10-Mbps and 100-Mbps Ethernet 319
Manchester encoding relies on the direction of the edge transition in the middle of the
timing window to determine the binary value for that bit period. In the encoding exam-
ple shown in Figure 6-3, one timing window is highlighted vertically through all four
waveform examples. The top waveform has a falling edge in the center of the timing
window, so it is interpreted as a binary 0.
The result is that in the center of the timing window for the second waveform, there is
a rising edge, which is interpreted as a binary 1.
Instead of a repeating sequence of the same binary value in the third waveform example,
there is an alternating binary sequence. In the first two examples, the signal must tran-
sition back between each bit period so that it can make the same-direction transition
each time in the center of the timing window. With alternating binary data, there is no
need to return to the previous voltage level in preparation for the next edge in the center
of the timing window. Thus, any time there is a long separation between one edge and
the next, you can be certain that both edges represent the middle of a timing window.
The fourth waveform example is random data that enables you verify that whenever
there is a wide separation between two transitions, both edges are in the center of a
timing window and represent the binary value for that timing window.
Legacy (10-Mbps) Ethernet has some common architectural features. All of these legacy
versions are referred to as shared Ethernet because they share a common collision domain.
It is not only allowed, but it is expected that an Ethernet network could contain multi-
ple types of media (for example, 10BASE5, 10BASE2, 10BASE-T, and so on). The
standard goes out of its way to ensure that interoperability is maintained. However,
when implementing a mixed-media network, it is important to pay particular attention
to the overall architecture design. It becomes easier to violate maximum delay limits as
the network grows and becomes more complex. The timing limits are based on param-
eters such as these:
■ Cable length and its propagation delay
■ Delay of repeaters
■ Delay of transceivers (including NICs, hubs, and switches)
■ Interframe gap shrinkage

■ Delays within the station
Lab Activity Waveform Decoding
The purpose of this lab is to integrate knowledge of networking media; OSI
Layers 1, 2, and 3; and Ethernet, by decoding a digital waveform of an
Ethernet frame.
1102.book Page 319 Tuesday, May 20, 2003 2:53 PM
320 Chapter 6: Ethernet Technologies and Ethernet Switching
5-4-3 Rule
10-Mbps Ethernet operates within the timing limits offered by a series of no more than
five segments separated by no more than four repeaters. That is, no more than four
repeaters can be connected in series between any two distant stations. The coaxial
implementations have a further requirement that there can be no more than three pop-
ulated segments between any two distant stations. The other two allowed coaxial seg-
ments are used to extend the diameter of the collision domain and are called link segments.
The primary characteristic of a link segment is that it has exactly two devices attached.
All twisted-pair links, such as 10BASE-T, meet the definition of a link segment.
10BASE5
The original (1980) Ethernet product (10BASE5) transmitted 10 Mbps over a single
thick coaxial cable bus, thus the name Thicknet. 10BASE5 is important for historical
reasons: It was the first medium used for Ethernet. 10BASE5 was part of the original
802.3 standard. It can be found today as part of legacy installations. It is not a preferred
choice for new networks because its primary benefit, length, can be accomplished in
other ways. Although 10BASE5 systems are inexpensive and require no configuration
(there is no need for hubs to extend the length of the system), basic components such
as NICs are very difficult to find, and the technology is very sensitive to signal reflections
on the cable. In addition, 10BASE5 systems are very cable-dependent across the whole
collision domain and thus represent a large single point of failure.
The timing, frame format, and transmission process were described previously in
Chapter 5, “Ethernet Fundamentals,” and are common to all 10-Mbps legacy Ethernet.
10BASE5 uses Manchester-encoded signals on thick coaxial cable. Figure 6-4 is an

