C) All rights reserved by Professor Wen-Tsuen Chen1 IEEE 802.5 Token Ring ä Easy fault detection....

C) All rights reserved by Professor C) All rights reserved by Professor Wen-Tsuen ChenWen-Tsuen Chen

11

IEEE 802.5 Token RingIEEE 802.5 Token Ring

Easy fault detection.Easy fault detection. Different transmission media can be used.Different transmission media can be used. Multiple rings allow the network to carry voice, video , Multiple rings allow the network to carry voice, video ,

facsimilie and graphics signals concurrently with data.facsimilie and graphics signals concurrently with data. The network can span large areas, since signal are The network can span large areas, since signal are

regenerated at each station ring interface.regenerated at each station ring interface. Fair and known upper bound on delay in channel accessFair and known upper bound on delay in channel access Multiple priority are implemented.Multiple priority are implemented.

IBM Zurick, Switzerland & Triangle Park, North Carolina, USA in April 1982IBM Zurick, Switzerland & Triangle Park, North Carolina, USA in April 1982


22

With a typical signal propagation speed With a typical signal propagation speed 200 200 mm//us, us, physical length of a bit is 200/R meters, where R is the physical length of a bit is 200/R meters, where R is the ring data rate in Mbps.ring data rate in Mbps.

For a 1-mbps ring with 1000 m in circumference, the ring For a 1-mbps ring with 1000 m in circumference, the ring can contain only 5 bits.can contain only 5 bits.

A transmitting station must drain the ring while it A transmitting station must drain the ring while it continues to transmit.continues to transmit.

A station holding the token “owns” the ring.A station holding the token “owns” the ring.


33

Unidirectional ringUnidirectional ring Active ring interface with singal regenerationActive ring interface with singal regeneration Short ring interface delay, data is copied or changed on the Short ring interface delay, data is copied or changed on the

fly.fly.


44

Token-Ring OperationToken-Ring Operation


55

Token Ring Token Ring

(1)(1) (2)(2)

(3)(3)


66

MAC Sublayer ProtocolMAC Sublayer Protocol

SD: Starting delimiter NN0NN000 , N = nondata bit.SD: Starting delimiter NN0NN000 , N = nondata bit.ED: Ending delimiter NN1NN1IE, where I is the intermediate-frame ED: Ending delimiter NN1NN1IE, where I is the intermediate-frame bit to indicate non-final frame in a multiple-frame transmission andbit to indicate non-final frame in a multiple-frame transmission and E is the error-detection bit which is set if any repeater detects an error.E is the error-detection bit which is set if any repeater detects an error.


77

AC: Access Control FieldAC: Access Control Field FC: Frame-Control FieldFC: Frame-Control Field

FS: Frame Status FieldFS: Frame Status Field


88

Token ring control framesToken ring control frames


99

A free token circulates the ring when all stations aA free token circulates the ring when all stations are idle.re idle.

A station wishing to transmit must wait until it detA station wishing to transmit must wait until it detects a free token passing by. ects a free token passing by.

It then seizes the token by changing the token bit tIt then seizes the token by changing the token bit to transform it into the start-of-frame sequence for o transform it into the start-of-frame sequence for a data frame. The data to be transmitted is then apa data frame. The data to be transmitted is then appended.pended.

The frame on the ring will make a round trip and tThe frame on the ring will make a round trip and then removed by the transmitting stationhen removed by the transmitting station


1010

The transmitting station will issue a free token if it The transmitting station will issue a free token if it has completed transmission of its frame and the has completed transmission of its frame and the leading edge of the transmitted frame has returned leading edge of the transmitted frame has returned to the station.to the station.

A station may hold the token for a token holding A station may hold the token for a token holding time, which is typically 10 time, which is typically 10 msec, msec, for multiple-frame transmission..


1111

Token-Ring Priority SchemeToken-Ring Priority Scheme


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Token-Ring Priority Token-Ring Priority

PPff = priority of frame to be transmitted by a station = priority of frame to be transmitted by a station

PPss = service priority : priority of current token = service priority : priority of current token

PPrr = value of Ps as contained in the last token = value of Ps as contained in the last token

received by this stationreceived by this station

RRss = reservation value in the current token = reservation value in the current token

RRrr = highest reservation value in the frames received = highest reservation value in the frames received

by this station during the last token rotationby this station during the last token rotation


1313

Token-Ring Priority(cont.) Token-Ring Priority(cont.)

The scheme works as follows:The scheme works as follows:A station wishing to transmit must wait for a token with A station wishing to transmit must wait for a token with

PPss P Pff..

