-distant stations on
the network both need to send a frame and when the second station does
not begin transmitting until just before the frame from the first
station arrives. The collision will be detected almost immediately by
the second station, but it will not be detected by the first station
until the corrupted signal has propagated all the way back to that
station. The maximum time that is required to detect a collision (the
collision window, or "slot time") is approximately equal to twice the
signal propagation time between the two most-distant stations on the
network.
This means that both the minimum frame length and the maximum collision
diameter are directly related to the slot time. Longer minimum frame
lengths translate to longer slot times and larger collision diameters;
shorter minimum frame lengths correspond to shorter slot times and
smaller collision diameters.
The trade-off was between the need to reduce the impact of collision
recovery and the need for network diameters to be large enough to
accommodate reasonable network sizes. The compromise was to choose a
maximum network diameter (about 2500 meters) and then to set the
minimum frame length long enough to ensure detection of all worst-case
collisions.
The compromise worked well for 10 Mbps, but it was a problem for higher
data-rate Ethernet developers. Fast Ethernet was required to provide
backward compatibility with earlier Ethernet networks, including the
existing IEEE 802.3 frame format and error-detection procedures, plus
all applications and networking software running on the
10-Mbps networks.
Although signal propagation velocity is essentially constant for all
transmission rates, the time required to transmit a frame is inversely
related to the transmission rate. At 100 Mbps, a minimum-length frame
can be transmitted in approximately one-tenth of the defined slot time,
and any collision that occurred during the transmission would not
likely be detected by the transmitting stations. This, in turn, meant
that the maximum network diameters specified for 10-Mbps networks could
not be used for 100-Mbps networks. The solution for Fast Ethernet was
to reduce the maximum network diameter by approximately a factor of 10
(to a little more than 200 meters).
The same problem also arose during specification development for
Gigabit Ethernet, but decreasing network diameters by another factor of
10 (to approximately 20 meters) for 1000-Mbps operation was simply not
practical. This time, the developers elected to maintain approximately
the same maximum collision domain diameters as 100-Mbps networks and to
increase the apparent minimum frame size by adding a variable-length
nondata extension field to frames that are shorter than the minimum
length (the extension field is removed during frame reception).
Figure 7-7 shows the MAC frame format with the gigabit extension field,
and Table 7-1 shows the effect of the trade-off between the
transmission data rate and the minimum frame size for 10-Mbps,
100-Mbps, and 1000-Mbps Ethernet.
Figure 7-7        MAC Frame with Gigabit Carrier Extension

  
Table 7-1        Limits for Half-Duplex Operation
      
Parameter
   
10 Mbps
   
100 Mbps
   
1000 Mbps
      
Minimum frame size
   
64 bytes
   
64 bytes
   
520 bytes1 (with extension field added)
      
Maximum collision diameter, DTE to DTE
   
100 meters UTP
   
100 meters UTP
412 meters fiber
   
100 meters UTP
316 meters fiber
      
Maximum collision diameter with repeaters
   
2500 meters
   
205 meters
   
200 meters
      
Maximum number of repeaters in network path
   
5
   
2
   
1
1 520
bytes applies to 1000Base-T implementations. The minimum frame size
with extension field for 1000Base-X is reduced to 416 bytes because
1000Base-X encodes and transmits 10 bits for each byte.
Another change to the
Ethernet CSMA/CD transmit specification was the addition of frame
bursting for gigabit operation. Burst mode is a feature that allows a
MAC to send a short sequence (a burst) of frames equal to approximately
5.4 maximum-length frames without having to relinquish control of the
medium. The transmitting MAC fills each interframe interval with
extension bits, as shown in Figure 7-8, so that other stations on the
network will see that the network is busy and will not attempt
transmission until after the burst is complete.
Figure 7-8        A Gigabit Frame-Burst Sequence

If the length of the first frame is less than the minimum frame length,
an extension field is added to extend the frame length to the value
indicated in Table 7-1. Subsequent frames
in
a frame-burst sequence do not need extension fields, and a frame burst
may continue as long as the burst limit has not been reached. If the
burst limit is reached after a frame transmission has begun,
transmission is allowed to continue until that entire frame has been
sent.
Frame extension fields are not defined, and burst mode is not allowed for 10 Mbps and 100 Mbps transmission rates.
Full-Duplex Transmission—An Optional Approach to Higher Network Efficiency
Full-duplex operation is an optional MAC capability that allows
simultaneous two-way transmission over point-to-point links. Full
duplex transmission is functionally much simpler than half-duplex
transmission because it involves no media contention, no collisions, no
need to schedule retransmissions, and no need for extension bits on the
end of short frames. The result is not only more time available for
transmission, but also an effective doubling of the link bandwidth
because each link can now support full-rate, simultaneous, two-way
transmission.
Transmission can usually
begin as soon as frames are ready to send. The only restriction is that
there must be a minimum-length interframe gap between successive
frames, as shown in Figure 7-9, and each frame must conform to Ethernet
frame format standards.
Figure 7-9        Full Duplex Operation Allows Simultaneous Two-Way Transmission on the Same Link

Flow Control
Full-duplex operation requires concurrent implementation of the
optional flow-control capability that allows a receiving node (such as
a network switch port) that is becoming congested to request the
sending node (such as a file server) to stop sending frames for a
selected short period of time. Control is MAC-to-MAC through the use of
a pause frame that is automatically generated by the receiving MAC. If
the congestion is relieved before the requested wait has expired, a
second pause frame with a zero time-to-wait value can be sent to
request resumption of transmission. An overview of the flow control
operation is shown in Figure 7-10.
Figure 7-10        An Overview of the IEEE 802.3 Flow Control Sequence