example of a 10BASE5 signal. It is transmitted from approximately 0V to –1V. 10BASE5
potentially could be idle (0V) for days if no station wanted to transmit. 10BASE5 is
asynchronous.
Figure 6-4 10BASE5 Signal Decoded
1102.book Page 320 Tuesday, May 20, 2003 2:53 PM
10-Mbps and 100-Mbps Ethernet 321
In Figure 6-4, timing marks have been added to aid you in recognizing the timing
windows from which the binary data was decoded. The y-axis is voltage; the x-axis is
time. Voltage has been measured between the central conductor and the outer sheath-
ing of the coaxial cable.
A 10BASE5 thick coax cable, as shown in Figure 6-5, has a solid central conductor, a
minimum nominal velocity of propagation (NVP) of 0.77c, and 50 ohms of impedance/
termination resistance; it uses N-style screw-on connections. Each of the maximum five
segments of thick coax can be up to 500m (1640 ft.) long, and each station is con-
nected to a transceiver on the coax via an Attachment Unit Interface (AUI) cable that
can be up to 50m (164 ft.) long. The cable is large, heavy, and difficult to install, but
the distance limitations were favorable; this prolonged its use in certain applications.
Figure 6-5 10BASE5 Thicknet Cable
Other specifications or limitations of 10BASE5 cable include the following:
■ Only one station can transmit at a time (or a collision will occur).
■ 10BASE5 can run only in half-duplex mode, subject to the CSMA/CD rules.
■ Up to 100 stations, including repeaters, can exist on any individual 10BASE5
segment.
10BASE2
10BASE2 (originally 802.3a-1985) was introduced in 1985 because its coaxial cable of
a smaller size, lighter weight, and greater flexibility made installation easier than 10BASE5.
Because of its use of thinner cable, 10BASE2 often is referred to as Thinnet. 10BASE2
still exists in legacy networks. Although there is little reason to install a 10BASE2 net-
work today, its low cost and lack of need for hubs are attractive. Essentially, 10BASE2
requires no configuration, although obtaining NICs is increasingly difficult. Just like

10BASE5 systems, 10BASE2 systems are very cable-dependent across the whole colli-
sion domain and represent a large single point of failure.
The timing, frame format, and transmission were described previously in Chapter 5
and are common to all 10-Mbps Legacy Ethernet.
1102.book Page 321 Tuesday, May 20, 2003 2:53 PM
322 Chapter 6: Ethernet Technologies and Ethernet Switching
10BASE2 uses Manchester-encoded signals on thin coaxial cable. A 10BASE2 signal
is transmitted from approximately 0V to –1V. (The y-axis is voltage; the x-axis is time.
Voltage is measured between the center conductor and the outer sheathing conductor.)
10BASE2 potentially could be idle (0V) for days if no station wanted to transmit. 10BASE2
is asynchronous.
The computers on the LAN were linked together like the beads of a necklace by an
unbroken series of coaxial cable lengths. These lengths of coaxial cable were attached
by British Naval Connectors (BNCs) to a T-shape connector on the NIC, as shown in
Figure 6-6. This single coaxial cable was the shared bus for the network. Workstations
easily could be moved and reattached, or new workstations could be added to the LAN.
Otherwise, 10BASE2 used the same original Ethernet half-duplex protocol.
A 10BASE2 thin coax cable, as shown in Figure 6-8, has a stranded central conductor.
(Be sure that stranded coax is specified when new cable is ordered. Some installers find
it hard to work with and use solid-core coax when possible.) It has a minimum nomi-
nal velocity of propagation (NVP) of 0.65c, has 50 ohms of impedance/termination
resistance, and uses BNC T-style connections. Each of the maximum five segments of
thin coax can be up to 185m long (600 ft.), and each station is connected directly to
the BNC T connector on the coax.
Figure 6-6 Thinnet and BNC Connector
10BASE-T
10BASE-T (originally 802.3i-1990) substituted the cheaper and easier-to-install UTP
copper cable for coaxial cable. This cable plugged into a central connection device, a
hub or a switch, that contained the shared bus. The type of cable used in 10BASE-T,
the distances that the cable could extend from the hub, and the way in which the UTP

