CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 36 potx

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 ■ Inter

Manchester encodingrelies 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 trantran-sition

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 exammulti-ple, 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

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

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 segseg-ments 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

One Bit Period One Bit Period

Preamble SFD Destination

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 0 0 0 0 0

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

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

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

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

1 TD+ (Transmit Data, positive-going differential signal)

2 TD– (Transmit Data, negative-going differential signal)

3 RD+ (Receive Data, positive-going differential signal)

6 RD– (Receive Data, negative-going differential signal)

RJ-45 Pin Label

1 RD+

2 RDÐ

3 TD+

4 NC

5 NC

6 TDÐ

7 NC

8 NC

RJ-45 Pin Label

1 TD+

2 TDÐ

3 RDÐ

4 NC

5 NC

6 RDÐ

7 NC

8 NC

Hub/Switch

Workstation

Server/Router

Workstation

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

RJ-45 Pin Label

1 RD+

2 RDÐ

3 TD+

4 NC

5 NC

6 TDÐ

7 NC

8 NC

RJ-45 Pin Label

1 TD+

2 TDÐ

3 RDÐ

4 NC

5 NC

6 RDÐ

7 NC

8 NC

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

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■ 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

Station to station, station to switch,

switch to switch

100m, with no limitations on daisy chaining

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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