The full-duplex operation and its companion flow control capability are
both options for all Ethernet MACs and all transmission rates. Both
options are enabled on a link-by-link basis, assuming that the
associated physical layers are also capable of supporting full-duplex
operation.
Pause frames are identified as MAC control frames by an exclusive
assigned (reserved) length/type value. They are also assigned a
reserved destination address value to ensure that an incoming pause
frame is never forwarded to upper protocol layers or to other ports in
a switch.
Frame Reception
Frame reception is essentially the same for both half-duplex and
full-duplex operations, except that full-duplex MACs must have separate
frame buffers and data paths to allow for simultaneous frame
transmission and reception.
Frame reception is the reverse of frame transmission. The destination
address of the received frame is checked and matched against the
station's address list (its MAC address, its group addresses, and the
broadcast address) to determine whether the frame is destined for that
station. If an address match is found, the frame length is checked and
the received FCS is compared to the FCS that was generated during frame
reception. If the frame length is okay and there is an FCS match, the
frame type is determined by the contents of the Length/Type field. The
frame is then parsed and forwarded to the appropriate upper layer.
The VLAN Tagging Option
VLAN tagging is a MAC option that provides three important capabilities
not previously available to Ethernet network users and network
managers:
•

Provides a means to expedite time-critical network traffic by setting transmission priorities for outgoing frames.  
•

Allows
stations to be assigned to logical groups, to communicate across
multiple LANs as though they were on a single LAN. Bridges and switches
filter destination addresses and forward VLAN frames only to ports that
serve the VLAN to which the traffic belongs.
•

Simplifies network management and makes adds, moves, and changes easier to administer.
A VLAN-tagged frame is
simply a basic MAC data frame that has had a 4-byte VLAN header
inserted between the SA and Length/Type fields, as shown in Figure
7-11.
Figure 7-11        VLAN-Tagged Frames Are Identified When the MAC Finds the LAN Type Value in the Normal Length/Type Field Location

The VLAN header consists of two fields:
•

A reserved 2-byte type value, indicating that the frame is a VLAN frame
•

A
two-byte Tag-Control field that contains both the transmission priority
(0 to 7, where 7 is the highest) and a VLAN ID that identifies the
particular VLAN over which the frame is to be sent
The receiving MAC reads the reserved type value, which is located in
the normal Length/Type field position, and interprets the received
frame as a VLAN frame. Then the following occurs:
•

If
the MAC is installed in a switch port, the frame is forwarded according
to its priority level to all ports that are associated with the
indicated VLAN identifier.
•

If
the MAC is installed in an end station, it removes the 4-byte VLAN
header and processes the frame in the same manner as a basic data
frame.
VLAN tagging requires that all network nodes involved with a VLAN group be equipped with the VLAN option.

The Ethernet Physical Layers
Because Ethernet devices implement only the bottom two layers of the
OSI protocol stack, they are typically implemented as network interface
cards (NICs) that plug into the host device's motherboard. The
different NICs are identified by a three-part product name that is
based on the physical layer attributes.
The naming convention is a concatenation of three terms indicating the
transmission rate, the transmission method, and the media type/signal
encoding. For example, consider this:
•

10Base-T = 10 Mbps, baseband, over two twisted-pair cables
•

100Base-T2 = 100 Mbps, baseband, over two twisted-pair cables
•

100Base-T4 = 100 Mbps, baseband, over four-twisted pair cables
•

1000Base-LX = 100 Mbps, baseband, long wavelength over optical fiber cable
A question sometimes
arises as to why the middle term always seems to be "Base." Early
versions of the protocol also allowed for broadband transmission (for
example, 10Broad), but broadband implementations were not successful in
the marketplace. All current Ethernet implementations use baseband
transmission.
Encoding for Signal Transmission
In baseband transmission,
the frame information is directly impressed upon the link as a sequence
of pulses or data symbols that are typically attenuated (reduced in
size) and distorted (changed in shape) before they reach the other end
of the link. The receiver's task is to detect each pulse as it arrives
and then to extract its correct value before transferring the
reconstructed information to the receiving MAC.
Filters and pulse-shaping circuits can help restore the size and shape
of the received waveforms, but additional measures must be taken to
ensure that the received signals are sampled at the correct time in the
pulse period and at same rate as the transmit clock:
•

The
receive clock must be recovered from the incoming data stream to allow
the receiving physical layer to synchronize with the incoming pulses.
•

Compensating measures must be taken for a transmission effect known as baseline wander.
Clock recovery requires
level transitions in the incoming signal to identify and synchronize on
pulse boundaries. The alternating 1s and 0s of the frame preamble were
designed both to indicate that a frame was arriving and to aid in clock
recovery. However, recovered clocks can drift and possibly lose
synchronization if pulse levels remain constant and there are no
transitions to detect (for example, during long strings of 0s).
Baseline wander results because Ethernet links are AC-coupled to the
transceivers and because AC coupling is incapable of maintaining
voltage levels for more than a short time. As a result, transmitted
pulses are distorted by a droop effect similar to the exaggerated
example shown in Figure 7-12. In long strings of either 1s or 0s, the
droop can become so severe that the voltage level passes through the
decision threshold, resulting in erroneous sampled values for the
affected pulses.
Figure 7-12        A Concept Example of Baseline Wander

Fortunately, encoding the
outgoing signal before transmission can significantly reduce the effect
of both these problems, as well as reduce the possibility of
transmission errors. Early Ethernet implementations, up to and
including 10Base-T, all used the Manchester encoding method, shown in
Figure 7-13. Each pulse is clearly identified by the direction of the
midpulse transition rather than by its sampled level value.
Figure 7-13        Transition-Based Manchester Binary Encoding

Unfortunately, Manchester encoding introduces some difficult
frequency-related problems that make it unsuitable for use at higher
data rates. Ethernet versions subsequent to 10Base-T all use different
encoding procedures that include some or all of the following
techniques:
•

Using data scrambling—A
procedure that scrambles the bits in each byte in an orderly (and
recoverable) manner. Some 0s are changed to 1s, some 1s are changed to
0s, and some bits are left the same. The result is reduced run-length
of same-value bits, increased transition density, and easier clock
recovery.
•

Expanding the code space—A
technique that allows assignment of separate codes for data and control
symbols (such as start-of-stream and end-of-stream delimiters,
extension bits, and so on) and that assists in transmission error
detection.
•

Using forward error-correcting codes—An
encoding in which redundant information is added to the transmitted
data stream so that some types of transmission errors can be corrected
during frame reception.

