figure 8.1 csma/cd worst-case collision detection

Figure 8.1 CSMA/CD worst-case collision detection.

© Pearson Education Limited 2001

Figure 8.2 Hub configuration principles: (a) topology; (b) repeater schematic.


Figure 8.3 Ethernet/IEEE802.3 characteristics: (a) frameformat; (b) operational parameters.


Type/Length

2

SA

2/6

SFD

1

DA

2/6

Preamble

7Bytes

FCS

4

Data

46/1500

60/1512 bytes

I/G

1

L/C

1Bits 14/46

SFD = start-of-frame delimiterDA/SA = source/destination addressI/G = individual (=0)/group (=1) address

FCS = frame check sequenceL/C = locally administered (=1)/

centrally administered (=0)

Bit rateSlot timeInterframe gapAttempt limitBack off limitJam sizeMaximum frame size (including FCS)Minimum frame size (including FCS)

10 Mbps (Manchester encoded)512 bit times9.6 microseconds161032 bits1518 bytes512 bits

(a)

(b)

Figure 8.4 CSMA/CD MAC sublayer operation: (a) transmit; (b) receive.


Figure 8.5 Token ring network: principle of operation.


Figure 8.6 Token ring wiring configurations: (a) single hub; (b) station coupling unit; (c) multiple hubs/concentrators.


MAC unit

Station 1

MAC unit

Station 2

MAC unit

Station N

SCU SCUSCU

Hub/concentrator

Shielded twisted-pair (STP)drop cables – 3 pairs per cable

including power

(a)

SCUSCU

Hub/concentrator

Inserted

(b)

SCUSCU SCUSCU

Bypassed

Trunk cable – STP or optical fiber

Trunk coupling unit(TCU)

to/from stations

to/from station to/from station

Bypassed Inserted

Station coupling unit(SCU)

Hub/concentrator

(c)

Figure 8.7 Token ring network frame formats and fielddescriptions: (a) token format; (b) frame format; (c) fielddescriptions.


Figure 8.8 Token ring MAC sublayer operation: (a) transmit; (b) receive.


Figure 8.9 Token generation and stack modifications: (a) tokengeneration [Note: Sx = 0 if stack empty]; (b) stackmodification.


Frame(s) queued and token received with P < Pm

Transmit queued (waiting) frame(s) with P = Pr and R = 0

Token (P = Rr/Pm, R = 0)PUSH Pr to SrPUSH P to Sx

Token (P = Rr/Pm, R = 0)POP SxPUSH P to Sx

Token (P = Pr, R = Rr/Pm)

Pr < Rr/Pm and Pr > Sx Pr < Rr/Pm and Pr = SxPr > Rr/Pm

Transmit token

(a)

Token received with P = Sx

Token (P = Sr, R = Rr)POP Sx and SrIf Sx and Sr empty, cease stacking

Token (P = Rr, R = 0)PUSH P to SxContinue stacking

Rr < SrRr > Sr

Transmit token

(b)

Figure 8.10 Token ring priority example.


Station1Pm = 2F/T (P, R)Rotation

Station7Pm = 2F/T (P, R)



T(0, 0)0 T(0, 0) T(0, 0) T(0, 0)

T(0, 0)Pr = 0, Rr = 0, Pm = 2Pm > PrF(0, 0)P = Pr = 0; Rr = 0

F(0, 0)Pr = 0, Rr = 0, Pm = 2Pm > RrF(0, 2)P = Pr = 0; R = Pm = 2

F(0, 2)Pr = 0, Rr = 2, Pm = 4Pm > PrF(0, 4)P = Pr = 0; R = Pm = 4

F(0, 4)Pr = 0, Rr = 4, Pm = 4Pm = RrF(0, 4)P = Pr = 0; R = Rr = 4

1

F(0, 4)Pr = 0, Rr = 4Rr > PrT(4, 0)P = Rr = 4; R = 0

T(4, 0)Pr = 4, Rr = 0, Pm = 2Pr > Pm > RrT(4, 2)P = Pr; R = Pm = 2

T(4, 2)Pr = 4, Rr = 2, Pm = 4Pm = PrF(4, 0)P = Pr = 4; R = 0

F(4, 0)Pr = 4, Rr = 0, Pm = 4Pm > RrF(4, 4)P = Pr; R = Pm = 4

2

Stacking St = 0, Sx = 4

F(4, 4)no frame totransmitF(4, 4)