was installed, interconnected, and tested were standardized in a “structured cabling
1102.book Page 322 Tuesday, May 20, 2003 2:53 PM
10-Mbps and 100-Mbps Ethernet 323
system,” which increasingly specified a star or extended star topology. 10BASE-T was
originally a half-duplex protocol, but full-duplex features were added later. The explo-
sion in Ethernet’s popularity in the 1990s—when Ethernet came to dominate LAN
technology—was 10BASE-T running on Category (Cat) 5 UTP. To reacquaint yourself
with network topologies and networking media, refer back to Chapter 2, “Networking
Fundamentals,” and Chapter 3, “Networking Media.”
The timing, frame format, and transmission were described previously and are common
to all 10-Mbps legacy Ethernet.
10BASE-T uses Manchester line-encoded signals over Category 3 (now 5, 5e, or
better) UTP.
10-Mbps Ethernet is asynchronous, and the cable often is completely idle (0V) for long
periods of time between transmissions. 10BASE-T links have a link pulse present about
every 125 milliseconds (eight times per second), but can otherwise be idle. 10BASE-T
networks are “alive” with link pulses.
A 10BASE-T unshielded twisted-pair (UTP) cable has a solid conductor for each wire
in the maximum 90m horizontal cable, which should be 0.4 mm to 0.6 mm (26 to 22
American Wire Gauge [AWG]) in diameter. The 10m of allowed patch cables use similar-
dimension stranded cable for durability because it is expected to experience repeated
flexing. Suitable UTP cable has a minimum NVP of 0.585c, has 100 ohms of impedance,
and uses eight-pin RJ-45 modular connectors as specified in ISO/IEC 8877. Cables
between a station and a hub generally are described as between 0m and 100m long
(0 ft. to 328 ft.), although the precise maximum length is determined by propagation
delay through the link segment (any length that does not exceed 1000 ns of delay is
acceptable). Usually, 0.5 mm (24 AWG) diameter UTP wire in a multipair cable will
meet the requirements at 100m.
Although Category 3 cable is adequate for use on 10BASE-T networks, it is strongly
recommended that any new cable installations be made with Category 5e or better

materials and wiring practices. Use all four pairs, and use either the T568A or T568B
cable pinout arrangement. With this type of cable installation, it should be possible to
operate many different media access protocols (including 1000BASE-T) over the same
cable plant, without rewiring.
1102.book Page 323 Tuesday, May 20, 2003 2:53 PM
324 Chapter 6: Ethernet Technologies and Ethernet Switching
Table 6-2 shows the pinout for a 10BASE-T connection. Notice that two separate
transmit/receive paths exists (whereas coaxial cable has only one).
Figure 6-7 shows conceptual and physical connections between two stations. A cross-
over cable is required, so Tx on device A sends signals to Rx on device B. Note that
two point-to-point connections exist (TxA to RxB, and TxB to RxA).
Figure 6-7 10BASE-T Station to Station
Table 6-2 10BASE-T Cable Pinouts
Pin Number Signal
1 TD+ (Transmit Data, positive-going differential signal)
2 TD– (Transmit Data, negative-going differential signal)
3 RD+ (Receive Data, positive-going differential signal)
4 Unused
5 Unused
6 RD– (Receive Data, negative-going differential signal)
7 Unused
8 Unused
1102.book Page 324 Tuesday, May 20, 2003 2:53 PM
10-Mbps and 100-Mbps Ethernet 325
Figure 6-8 shows the connection between stations and repeaters, multiport repeaters
(hubs), or switches. The same connection would be used between a router and a hub
or a switch. A straight-through cable is used. Note that inside the hub is a bus topol-
ogy, which is a collision domain. When a workstation is connected to a switch using a
straight-through cable, all individual links are point-to-point. The switch fabric circuitry
allows full bandwidth simultaneously between pairs of ports without collisions.

Figure 6-8 10BASE-T Straight-Through Cable
Because station-to-station, switch-to-switch, and station-to-switch connections all are
point-to-point links, they have two physically separate communication pathways/
channels on two separate UTP wire pairs. In this case, collisions are not physical events,
but rather the result of the decision to not allow simultaneous Tx and Rx. Thus, either
half duplex (subject to the administrative imposition of CSMA/CD) or full duplex (no
physical collisions occur) is a configuration choice. Most of the time, you run these
connections in full duplex, which not only eliminates collisions, but also doubles the
throughput of the connection. When first introduced, the relevant IEEE standard was
entitled 802.3x-1997 Full-Duplex. However, station-to-hub connections involve the
bus topology within the hub, an actual physical collision domain. Hence, this connec-
tion can run only half duplex and is subject to CSMA/CD because of the physical
nature of the structure.
10BASE-T carries 10 Mbps of traffic in half-duplex mode; however, 10BASE-T in full-
duplex mode actually can exchange 20 Mbps of traffic (although, again, some of this is
overhead, not user data). This concept will become increasingly important with the
desire to increase the speed of Ethernet links.
1102.book Page 325 Tuesday, May 20, 2003 2:53 PM
326 Chapter 6: Ethernet Technologies and Ethernet Switching
10BASE-T Architecture
10BASE-T links generally consist of a connection between the station and a hub or
switch. Hubs should be thought of as multiport repeaters and count toward the limit
on repeaters between distant stations. Switches can be thought of as multiport bridges
and are subject to 100m length limitations but no limit on switches between distant
stations.
Although hubs can be linked in series (sometimes called daisy-chaining, or cascading),
it is best to avoid this arrangement when possible, to keep from violating the limit for
maximum delay between distant stations. The physical size of a 10BASE-T network is
subject to the same rules as 10BASE5 and 10BASE2 concerning the number of repeaters.
When multiple hubs are required, it is best to arrange them in hierarchical order, to