Note       

Forward
error-correcting codes are used in 1000Base-T to achieve an effective
reduction in the bit error rate. Ethernet protocol limits error
handling to detection of bit errors in the received frame. Recovery of
frames received with uncorrectable errors or missing frames is the
responsibility of higher layers in the protocol stack.
The 802.3 Physical Layer Relationship to the ISO Reference Model
Although the specific logical model of the physical layer may vary from
version to version, all Ethernet NICs generally conform to the generic
model shown in Figure 7-14.
Figure 7-14        The Generic Ethernet Physical Layer Reference Model

The physical layer for each transmission rate is divided into sublayers
that are independent of the particular media type and sublayers that
are specific to the media type or signal encoding.
•

The reconciliation sublayer and the optional media-independent interface (MII in
10-Mbps
and 100-Mbps Ethernet, GMII in Gigabit Ethernet) provide the logical
connection between the MAC and the different sets of media-dependent
layers. The MII and GMII are defined with separate transmit and receive
data paths that are bit-serial for 10-Mbps implementations,
nibble-serial (4 bits wide) for 100-Mbps implementations, and
byte-serial (8 bits wide) for 1000-Mbps implementations. The
media-independent interfaces and the reconciliation sublayer are common
for their respective transmission rates and are configured for
full-duplex operation in 10Base-T and all subsequent Ethernet versions.
•

The
media-dependent physical coding sublayer (PCS) provides the logic for
encoding, multiplexing, and synchronization of the outgoing symbol
streams as well symbol code alignment, demultiplexing, and decoding of
the incoming data.
•

The
physical medium attachment (PMA) sublayer contains the signal
transmitters and receivers (transceivers), as well as the clock
recovery logic for the received data streams.
•

The medium-dependent interface (MDI) is the cable connector between the signal transceivers and the link.
•

The
Auto-negotiation sublayer allows the NICs at each end of the link to
exchange information about their individual capabilities, and then to
negotiate and select
the most favorable operational mode that they
both are capable of supporting. Auto-negotiation is optional in early
Ethernet implementations and is mandatory in later versions.
Depending on which type of signal encoding is used and how the links
are configured, the PCS and PMA may or may not be capable of supporting
full-duplex operation.
10-Mbps Ethernet—10Base-T
10Base-T provides Manchester-encoded 10-Mbps bit-serial communication
over two unshielded twisted-pair cables. Although the standard was
designed to support transmission over common telephone cable, the more
typical link configuration is to use two pair of a four-pair Category 3
or 5 cable, terminated at each NIC with an 8-pin RJ-45 connector (the
MDI), as shown in Figure 7-15. Because each active pair is configured
as a simplex link where transmission is in one direction only, the
10Base-T physical layers can support either half-duplex or full-duplex
operation.
Figure 7-15        The Typical 10Base-T Link Is a Four-Pair UTP Cable in Which Two Pairs Are Not Used

Although 10Base-T may be considered essentially obsolete in some
circles, it is included here because there are still many 10Base-T
Ethernet networks, and because full-duplex operation has given 10BaseT
an extended life.
10Base-T was also the first Ethernet version to include a link
integrity test to determine the health of the link. Immediately after
powerup, the PMA transmits a normal link pulse (NLP) to tell the NIC at
the other end of the link that this NIC wants to establish an active
link connection:
•

If the NIC at the other end of the link is also powered up, it responds with its own NLP.
•

If
the NIC at the other end of the link is not powered up, this NIC
continues sending an NLP about once every 16 ms until it receives a
response.
The link is activated only after both NICs are capable of exchanging valid NLPs.
100 Mbps—Fast Ethernet
Increasing the Ethernet transmission rate by a factor of ten over
10Base-T was not a simple task, and the effort resulted in the
development of three separate physical layer standards for 100 Mbps
over UTP cable: 100Base-TX and 100Base-T4 in 1995, and 100Base-T2 in
1997. Each was defined with different encoding requirements and a
different set of media-dependent sublayers, even though there is some
overlap in the link cabling. Table 7-2 compares the physical layer
characteristics of 10Base-T to the various 100Base versions.
  
Table 7-2        Summary of 100Base-T Physical Layer Characteristics
      
Ethernet Version
   
Transmit Symbol Rate1
   
Encoding
   
Cabling
   
Full-Duplex Operation
      
10Base-T
   
10 MBd
   
Manchester
   
Two pairs of UTP Category -3 or better
   
Supported
      
100Base-TX
   
125 MBd
   
4B/5B
   
Two pairs of UTP Category -5 or Type 1 STP
   
Supported
      
100Base-T4
   
33 MBd
   
8B/6T
   
Four pairs of UTP Category -3 or better
   
Not supported
      
100Base-T2
   
25 MBd
   
PAM5x5
   
Two pairs of UTP Category -3 or better
   
Supported
1
One baud = one transmitted symbol per second, where the transmitted
symbol may contain the equivalent value of 1 or more binary bits.
Although not all three 100-Mbps versions were successful in the
marketplace, all three have been discussed in the literature, and all
three did impact future designs. As such, all three are important to
consider here.
100Base-X
100Base-X was designed to support transmission over either two pairs of
Category 5 UTP copper wire or two strands of optical fiber. Although
the encoding, decoding, and clock recovery procedures are the same for
both media, the signal transmission is different—electrical pulses in
copper and light pulses in optical fiber. The signal transceivers that
were included as part of the PMA function in the generic logical model
of Figure 7-14 were redefined as the separate physical media-dependent
(PMD) sublayers shown in Figure 7-16.
Figure 7-16        The 100Base-X Logical Model