While waiting , a station may reserve a future token at its While waiting , a station may reserve a future token at its priority level(Ppriority level(Pff). If a data frame goes by, and if the ). If a data frame goes by, and if the

reservation field is less than its priority (Rreservation field is less than its priority (Rss < P < Pff), then the ), then the

station may set the reservation field of the frame to its station may set the reservation field of the frame to its

priority(Rpriority(Rss P Pff). If a token frame goes by, and if(R). If a token frame goes by, and if(Rss < P < Pff

and Pand Pff < P < Pss), then the station sets the reservation field of ), then the station sets the reservation field of

the frame to its priority(Rthe frame to its priority(Rss P Pff).This has the effect of ).This has the effect of

preempting any lower-priority reservation.preempting any lower-priority reservation.


1414

Token-Ring Priority (cont.) Token-Ring Priority (cont.)

When a station seizes a token, it sets the token bit to 1 to When a station seizes a token, it sets the token bit to 1 to start a data frame, sets the reservation field of the data start a data frame, sets the reservation field of the data frame to 0, and leaves the priority field unchanged(the frame to 0, and leaves the priority field unchanged(the same as that of the incoming token frame).same as that of the incoming token frame).

Following transmission of one or more data frames, a Following transmission of one or more data frames, a station issues a new token with the priority and reservation station issues a new token with the priority and reservation fields set as indicated in Table 9.7.fields set as indicated in Table 9.7.


1515

Token-Ring Priority (cont.) Token-Ring Priority (cont.)

SSxx= stack used to store new values of token priority= stack used to store new values of token priority

Sr = stack used to store new values of token prioritySr = stack used to store new values of token priority


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Centralized Approach(IBM) Centralized Approach(IBM)

A station is designated as active token monitor.A station is designated as active token monitor. The monitor detects the lost token condition by using The monitor detects the lost token condition by using

a time-out greater than the time required for the a time-out greater than the time required for the largest frame to completely traverse the ring. To largest frame to completely traverse the ring. To recover, purge the ring and issue a free token.recover, purge the ring and issue a free token.

To detect a circulating busy token, the monitor set the To detect a circulating busy token, the monitor set the monitor bit to 1 on any passing busy token. If it sees a monitor bit to 1 on any passing busy token. If it sees a busy token with a bit set,busy token with a bit set, transmitting station failed transmitting station failed to purge its frame. The monitor changes the busy to purge its frame. The monitor changes the busy token to a free token.token to a free token.


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Token Maintenance Token Maintenance

Errors : Errors : no token circulatingno token circulating Persistent busy tokenPersistent busy token

Decentralized Approach(IEEE 802)Decentralized Approach(IEEE 802) If a station has a frame to transmit and has not seen a token If a station has a frame to transmit and has not seen a token

for a time TNT(no-token time), it assumes a token is lost, for a time TNT(no-token time), it assumes a token is lost, purge the ring and issues a token.purge the ring and issues a token.

A station also keeps track of the amount of time since the A station also keeps track of the amount of time since the last valid data frame or token. When this exceeds TVX last valid data frame or token. When this exceeds TVX (Valid frame timer)(Valid frame timer)


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IEEE 802.5 Physical-Layer MediumIEEE 802.5 Physical-Layer Medium

Shielded-twisted pair : 4Mbps and 16Mbps.Shielded-twisted pair : 4Mbps and 16Mbps. unshielded-twisted pair : 4Mbps.unshielded-twisted pair : 4Mbps. Use differential Manchester encoding.Use differential Manchester encoding.


1919

Slotted RingSlotted Ring

E

E

E

E

E F

F

F


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Slotted Ring (cont.)Slotted Ring (cont.)

Cambridge Ring : University of CambridgeCambridge Ring : University of Cambridge Each slot has a leading bit to indicate the slot as Each slot has a leading bit to indicate the slot as

empty or full.empty or full. A station with data to send, waits until an empty slot A station with data to send, waits until an empty slot

arrives, marks the slot full and inserts a frame of data.arrives, marks the slot full and inserts a frame of data. The station cannot transmit another frame until this The station cannot transmit another frame until this

slot returns.slot returns.


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Cambridge RingCambridge Ring

Each slot : Each slot : 1 Source address byte1 Source address byte 1 Destination address byte1 Destination address byte 2 data bytes2 data bytes 5 control bits5 control bits Total : 37 bitsTotal : 37 bits

Advantage : simplicityAdvantage : simplicity Disadvantage : waste of bandwidthDisadvantage : waste of bandwidth


2222

Comparison of Local Area NetworksComparison of Local Area Networks

CSMA/CD vs. Token-passing :CSMA/CD vs. Token-passing : CSMA/CD non-deterministic access time not suitable for CSMA/CD non-deterministic access time not suitable for

real time application.real time application. Token-passing permits multiple levels of priorityToken-passing permits multiple levels of priority CDMA/CD is suitable for fast message transmission during CDMA/CD is suitable for fast message transmission during

light load.light load. Token-passing has better throughput during heavy load.Token-passing has better throughput during heavy load.