F(4, 4)Pr = 4, Rr = 4, Pm = 2Pr < RrF(4, 4)P = Pr; R = Rr

F(4, 4)Pr = 4, Rr = 4Rr = PrT(4, 4)P = Pr = 4; R = Rr = 4

T(4, 4)Pr = 4, Rr = 4, Pm = 4Pm = PrF(4, 0)P = Pr; R = 0

3

Sr = 0, Sx = 4


F(4, 0)Pr = 4, Rr = 0, Pm = 2Pm > RrF(4, 2)P = Pr; R = Pm


F(4, 2)Pr = 4, Rr = 2Rr < PrT(4, 2)P = Pr; R = Rr

4

Sr = 0, Sx = 4

T(4, 2)Pr = 4, Rr = 2, Sr = 0, Sx = 4Pr = Sx, Rr > SrT(2, 0)P = Pr = 2; R = 0

T(2, 0)Pr = 2, Rr = 0, Pm = 2Pr = PrF(2, 0)P = Pr; R = Rr



5

Sr = 0, Sx = 2


F(2, 0)Pr = 2, Rr = 0Rr < PrT(2, 0)P = Pr; R = Rr

T(2, 0)no frame totransmitF(2, 0)


6

Sr = 0, Sx = 2

T(2, 0)Pr = 2, Rr = 2, Sr = 0, Sx = 2Pr = Sx, Rr < SrT(0, 0)P = Sr; R = Rr

T(0, 0)no frame totransmitT(0, 0)

T(0, 0)no frame totransmitT(0, 0

7

Cease stacking

T(0, 0)no frame totransmitT(0, 0

T(0, 0) T(0, 0) T(0, 0)8 T(0, 0)

Figure 8.11 LAN interconnection: (a) repeaters; (b) bridges.


Figure 8.12 Transparent bridge schematic: (a) architecture; (b) application example.


Figure 8.13 Effect of dual paths on learning algorithm.


Figure 8.14 Active topology derivation example: (a) LANtopology; (b) root port selection; (c) designated port selection; (d) active topology.


Figure 8.14 Continued.


Figure 8.15 An example source routing bridged LAN: (a) topology; (b) routing table entries.


Figure 8.16 Token ring frame format: (a) position of routinginformation field; (b) structure of routing information field.


Figure 8.17 Source routing example: (a) topology; (b) spanningtree.


Figure 8.18 Typical establishmentwide LAN.


Figure 8.19 FDDI networking components.


Figure 8.20 Ring fault detection and isolation: (a) failuredetection; (b) redundant ring configuration; (c) segmentisolation; (d) station isolation.


TCU = trunk coupling units

Figure 8.21 FDDI wiring schematic: (a) building; (b) establishment.


Figure 8.22 FDDI physical interface schematic.


Figure 8.23 FDDI line coding and framing detail: (a) 4B5Bcodes; (b) frame formats.


Figure 8.24 FDDI transmission example.


Figure 8.25 FDDI timed token rotation protocol example.


4 + Tt + Tl

0 0 0

0 0 0

Tokenrotation

Station 1

TRT XMIT

Station 2

TRT XMIT

Station 3

TRT XMIT

Station 4

TRT XMIT

Tt + Tl

0

4 + Tt + Tl0

4 + Tt + Tl0

4 + Tt + Tl0

Tt + Tl4

4 + Tt + Tl0

4 + Tt + Tl0

4 + Tt + Tl0

Tt + Tl4

4 + Tt + Tl0

4 + Tt + Tl0

4 + Tt + Tl0

Tt + Tl44

3

2

1

0

TRT = token rotation timeXMIT = number of frames transmitted on this rotation of

the tokenTTRT = target token rotation time

Tt + Tl Tt + Tl Tt + Tl

Tt + Tl 4 + Tt + Tl 4 + Tt + Tl

Tt = time to transmit the tokenTl = ring latencyTTRT = 4 + Tt + Tl

4

Figure 8.26 100 Base T: (a) use of wire pairs; (b) 8B6Tencoding.


Pair 1

Pair 2

Pair 3

Pair 4

Station–Hubtransmissions

Hub–Stationtransmissions

Carrier sense/collision detect

Station

Station interface

Carrier sense/collision detect

Hub

Hub interface

+

–

+

0

–

1 0 1 1 1 0 1 0

40ns= 25 MTps

(mega ternary (signals)per second)

= 25 Mbaud

Line idle

8-bit byte

6-ternary symbol

(a)

(b)

Figure 8.27 100BaseT transmission detail: (a) DC balancetransmission rules; (b) 8B6T encoding sequence; (c) end ofstream encoding.


Figure 8.28 Start-of-frame detail: (a) effect of NEXT; (b) preamble sequence.