The 100Base-X encoding procedure is based on the earlier FDDI optical
fiber physical media-dependent and FDDI/CDDI copper twisted-pair
physical media-dependent signaling standards developed by ISO and ANSI.
The 100Base-TX physical media-dependent sublayer (TP-PMD) was
implemented with CDDI semiconductor transceivers and RJ-45 connectors;
the fiber PMD was implemented with FDDI optical transceivers and the
Low Cost Fibre Interface Connector (commonly called the duplex SC
connector).
The 4B/5B encoding procedure is the same as the encoding procedure used
by FDDI, with only minor adaptations to accommodate Ethernet frame
control. Each 4-bit data nibble (representing half of a data byte) is
mapped into a 5-bit binary code-group that is transmitted bit-serial
over the link. The expanded code space provided by the 32 5-bit
code-groups allow separate assignment for the following:
•

The 16 possible values in a 4-bit data nibble (16 code-groups).
•

Four
control code-groups that are transmitted as code-group pairs to
indicate the start-of-stream delimiter (SSD) and the end-of-stream
delimiter (ESD). Each MAC frame is "encapsulated" to mark both the
beginning and end of the frame. The first byte of preamble is replaced
with SSD code-group pair that precisely identifies the frame's
code-group boundaries. The ESD code-group pair is appended after the
frame's FCS field.
•

A
special IDLE code-group that is continuously sent during interframe
gaps to maintain continuous synchronization between the NICs at each
end of the link. The receipt of IDLE is interpreted to mean that the
link is quiet.
•

Eleven
invalid code-groups that are not intentionally transmitted by a NIC
(although one is used by a repeater to propagate receive errors).
Receipt of any invalid code-group will cause the incoming frame to be
treated as an invalid frame.
Figure 7-17 shows how a MAC frame is encapsulated before being transmitted as a 100Base-X code-group stream.  
Figure 7-17        The 100Base-X Code-Group Stream with Frame Encapsulation

100Base-TX transmits and receives on the same link pairs and uses the
same pin assignments on the MDI as 10Base-T. 100Base-TX and 100Base-FX
both support half-duplex and full-duplex transmission.
100Base-T4
100Base-T4 was developed
to allow 10BaseT networks to be upgraded to 100-Mbps operation without
requiring existing four-pair Category 3 UTP cables to be replaced with
the newer Category 5 cables. Two of the four pairs are configured for
half-duplex operation and can support transmission in either direction,
but only in one direction at a time. The other two pairs are configured
as simplex pairs dedicated to transmission in one direction only. Frame
transmission uses both half-duplex pairs, plus the simplex pair that is
appropriate for the transmission direction, as shown in Figure 7-18.
The simplex pair for the opposite direction provides carrier sense and
collision detection. Full-duplex operation cannot be supported on
100Base-T4.
Figure 7-18        The 100Base-T4 Wire-Pair Usage During Frame Transmission

100Base-T4 uses an 8B6T
encoding scheme in which each 8-bit binary byte is mapped into a
pattern of six ternary (three-level: +1, 0, -1) symbols known as 6T
code-groups. Separate 6T code-groups are used for IDLE and for the
control code-groups that are necessary for frame transmission. IDLE
received on the dedicated receive pair indicates that the link is
quiet.
During frame transmission, 6T data code-groups are transmitted in a
delayed round-robin sequence over the three transmit wire-pairs, as
shown in Figure 7-19. Each frame is encapsulated with start-of-stream
and end-of-packet 6T code-groups that mark both the beginning and end
of the frame, and the beginning and end of the 6T code-group stream on
each wire pair. Receipt of a non-IDLE code-group over the dedicated
receive-pair any time before the collision window expires indicates
that a collision has occurred.
Figure 7-19        The 100Base-T4 Frame Transmission Sequence

100Base-T2
The 100Base-T2 specification was developed as a better alternative for
upgrading networks with installed Category 3 cabling than was being
provided by 100Base-T4. Two important new goals were defined:
•

To provide communication over two pairs of Category 3 or better cable
•

To support both half-duplex and full-duplex operation
100Base-T2 uses a different signal transmission procedure than any
previous twisted-pair Ethernet implementations. Instead of using two
simplex links to form one full-duplex link, the 100Base-T2 dual-duplex
baseband transmission method sends encoded symbols simultaneously in
both directions on both wire pairs, as shown in Figure 7-20. The term
"TDX" indicates the 2 most significant bits in the nibble
before encoding and transmission. "RDX" indicates the same 2
bits after receipt and decoding.
Figure 7-20        The 100Base-T2 Link Topology

Dual-duplex baseband transmission requires the NICs at each end of the
link to be operated in a master/slave loop-timing mode. Which NIC will
be master and which will be slave
is
determined by autonegotiation during link initiation. When the link is
operational, synchronization is based on the master NIC's internal
transmit clock. The slave NIC uses the recovered clock for both
transmit and receive operations, as shown in Figure 7-21.
Each
transmitted frame is encapsulated, and link synchronization is
maintained with a continuous stream of IDLE symbols during interframe
gaps.
Figure 7-21        The 100Base-T2 Loop Timing Configuration