create a tree structure instead of a chain. Also, performance will be improved if fewer
repeaters separate stations. “Stackable” hubs, or concentrators with common backplanes
that will support several multiport adapter cards, permit large numbers of stations to
be connected to a device that counts as a single hub (repeater). Daisy-chaining switches
is fine and is not subject to restrictions.
All distances between stations are acceptable, although in one direction, the architecture
is at its limit. The most important aspect to consider is how to keep the delay between
distant stations to a minimum—regardless of the architecture and media types involved.
A shorter maximum delay provides better overall performance. Consider the following
architectures:
■ In Figure 6-9, there are five segments and four repeaters from Station 1 to any
other station in these paths. For 10BASE-T connections, the maximum of three
segments with stations does not apply because no other stations are on the same
cable. Each connection is described as a link segment.
Figure 6-9 Example 10-Mbps Mixed Architecture 1
1102.book Page 326 Tuesday, May 20, 2003 2:53 PM
10-Mbps and 100-Mbps Ethernet 327
■ In Figure 6-10, from any station (except Station 1) to any other station, the
path is only three repeaters. Because these alternate paths include 10BASE5 and
10BASE2 links, the other requirements still apply there (such as only three seg-
ments with stations).
Figure 6-10 Example 10-Mbps Mixed Architecture 2
10BASE-T links can have unrepeated distances up to 100m. This might seem like a
long distance, but it typically is used up quickly when wiring an actual building. Hubs
can solve this distance issue, although a maximum of four repeaters could be chained
together because of timing considerations. The widespread introduction of switches
has made this distance limitation less important. As long as workstations are located
within 100m of a switch, the 100m distance starts over at the switch, which could
be connected via another 100m to another switch, and so on. Because most modern
10BASE-T Ethernet is switched, these are the practical limits between devices. Ring,

star, and extended star topologies all are allowed. The issue then becomes one of logi-
cal topology and data flow, not timing or distance limitations.
Table 6-3 shows a chart of the 10BASE-T link characteristics.
Table 6-3 10BASE-T Link Characteristics Chart
Connection Maximum Segment
Station to station, station to switch,
switch to switch
100m, with no limitations on daisy
chaining
Station to hub 100m, but subject to four-repeater rule
1102.book Page 327 Tuesday, May 20, 2003 2:53 PM
328 Chapter 6: Ethernet Technologies and Ethernet Switching
100-Mbps Versions of Ethernet
100-Mbps Ethernet, also known as Fast Ethernet (in comparison to the original 10-Mbps
Ethernet), was a series of technologies. The two technologies that became commercially
important are 100BASE-TX (copper UTP-based) and 100BASE-FX (multimode optical
fiber-based). This section examines the commonalities between these two technologies
and then examines their differences individually.
Three things are common to 100BASE-TX and 100BASE-FX:
■ The timing parameters
■ The frame format
■ Parts of the transmission process
Table 6-4 shows the parameters for 100-Mbps Ethernet operation.
100BASE-TX and 100BASE-FX both share timing parameters. Note that 1 bit-time in
1000-Mbps Ethernet is 10 nsec = .01 microseconds = 1 100-millionth of a second.
The 100-Mbps frame format is the same as the 10-Mbps frame. Unlike 10-Mbps
Ethernet, in which the process was the same for all technologies until the signal was
applied to the medium.
Fast Ethernet represents a tenfold increase in speed. With this increase in speed comes
extra requirements. The bits being sent get shorter in duration and occur more frequently.

They require more careful timing, and their transmission requires frequencies closer to
Table 6-4 Parameters for 100-Mbps Ethernet Operation
Parameter Value
Bit-time 10 nsec
Slot time 512 bit-times
Interframe spacing 96 bits
Collision attempt limit 16
Collision backoff limit 10
Collision jam size 32 bits
Maximum untagged frame size 1518 octets
Minimum frame size 512 bits (64 octets)
1102.book Page 328 Tuesday, May 20, 2003 2:53 PM