Pair 1

Pair 2

Pair 3

Pair 4

NEXT = near-end crosstalk

Collision-detectline

Station Hub

(a)

NEXT

Pair 1

Pair 3

Pair 4

SOS-1 SOS-1 SFD Data

SOS-1 SOS-1 SFD Data

SOS-1 SFD Data

(b)

Station

SOS = start of streamSFD = start frame delimiter

Time

Figure 8.29 Fast Ethernet switch schematic.


Switch memory LOU 1 LOU NLIU N

Control processor

StationN

Station1

Dual 100 Mbps drop cables

LIU = line input unitLOU = line output unit

Fast Ethernet switch

LIU 1

Figure 8.30 Example network configuration with a FastEthernet switch and 10/100BaseT hubs.


Fast Ethernet switch

10 Base T Hub

Stations

100 Base T Hub

Stations

100 Base T Hub

Stations

ServerServer

10/100Mbps (CSMA/CD) half-duplex lines100Mbps duplex lines

Figure 8.31 LAN protocols: (a) protocol framework; (b) examples.


Transmission medium

Physical medium-dependent sublayer

Convergence sublayer

MAC sublayer

LLC sublayer

Network layer

Physical layer

Link layer

(b)

(a)

IEEE 802.1 Station management802.1d Transparent bridges

802.2 Logical link control (LLC)

802.3 CSMA/CD (Ethernet) bus802.3u Fast Ethernet802.3x Hop-by-hop switch flow control802.3z Gigabit Ethernet

802.5 Token ring

Figure 8.32 Fast Ethernet media-independent interface.


Transmit data

Transmit enable

Carrier sense

Collision detect

Receive data

Receive error

Transmit clock (TxClk)

Transmit data (TxD<0..3>)

Transmit enable (TxEn)

Carrier sense (CRS)

Collision detect (CD)

Receive clock (RxClk)

Receive data (RxD<0..3>)

Receive data available (RDv)

Receive error (RErr)

CSMA/CDMAC sublayer

interface

Media-independentinterface

MACsublayer

Convergencesublayer

PMDSublayer

Figure 8.33 MAC user service primitives: (a) CSMA/CD; (b) token ring.


LLC layer Correspondent’sLLC layer

MAC layer

MA_UNITDATA.request

MA_UNITDATA.confirm

MA_UNITDATA.request

MA_UNITDATA.confirm

MA_UNITDATA.indication

MA_UNITDATA.indication

(a)

(b)

Figure 8.34 LLC/MAC sublayer interactions.


MAC LLC

MA_UNITDATA.ind(UI)

MA_UNITDATA.req(UI)

Network

L_DATA.indication(NPDU)

L_DATA.request(NPDU)

Network

L_DATA.indication(NPDU)

L_DATA.request(NPDU)

LLC

MA_UNITDATA.ind(UI)

MA_UNITDATA.req(UI)

Source DTE Destination DTE

NPDU = network layer protocol data unit

Figure 8.35 Interlayer primitives and parameters.


ECB = event control block

Figure 8.36 Example enterprise network architecture.


GW

Site LAN

SiteLANGWGW

SiteLAN

Intersite communications facility

GW = intersite gateway (remote bridge or IP/IPX router)

Figure 8.37 Inverse multiplexing: (a) principle of operation; (b) reassembly schematic; (c) bonding protocol.


Figure 8.38 Public frame-relay network schematic.


Figure 8.39 Frame relay principles: (a) frame format; (b) framerouting; (c) frame relay schematic.


Figure 8.40 Schematic of large multisite enterprise networkbased on multiplexers and high bit rate leased circuits.


PBX

Sitetelephony

RB

SiteLAN

SMFRA

Site C

PBXSite

telephony

RBSiteLAN

SM

FRA

Site B

PBXSite

telephony

RBSiteLAN

SM

FRA

Site A

Intersiteenterprise backbone network

DS1/3 or E1/3digital leased circuits

MUXMUX

MUX

SM = subrate multiplexerFRA = frame relay adapter

RB = remote bridgeMUX = site multiplexer

Figure 8.41 Summary of the topics discussed in this chapterrelating to enterprise networks.


Summary

Enterprise networks

LANs (data)PBX (telephony)(Chapter 7)

Legacy LANs

Ethernet Token ring

LAN interconnection

Transparentbridges

Source routingbridges

FDDI backbonenetworks

High-speed LANs

Fast Ethernethubs/repeaters

GigabitEthernet

Fast Ethernetswitches

Enterprise LAN interconnection technologies

Remote bridges/routers(Chapter 9)

Multisite enterprise network example

ISDN connections Frame relay High bit rate leased circuits

Example 8.1


A token ring network has been configured to operate with four priorityclasses: 0, 2, 4, and 8, with 8 the highest priority. After a period of inac-tivity when no transmissions occur on successive rotations of the token,four stations generate frames to send as follows:

Station 1 1 frame of priority 2




Assuming the order of stations on the ring is in increasing numericalorder and that station 1 receives the token first with a zero priority andreservation field, derive and show in table form the transmissions madeby each station for the next eight rotations of the token. Include in yourtable the values in the priority and reservation fields both as each newtoken is generated and as each frame circulates around the ring. Alsoinclude the actions taken by the stacking station.