The 100Base-T2 encoding
process first scrambles the data frame nibbles to randomize the bit
sequence. It then maps the two upper bits and the two lower bits of
each nibble into two five-level (+2, +1, 0, -1, -2) pulse
amplitude-modulated (PAM5) symbols that are simultaneously transmitted
over the two wire pairs (PAM5x5). Different scrambling procedures for
master and slave transmissions ensure that the data streams traveling
in opposite directions on the same wire pair are uncoordinated.
Signal reception is essentially the reverse of signal transmission.
Because the signal on each wire pair at the MDI is the sum of the
transmitted signal and the received signal, each receiver subtracts the
transmitted symbols from the signal received at the MDI to recover the
symbols in the incoming data stream. The incoming symbol pair is then
decoded, unscrambled, and reconstituted as a data nibble for transfer
to the MAC.
1000 Mbps—Gigabit Ethernet
The Gigabit Ethernet
standards development resulted in two primary specifications:
1000Base-T for UTP copper cable and 1000Base-X STP copper cable, as
well as single and multimode optical fiber (see Figure 7-22).
Figure 7-22        Gigabit Ethernet Variations

1000Base-T
1000Base-T Ethernet provides full-duplex transmission over four-pair
Category 5 or better UTP cable. 1000Base-T is based largely on the
findings and design approaches that led to the development of the Fast
Ethernet physical layer implementations:
•

100Base-TX proved that binary symbol streams could be successfully transmitted over Category 5 UTP cable at 125 MBd.
•

100Base-T4 provided a basic understanding of the problems related to sending multilevel signals over four wire pairs.
•

100Base-T2
proved that PAM5 encoding, coupled with digital signal processing,
could handle both simultaneous two-way data streams and potential
crosstalk problems resulting from alien signals on adjacent wire pairs.
1000Base-T scrambles each
byte in the MAC frame to randomize the bit sequence before it is
encoded using a 4-D, 8-State Trellis Forward Error Correction (FEC)
coding in which four PAM5 symbols are sent at the same time over four
wire pairs. Four of the five levels in each PAM5 symbol represent 2
bits in the data byte. The fifth level is used for FEC coding, which
enhances symbol recovery in the presence of noise and crosstalk.
Separate scramblers for the master and slave PHYs create essentially
uncorrelated data streams between the two opposite-travelling symbol
streams on each wire pair.
The1000Base-T link topology is shown in Figure 7-23. The term
"TDX" indicates the 2 most significant bits in the data byte
before encoding and transmission. "RDX" indicates the same 2
bits after receipt and decoding.
Figure 7-23        The 1000Base-T Link Topology

The clock recovery and master/slave loop timing procedures are
essentially the same as those used in 100Base-T2 (see Figure 7-24).
Which NIC will be master (typically the NIC in a multiport intermediate
network node) and which will be slave is determined during
autonegotiation.
Figure 7-24        1000Base-T Master/Slave Loop Timing Configuration

Each transmitted frame is encapsulated with start-of-stream and
end-of-stream delimiters, and loop timing is maintained by continuous
streams of IDLE symbols sent on each wire pair during interframe gaps.
1000Base-T supports both half-duplex and full-duplex operation.
1000Base-X
All three 1000Base-X versions support full-duplex binary transmission
at 1250 Mbps over two strands of optical fiber or two STP copper
wire-pairs, as shown in Figure 7-25. Transmission coding is based on
the ANSI Fibre Channel 8B/10B encoding scheme. Each 8-bit data byte is
mapped into a 10-bit code-group for bit-serial transmission. Like
earlier Ethernet versions, each data frame is encapsulated at the
physical layer before transmission, and link synchronization is
maintained by sending a continuous stream of IDLE code-groups during
interframe gaps. All 1000Base-X physical layers support both
half-duplex and full-duplex operation.
Figure 7-25        1000Base-X Link Configuration

The principal differences
among the 1000Base-X versions are the link media and connectors that
the particular versions will support and, in the case of optical media,
the wavelength of the optical signal (see Table 7-3).
  
Table 7-3        1000Base-X Link Configuration Support
      
Link Configuration
   
1000Base-CX
   
1000Base-SX (850 nm Wavelength)
   
1000Base-LX (1300 nm Wavelength)
      
150 W STP copper
   
Supported
   
Not supported
   
Not supported
      
125/62.5 mm multimode optical fiber1
   
Not supported
   
Supported
   
Supported
      
125/50 mm multimode optical fiber
   
Not supported
   
Supported
   
Supported
      
125/10 mm single mode optical fiber
   
Not supported
   
Not supported
   
Supported
      
Allowed connectors
   
IEC style 1 or Fibre Channel style 2
   
SFF MT-RJ  or Duplex SC
   
SFF MT-RJ  or Duplex SC
1 The 125/62.5 mm specification refers to the cladding and core diameters of the optical fiber.
Network Cabling—Link Crossover Requirements
Link compatibility requires that the transmitters at each end of the
link be connected to the receivers at the other end of the link.
However, because cable connectors at both ends of the link are keyed
the same, the conductors must cross over at some point to ensure that
transmitter outputs are always connected to receiver inputs.
Unfortunately, when this requirement first came up in the development
of 10Base-T, IEEE 802.3 chose not to make a hard rule as to whether the
crossover should be implemented in the cable as shown in Figure 7-26a
or whether it should be implemented internally as shown in Figure
7-26b.
Figure 7-26        Alternative Ways for Implementing the Link Crossover Requirement

Instead, IEEE 802.3 defined two rules and made two recommendations:
•

There must be an odd number of crossovers in all multiconductor links.
•

If a PMD is equipped with an internal crossover, its MDI must be clearly labeled with the graphical X symbol.
•

Implementation of an internal crossover function is optional.
•

When
a DTE is connected to a repeater or switch (DCE) port, it is
recommended that the crossover be implemented within the DCE port.
The eventual result was
that ports in most DCEs were equipped with PMDs that contained internal
crossover circuitry and that DTEs had PMDs without internal crossovers.
This led to the following oft-quoted de facto "installation rule":
•

Use a straight-through cable when connecting DTE to DCE. Use a crossover cable when connecting DTE to DTE or DCE to DCE.
Unfortunately, the de facto rule does not apply to all Ethernet
versions that have been developed subsequent to 10Base-T. As things now
stand, the following is true:
•

All fiber-based systems use cables that have the crossover implemented within the cable.
•

All 100Base systems using twisted-pair links use the same rules and recommendations as 10Base-T.
•

1000Base-T
NICs may implement a selectable internal crossover option that can be
negotiated and enabled during autonegotiation. When the selectable
crossover option is not implemented, 10Base-T rules and recommendations
apply.