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Comparison of Local Area Networks (cont.)Comparison of Local Area Networks (cont.)

Bus vs. RingBus vs. Ring Ring networks can span larger areas.Ring networks can span larger areas. Ring allows easy fault detect.Ring allows easy fault detect. Bus network is more reliable when a station fails.Bus network is more reliable when a station fails.


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FDDI: Fiber Distributed Data InterfaceFDDI: Fiber Distributed Data Interface


2525


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FDDI frame formatFDDI frame format


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FDDI MAC protocolFDDI MAC protocol


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Two key differences between FDDI MAC protocol Two key differences between FDDI MAC protocol and IEEE 802.5 MAC Protocol:and IEEE 802.5 MAC Protocol: In FDDI, a station waiting for a token seizes the token by In FDDI, a station waiting for a token seizes the token by

aborting the token transmission as soon as the token frame aborting the token transmission as soon as the token frame is recognized.is recognized.

In FDDI, a station that has been transmitting data frames In FDDI, a station that has been transmitting data frames releases a new token as soon as it completes data frame releases a new token as soon as it completes data frame transmission.Allow multiple frames in a ring. The priority transmission.Allow multiple frames in a ring. The priority scheme cannot be implemented.scheme cannot be implemented.

Class of Services scheme is similar to that of Token Bus.Class of Services scheme is similar to that of Token Bus.


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Performance of Token Passing vs. CSMA/CDPerformance of Token Passing vs. CSMA/CD

To find the maximum achievable utilizationTo find the maximum achievable utilization Assume a LAN with N active stationsAssume a LAN with N active stations aa = maximum normalized propagation delay = maximum normalized propagation delay Assume each station is always prepared to transmit Assume each station is always prepared to transmit

a framea frame

Token Passing:Token Passing:

where Twhere T11= average time to transmit a data frame = average time to transmit a data frame

TT2 2 = average time to pass a token= average time to pass a token

21

1

TT

T


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Performance of Token Passing vs. CSMA/CD (cont.)Performance of Token Passing vs. CSMA/CD (cont.)

CSMA/CDCSMA/CD Time on the medium consists of transmission Time on the medium consists of transmission

intervals and contention intervalsintervals and contention intervals A transmission interval lasts A transmission interval lasts 1/2a1/2a time slots time slots The probability The probability AA that exactly one station attempts a that exactly one station attempts a

transmission in a slot istransmission in a slot is

where p = 1/N where p = 1/N 1max

1

)1

1(

)1(

N

N

NA

PNPA


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Performance of Token Passing vs. CSMA/CD (cont.)Performance of Token Passing vs. CSMA/CD (cont.)

mean length of a contention interval mean length of a contention interval ww is is

AA

a

au

A

AAAiNE

i

i

)1(2

12

1

1)1(][

1


3333

)1

1(

1

1

1

Na

Na

u

1a

1a


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3535

IEEE 802.6 Distributed Queue Dual BusIEEE 802.6 Distributed Queue Dual Bus

For metropolitan area networkFor metropolitan area network


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IEEE 802.6 Distributed Queue Dual Bus (cont.)IEEE 802.6 Distributed Queue Dual Bus (cont.)

Two parallel, unidirectional buses with stations Two parallel, unidirectional buses with stations attached to both buses in parallel.attached to both buses in parallel.

Each bus has a head-end, which generated a Each bus has a head-end, which generated a steady stream of 53-byte cells.steady stream of 53-byte cells.

Each station can send cells to stations which is in Each station can send cells to stations which is in the downstream of a bus. The stations uses the the downstream of a bus. The stations uses the other bus for reservationother bus for reservation..


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Distributed Queue Access ProtocolDistributed Queue Access Protocol

Each cell of size 53 bytes carries 44-bytes Each cell of size 53 bytes carries 44-bytes payload.payload.

Each cell has two bits, Each cell has two bits, BusyBusy to indicate that the to indicate that the cell is occupied , and cell is occupied , and RequestRequest, which can be set , which can be set when a station wants to make a request.when a station wants to make a request.

Stations queue up in a distributed FIFO queue.Stations queue up in a distributed FIFO queue. Use two counters, RC (request counter) and CD Use two counters, RC (request counter) and CD

(countdown counter) to implement the distributed (countdown counter) to implement the distributed queue.queue.


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Distributed Queue Access Protocol (cont.)Distributed Queue Access Protocol (cont.)