Answer:

The transmissions made by each station for the next eight rotations ofthe token are shown in Figure 8.10.

On the first rotation of the token, station 1 seizes the token and initiatesthe transmission of its waiting frame. Also on this rotation the reser vationfield in the frame is raised by station 7 to 2 and then by station 15 to 4.

8.1 Continued


On the second rotation, station 1 reads the reservation field from theframe and determines it must release the token with a priority of 4.Since it is raising the ring priority, it must become a stacking station andsaves the current ring priority (0) on stack St and the new priority (4)on Sx. The token then rotates and is seized by station 15. Also on thisrotation station 17 raises the reservation field from 0 to 4.

On the third rotation, station 15 releases the token with a priority andreservation field of 4. Station 17 therefore seizes the token and initiatesthe transmission of its waiting frame.

On the fourth rotation, station 7 updates the reservation field from 0 to2 and this causes the token to be released by station 17 with the samepriority (4) but a reservation value of 2.

On the fifth rotation, since station 1 is a stacking station, it detects Rr isgreater than Sr and hence lowers the priority of the token/ring from 4to 2 and saves the lower priority on the stack. Station 7 is therefore ableto transmit its waiting frame.

On the sixth rotation, station 7 releases the token with the same prioritysince no reservations have been made.

On the seventh rotation, station 1 detects the reservation field in thetoken is less than the priority field and hence reduces the priority to 0 andthereby ceases to be a stacking station. The token has thus returned to itsinitial state and continues rotating until further frames are generated.

Example 8.2


To illustrate how the various elements of the spanning tree algorithmwork, consider the bridged LAN shown in Figure 8.14(a). The uniqueidentifier of each bridge is shown inside the box representing thebridge together with the port numbers in the inner boxes connectingthe bridge to each segment. Typically, the additional bridges on eachsegment are added to improve reliability in the event of a bridge fail-ure. Also, assume that the LAN is just being brought into service, allbridges have equal priority, and all segments have the same designatedcost (bit rate) associated with them. Determine the active (spanningtree) topology.

Answer:

(i) First the exchange of configuration BPDUs will establish bridge B1as the root bridge since this has the lowest identifier.

(ii) After the exchange of configuration BPDUs, the root path cost(RPC) of each port will have been computed. These are shown inFigure 8.14(b).

(iii) The root port (RP) for each bridge is then chosen as the port withthe lowest RPC. For example, in the case of bridge B3, port 1 hasan RPC of 1 and port 2 an RPC of 2, so port 1 is chosen. In the caseof B2, both ports have the same RPC and hence port 1 is chosensince this has the smaller identifier. The selected RPs are alsoshown in the figure.

(iv) B1 is the root bridge so all its ports have a designated port cost(DPC) of 0. Hence they are the designated ports for segments S1,S2, and S3.

(v) For S4, port 1 of B5 is an RP and hence is not involved in selectingthe designated port. The two other ports connected to S4 bothhave a DPC of 1. Hence port 2 of B3 is selected as the designatedport because it has a lower identifier than B4.

(vi) For S5, the only port connected to it is port 2 of B5 and hence thisis selected.

(vii)Finally, for S6 both ports have a DPC of 1, so port 2 of B4 isselected rather than port 2 of B6.

The DPCs are shown in Figure 8.14(c) and the resulting active topologyis thus as shown in Figure 8.14(d). Note that the DPC of a port is alwaysequal to the RPC of the root port of the bridge.

Example 8.3


Assume the bridged LAN shown in Figure 8.17(a) is to operate usingsource routing. Also assume that all bridges have equal priority and allrings have the same designated cost (bit rate). Derive the followingwhen station A wishes to send a frame to station B:

(i) the active spanning tree for the LAN,

(ii) all the paths followed by the single-root broadcast frame(s),

(iii) all the paths followed by the all-routes broadcast frame(s),

(iv) the route (path) selected by A.

Answer:

(i) (a) Bridge B1 has the lowest identifier and is selected as the route.

(b) The root ports for each bridge are then derived as shown.

(c) The designated ports for each segment can now be derivedand these are as shown.