System Considerations
Given all the choices discussed previously, it might seem that it would
be no problem to upgrade an existing network or to plan a new network.
The problem is twofold. Not all the choices are reasonable for all
networks, and not all Ethernet versions and options are available in
the market, even though they may have been specified in the standard.
Choosing UTP-Based Components and Media Category
By now, it should be
obvious that UTP-based NICs are available for 10-Mbps, 100-Mbps, and
1000-Mbps implementations. The choice is relatively simple for both
10-Mbps and 1000-Mbps operation: 10Base-T and 1000Base-T. From the
previous discussions, however, it would not seem to be that simple for
100-Mbps implementations.
Although three UTP-based NICs are defined for 100 Mbps, the market has
effectively narrowed the choice to just 100Base-TX, which became widely
available during the first half of 1995:
•

By
the time 100Base-T4 products first appeared on the market, 100Base-TX
was well entrenched, and development of the full-duplex option, which
100Base-T4 could not support, was well underway.
•

The
100Base-T2 standard was not approved until spring 1997, too late to
interest the marketplace. As a result, 100Base-T2 products were not
even manufactured.
Several choices have also been specified for UTP media: Category 3, 4,
5, or 5E. The differences are cable cost and transmission rate
capability, both of which increase with the category numbers. However,
current transmission rate requirements and cable cost should not be the
deciding factors in choosing which cable category to install. To allow
for future transmission rate needs, cables lower than Category 5 should
not even be considered, and if gigabit rates are a possible future
need, Category 5E should be seriously considered:
•

Installation labor costs are essentially constant for all types of UTP four-pair cable.
•

Labor
costs for upgrading installed cable (removing the existing and
installing new) are typically greater than the cost of the original
installation.
•

UTP cable is backward-compatible. Higher-category cable will support lower-category NICs, but not vice versa.
•

The physical life of UTP cable (decades) is much longer than the useable life of the connected equipment.  
Auto-negotiation—An Optional Method for Automatically Configuring Link Operational Modes
The purpose of
autonegotiation is to find a way for two NICs that share a UTP link to
communicate with each other, regardless of whether they both
implemented the same Ethernet version or option set.
Autonegotiation is performed totally within the physical layers during
link initiation, without any additional overhead either to the MAC or
to higher protocol layers. Autonegotiation allows UTP-based NICs to do
the following:
•

Advertise their Ethernet version and any optional capabilities to the NIC at the other end of the link  
•

Acknowledge receipt and understanding of the operational modes that both NICs share
•

Reject any operational modes that are not shared
•

Configure each NIC for highest-level operational mode that both NICs can support
Autonegotiation is
specified as an option for 10Base-T, 100Base-TX, and 100Base-T4, but it
is required for 100Base-T2 and 1000Base-T implementations. Table 7-4
lists the defined selection priority levels (highest level = top
priority) for UTP-based Ethernet NICs.
  
Table 7-4        The Defined Autonegotiation Selection Levels for UTP NICs
      
Selection Level
   
Operational Mode
   
Maximum Total Data Transfer Rate (Mbps)1
      
9
   
1000Base-T full-duplex
   
2000
      
8
   
1000Base-T half-duplex
   
1000
      
7
   
100Base-T2 full-duplex
   
200
      
6
   
100Base-TX full-duplex
   
200
      
5
   
100Base-T2 half-duplex
   
100
      
4
   
100Base-T4 half-duplex
   
100
      
3
   
100Base-TX half-duplex
   
100
      
2
   
10Base-T full-duplex
   
20
      
1
   
10Base-T half-duplex
   
10
1 Because full-duplex operation allows simultaneous two-way
transmission, the maximum total transfer rate for full-duplex operation
is double the half-duplex transmission rate.
The autonegotiation
function in UTP-based NICs uses a modified 10Base-T link integrity
pulse sequence in which the NLPs are replaced by bursts of fast link
pulses (FLPs), as shown in Figure 7-27. Each FLP burst is an
alternating clock/data sequence in which the data bits in the burst
identify the operational modes supported by the transmitting NIC and
also provide information used by the autonegotiation handshake
mechanism. If the NIC at the other end of the link is a compatible NIC
but does not have autonegotiation capability, a parallel detection
function still allows it to be recognized. A NIC that fails to respond
to FLP bursts and returns only NLPs is treated as a 10Base-T
half-duplex NIC.
Figure 7-27        Autonegotiation FLP Bursts Replace NLPs During Link Initiation

At first glance, it may
appear that the autonegotiation process would always select the mode
supported by the NIC with the lessor capability, which would be the
case if both NICs use the same encoding procedures and link
configuration. For example, if both NICs are 100Base-TX but only one
supports full-duplex operation, the negotiated operational mode will be
half-duplex 100Base-TX. Unfortunately, the different 100Base versions
are not compatible with each other at 100 Mbps, and a 100Base-TX
full-duplex NIC would autonegotiate with a 100Base-T4 NIC to operate in
10Base-T half-duplex mode.
Autonegotiation in 1000Base-X NICs is similar to autonegotiation in
UTP-based systems, except that it currently applies only to compatible
1000Base-X devices and is currently constrained to negotiate only
half-duplex or full-duplex operation and flow control direction.
Network Switches Provide a Second, and Often Better, Alternative to Higher Link Speeds in CSMA/CD Network Upgrades
Competitively priced network switches became available on the market
shortly after the mid-1990s and essentially made network repeaters
obsolete for large networks. Although repeaters can accept only one
frame at a time and then send it to all active ports (except the port
on which it is being received), switches are equipped with the
following:
•