(d) The active topology is as shown in Figure 8.17(b).

(ii) Paths of single-route broadcast frames:

R1→B1→R2→B2→R3

R2→B3→R5→B6→R6

R1→B4→R4→B5

(iii)Paths of all-routes broadcast frames:

R6→B6→R5→B3→R2→B2→R3

B2→B1→R1

B1→B4→R4→B5→R5→B3

(iv) Since each ring has the same bit rate, the route (path) selected iseither:

R1→B1→R2→B3→R5→B6→R6

or

R1→B4→R4→B5→R5→B6→R6

Example 8.4


Assuming a signal propagation delay in the fiber of 5 µs per 1km, derivethe latency of the following FDDI ring configurations in both time andbits assuming a usable bit rate of 100Mbps.

(i) 2 km ring with 20 stations,

(ii) 20 km ring with 200 stations,

(iii) 100 km ring with 500 stations.

Answer:

Ring latency, Tl = Signal propagation delay, Tp + N × station latency,Ts where N is the number of stations.

(i) Tl = 2 × 5 + 20 × 1

= 30 µs or 3000 bits

(ii) Tl = 20 × 5 + 200 × 1

= 300 µs or 30000 bits

(iii) Tl = 100 × 5 + 500 × 1

= 1000 µs or 100000 bits

Note that the above values assume that the primary ring only is in use. Ifa fault occurred, the three signal propagation delay values would eachbe doubled. Also, for dual attach stations, the station latency would bedoubled.

Example 8.5


The timed token rotation protocol is to be used to control access to afour-station FDDI ring network. All frames to be transmitted are of thesame length and the TTRT to be used is equivalent to the transmissionof four frames plus any ring latency. After an idle period when noframes are ready to send, all four stations receive a block of frames tosend. Assuming the time for the token to rotate around the ring in theidle state is equal to the time to transmit the token, Tt, plus the ringlatency T1, derive and show in table form the number of frames eachstation can transmit for the next four rotations of the token.

Answer:

The number of frames that each station can transmit on the first fourrotations of the token are shown in diagrammatic form in Figure 8.25.They are derived as follows.

After the idle period, the TRT of all stations will be Tt + T1 since noframes have been transmitted. Once frames become ready to send, onreceipt of the token station 1 computes a THT of 4 and hence transmits(XMIT) four waiting frames before passing on the token. However,since station 1 has sent four frames, none of the other stations is able tosend any frames for this rotation of the token. This is so since their TRTis now greater than 4 in each case and hence their corresponding com-puted THT is negative.

On the second rotation of the token, the TRT of station 1 has incre-mented to 4 plus the ring latency, hence it is not able to send anywaiting frames. This means that the TRT of station 2 is less than theTTRT and the computed THT is again 4 hence it can send four waitingframes. Again this will block stations 3 and 4 from sending any waitingframes on this rotation of the token.

On the third rotation of the token, the THT of stations 1 and 2 are both 4plus the ring latency and hence neither can send frames. The TRT of sta-tion 3, however, is this time less than the TTRT and the computed THT is4 hence it can transmit four waiting frames. Again station four is blocked.

Finally, on the fourth rotation of the token, stations 1, 2, and 3 are stillall blocked and station 4 can transmit four frames. This simple exampleshows that the available transmission capacity is shared in an equitableway between all four stations.

Example 8.6


Derive the maximum obtainable throughput and the maximum accessdelay for the following three ring configurations. Assume a TTRT of4ms has been chosen for each configuration.

(i) 2km ring with 20 stations,

(ii) 20 km ring with 200 stations,

(iii) 100km ring with 500 stations.

Answer:

The three ring configurations are the same as those used in Example 8.4and hence the same computed ring latency times will be used here. Now:

n(TTRT – Tl)Maximum available thoughput, Umax =nTTRT + Tl

Maximum access delay, Amax = (n – 1) TTRT + 2Tl

(i) From Example 8.4, Tl = 0.03ms. Hence:

20(4 – 0.03)Umax = = 0.99

20 × 4 + 0.03

and

Amax = 19 × 4 + 0.06 = 76.06ms

8.6 Continued


(ii) From Example 8.4, Tl = 0.3ms. Hence:

200 (4 – 0.3)Umax = = 0.92

200 × 4 + 0.3

and

Amax = 199 × 4 + 0.6 = 796.6ms

(iii) From Example 8.4, Tl = 1ms. Hence:

500 (4 – 1)Umax = = 0.75

500 × 4 + 1

and

Amax = 499 × 4 + 2 = 1.998s

figure 8.1 csma/cd worst-case collision detection

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