MAC-based
ports with I/O frame buffers that effectively isolate the port from
traffic being sent at the same time to or from other ports on the
switch
•

Multiple internal data paths that allow several frames to be transferred between different ports at the same time
These may seem like small
differences, but they produce a major effect in network operation.
Because each port provides access to a high-speed network bridge (the
switch), the collision domain in the network is reduced to a series of
small domains in which the number of participants is reduced to two—the
switch port and the connected NIC (see Figure 7-28). Furthermore,
because each participant is now in a private collision domain, his or
her available bandwidth has not only been markedly increased, it was
also done without having to change the link speed.
Consider, for example, a
48-station workgroup with a couple of large file servers and several
network printers on a 100-Mbps CSMA/CD network. The average available
bandwidth, not counting interframe gaps and collision recovery, would
be 100 ³
50 = 2 Mbps (network print servers do not generate network traffic). On
the other hand, if the same workgroup were still on a 10Base-T network
in which the repeaters had been replaced with network switches, the
bandwidth available to each user would be 10 Mbps.
Clearly, network configuration is as important as raw link speed.  

Note       

To
ensure that each end station will be capable of communicating at full
rate, the network switches should be nonsaturating (be capable of
accepting and transferring data at the full rate from each port
simultaneously).
Multispeed NICs
Auto-negotiation opened
the door to the development of low-cost, multispeed NICs that, for
example, support both half- and full-duplex operation under either
100Base-TX or 10Base-T signaling procedures. Multispeed NICs allow
staged network upgrades in which the 10Base-T half-duplex end stations
can be connected to 100Base-TX full-duplex switch ports without
requiring the NIC in the PC to be changed. Then, as more bandwidth is
needed for individual PCs, the NICs in those PCs can be upgraded to
100Base-TX full-duplex mode.
Figure 7-28        Replacing the Network Repeaters with Switches Reduces the Collision Domains to Two NICs Each

Choosing 1000Base-X Components and Media
Although Table 7-3 shows
that there is considerable flexibility of choice in the 1000Base-X link
media, there is not total flexibility. Some choices are preferred over
others:
•

NICs
at both ends of the link must be the same 1000Base-X version (CX, LX,
or SX), and the link connectors must match the NIC connectors.
•

The
1000Base-CX specification allows either style 1 or style 2 connectors,
but style 2 is preferred because some style 1 connectors are not
suitable for operation at 1250 Mbps. 1000Base-CX links are intended for
patch-cord use within a communications closet and are limited to 25
meters.
•

The
1000Base-LX and 1000Base-SX specifications allow either the small form
factor SFF MT-RJ or the larger duplex SC connectors. Because SFF MT-RJ
connectors are only about half as large as duplex SC connectors, and
because space is a premium, it follows that SFF MT-RJ connectors may
become the predominant connector.
•

1000Base-LX transceivers generally cost more than 1000Base-SX transceivers.
•

The
maximum operating range for optical fibers depends on both the
transmission wavelength and the modal bandwidth (MHz.km) rating of the
fiber. See Table 7-5.
  
Table 7-5        Maximum Operating Ranges for Common Optical Fibers
      
Fiber Core Diameter/Modal Bandwidth
   
1000Base-SX
(850 nm Wavelength)
   
1000Base-LX
(1300 nm Wavelength)
      
62.5 mm multimode fiber (200/500) MHz.km
   
275 meters
   
550 meters1
      
50 mm multimode fiber (400/400) MHz.km
   
500 meters
   
550 meters1
      
50 mm multimode fiber (500/500) MHz.km
   
550 meters
   
550 meters1
      
10 mm single-mode fiber
   
Not supported
   
5000 meters
1 1000Base-LX
transceivers may also require use of an offset-launch,
mode-conditioning patch cord when coupling to some existing multimode
fibers.
The operating ranges shown in Table 7-5 are those specified in the IEEE 802.3 standard.
In practice, however, the maximum operating range for LX transceivers over 62.5 mm
multimode fiber is approximately 700 meters, and some LX transceivers
have been qualified to support a 10,000-meter operating range over
single-mode fiber.
Multiple-Rate Ethernet Networks
Given the opportunities
shown by the example in the previous sections, it is not surprising
that most large Ethernet networks are now implemented with a mix of
transmission rates and link media, as shown in the cable model in
Figure 7-29.
Figure 7-29        An Example Multirate Network Topology—the ISO/IEC 11801 Cable Model

The ISO/IEC 11801 cable model is the network model on which the IEEE 802.3 standards are based:  
•

Campus distributor—The term campus
refers to a facility with two or more buildings in a relatively small
area. This is the central point of the campus backbone and the telecom
connection point with the outside world. In Ethernet LANs, the campus
distributor would typically be a gigabit switch with telecom interface
capability.
•

Building distributor—This
is the building's connection point to the campus backbone. An Ethernet
building distributor would typically be a 1000/100- or 1000/100/10-Mbps
switch.
•

Floor distributor—This
is the floor's connection point to the building distributor. ISO/IEC
11801 recommends at least one floor distributor for every 1000 m2
of floor space in office environments, and, if possible, a separate
distributor for each floor in the building. An Ethernet floor
distributor would typically be a 1000/100/10- or 100/10-Mbps switch.
•

Telecom outlet—This
is the network connection point for PCs, workstations, and print
servers. File servers are typically colocated with and directly
connected to the campus, building, or floor distributors, as
appropriate for their intended use.
•

Campus backbone cabling—This
is typically single- or multimode cable that interconnects the central
campus distributor with each of the building distributors.
•

Building backbone cabling—This
is typically Category 5 or better UTP or multimode fiber cable that
interconnects the building distributor with each of the floor
distributors in the building.
•

Horizontal cabling—This is predominantly Category 5 or better UTP cable, although a few installations are using multimode fiber.
As with UTP cable
selection, the choice of link media and intermediate network nodes
should always be made with an eye to future transmission rate needs and
the life expectancy of the network elements, unpredictable though they
may be. In the 1990s, LAN transmission rates increased 100 times and,
by 2002, will increase yet another 10 times.
This does not mean that all—or even some—end stations and their
interconnecting links will require gigabit capability. It does mean,
however, that more central network nodes (such as most campus
distributors and many building distributors) should be equipped with
gigabit capability, and that all floor distributors should have at
least 100 Mbps capability. It also means that all network switches
should be nonblocking and that all ports should have full-duplex
capability, and that any new campus backbone links should be installed
with single-mode fiber.
Link Aggregation—Establishing Higher-Speed Network Trunks
Link aggregation is a
recent optional MAC capability that allows several physical links to be
combined into one logical higher-speed trunk. It provides the means to
increase the effective data rate between two network nodes in unit
multiples of the individual link transmission rate rather than in an
order-of-magnitude step.
Link aggregation can be a cost-effective way to provide higher-speed
connections in Ethernet LANs that are reaching saturation with 100 Mbps
transmission rates but that won't require gigabit capability, at least
in the short term. For example, the maximum length for 62.5 mm
multimode fiber links is 2000 meters at 100 Mbps, and multimode fiber
has been often used for campus backbone links. The logical upgrade
would seem to be to reuse these links for 1000 Mbps operation, but the
maximum supportable length for multimode fiber is only 700 meters and
only with 1000Base-LX. If the existing links are longer than 700
meters, aggregating n existing links will support an effective transmission rate of (100 n) Mbps.
Link aggregation should be viewed as a network configuration option
that is primarily used in the few interconnections that require higher
data rates than can be provided by single links, such as
switch-to-switch and in switch-to-file server. It can also be used to
increase the reliability of critical links. Aggregated links can be
rapidly reconfigured (typically in about 1 second or less) in case of
link failure, with low risk of duplicated or reordered frames.
Link aggregation does not affect either the IEEE 802.3 data frame
format(s) or any higher layers in the protocol stack. It is
backward-compatible with "aggregation-unaware" devices and can be used
with any Ethernet data rate (although it does not make sense for 10
Mbps because it would likely cost less to procure a pair of 100-Mbps
NICs). Link aggregation can be enabled only on parallel point-to-point
links and those that support full-duplex same-speed operation.
Network Management
All higher-speed Ethernet specifications include definitions for
managed objects and control agents that are compatible with Simple
Network Management Protocol (SNMP) and that can be used to gather
statistics about the operation of the network nodes and to assist in
network management. Because user information is anecdotal at best and
usually comes long after the fact, all larger networks should at least
be configured with managed switches and network servers to ensure that
potential problems and bottlenecks can be identified before they cause
serious network deterioration.
Migrating to Higher-Speed Networks
By now, it should be apparent that upgrading existing networks
typically does not require wholesale equipment or media changes, but it
does require knowledge of the current network configuration and the
network location of potential problems. This means that a network
management system should be in place and that a cable plant database
should be both available and accurate. It is time-consuming and often
difficult to determine link type and availability after the cables have
been pulled through conduit, buried in walls, and layered in cable
trays.
Links are often the limiting factors in network upgrades. Existing
Category 5 links should support all current Ethernet rates from 10 Mbps
to 1000 Mbps, although they should be tested to ensure their capability
to support gigabit rates. If the network is equipped with only Category
3 cable, some links will have to be replaced before upgrading to 1000
Mbps. A similar situation exists with single- and multimode fiber.
Multimode fiber cannot be used for all backbone installations.
Single-mode fiber, on the other hand, not only can support all backbone
lengths up to 10,000 meters at 1000 Mbps, but it also will be capable
of supporting backbone use at 10-gigabit data rates in the future.
Switch replacement can begin as soon as the necessary links are
available. Existing switches at the campus and building distributor
levels can often be reused at the building or floor distributor level.
NICs can generally be replaced to extend the useful life of end
stations. And so on.

Summary
The chapter began with an overview of the Ethernet technology, the
network building blocks, and Ethernet's relationship to the ISO
seven-layer reference model. The requirements for MAC and PHY
compatibility also were introduced.
The basic MAC responsibilities were defined:
•

Data encapsulation—Assembling
the frame into the defined format before transmission begins, and
disassembling the frame after it has been received and checked for
transmission errors.
•

Media access control—In the required CSMA/CD half-duplex mode, and in the optional full-duplex mode.
Two optional MAC capability extensions and their associated frame
formats were discussed. The VLAN tagging option allows network nodes to
be defined with logical as well as physical addresses, and provides a
means to assign transmission priorities on a frame-by-frame basis. A
specific format for the pause frame, which is used for short-term link
flow control, is defined in the standard but was not covered here
because it is automatic MAC capability that is invoked as needed to
prevent input buffer overrun.
The PHY layer discussions
included descriptions of the signaling procedures and media
requirements/limitations for the following:
•

10Base-T
•

100Base-TX, 100Base-T4, and 100Base-T2
•

1000Base-T, 1000Base-CX, 1000Base-LX, and 1000Base-SX
Although 100Base-FX was
not specifically discussed, it uses the same signaling procedure as
100Base-TX, but over optical fiber media rather than UTP copper.
The remaining sections of
the chapter were devoted to systems considerations for both
twisted-pair and optical fiber LAN implementations:
•

Link crossover requirements in UTP networks
•

Matching of PMDs and network media to ensure desired data rates
•

Use of link aggregation to create higher-speed logical trunks
•

Implementation of multispeed networks
After essentially
finishing the chapter, you should have a reasonable working knowledge
of the Ethernet protocol and network technology. The next section
should help determine whether you need to go back and reread the
chapter.
               
               
               
               
               

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