RC of a station counts the number of a RC of a station counts the number of a downstream requests pending until the stations downstream requests pending until the stations itself has a frame to send. At that point, CDitself has a frame to send. At that point, CDRC RC and RCand RC0; and count the number of request 0; and count the number of request made after that station becomes ready.made after that station becomes ready.


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When a station has a cell queued, CD represents When a station has a cell queued, CD represents its position in the distributed queue, with 0 being its position in the distributed queue, with 0 being the front of the queue.the front of the queue.

CD is decreased each time an empty cell passes by CD is decreased each time an empty cell passes by until CD is 0, at which point, the queued frame is until CD is 0, at which point, the queued frame is sent in the next empty cell.sent in the next empty cell.


4040


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IEEE 802.2 Logical Link ControlIEEE 802.2 Logical Link Control

LCC provides three service option: unreliable LCC provides three service option: unreliable datagram service, acknowledged datagram service, datagram service, acknowledged datagram service, and reliable connection-oriented service.and reliable connection-oriented service.

LCC header is based on the HDLC protocolLCC header is based on the HDLC protocolbitsbits 88 88 8 or 168 or 16 88XXN (N>0)N (N>0)

DSAPDSAP SSAPSSAP ControlControl InformationInformation

DSAP (SSAP): Destination (Source) Service Access PointDSAP (SSAP): Destination (Source) Service Access Point Control field is identical to that of HDLCControl field is identical to that of HDLC


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4343

BridgesBridges

For interconnecting multiple LANs in the data link For interconnecting multiple LANs in the data link layer.layer.

Reasons for bridgingReasons for bridging Autonomy of owners of LANs.Autonomy of owners of LANs. Geographically spread organizations.Geographically spread organizations. To accommodate the traffic load in multiple LANs.To accommodate the traffic load in multiple LANs. ReliabilityReliability SecuritySecurity


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4545


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Problems for Bridging Different Types of LANsProblems for Bridging Different Types of LANs

Different frame format.Different frame format. Different data rate.Different data rate. Different maximum frame length.Different maximum frame length. Priority handling. ,etc.Priority handling. ,etc.


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4848

Transparent bridgeTransparent bridge

Spanning tree bridge by R. PerlmanSpanning tree bridge by R. Perlman Each bridge has a routing table which is updated Each bridge has a routing table which is updated

using a backward learning algorithm.using a backward learning algorithm. The routing procedure for an incoming frame,The routing procedure for an incoming frame,

1. If destination and source LANs are the same, discard the 1. If destination and source LANs are the same, discard the frame.frame.

2. If destination and source LANs are different, forward the 2. If destination and source LANs are different, forward the frame (according to the routing table)frame (according to the routing table)

3. If the destination LAN is unknown (in the routing table), 3. If the destination LAN is unknown (in the routing table), flood the frameflood the frame


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11

22

22 33

44


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Routing Table of a BridgeRouting Table of a Bridge Filtering DatabaseFiltering Database

Dest. Port Time Dest. Port TimeDest. Port Time Dest. Port Time

ABCDEFG

H

11222222

ABCDEFG

H

22233444

Dest. :Destination addressPort :Bridge port numberTime :Entry Time. An entry will be deleted after a certain time limit.

B1 B2


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Construct a spanning tree of bridges and LANs.Construct a spanning tree of bridges and LANs.

• Nodes: LANs and BridgesNodes: LANs and Bridges• An arc connects a LAN to a bridgeAn arc connects a LAN to a bridge


5252


5353

To Build the Spanning TreeTo Build the Spanning Tree

The bridge with the lowest serial number becomes The bridge with the lowest serial number becomes the root. Each broadcast its serial number. the root. Each broadcast its serial number.

A tree of shortest paths from the root to every A tree of shortest paths from the root to every LAN is constructed.LAN is constructed.

A distributed algorithm for constructing the A distributed algorithm for constructing the minimum spanning tree was invented by R. minimum spanning tree was invented by R. Perlman.Perlman. Determine the root bridgeDetermine the root bridge


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To Build the Spanning Tree(Cont.)To Build the Spanning Tree(Cont.)

Determine the root port on all other bridges. Determine the root port on all other bridges. The root port of a bridge is the port on the The root port of a bridge is the port on the minimum-cost path to the root bridgeminimum-cost path to the root bridge

Determine the designated port on each LAN. Determine the designated port on each LAN. The designated port of a LAN is the port of the The designated port of a LAN is the port of the bridge attached to the LAN, called the bridge attached to the LAN, called the designated bridge that provides the minimal designated bridge that provides the minimal cost path to the root bridge.cost path to the root bridge.


5555


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Source Routing BridgesSource Routing Bridges

Mostly used in token ring networks.Mostly used in token ring networks. The sender includes in its frame header the exact path The sender includes in its frame header the exact path

(route) that the frame will follow.(route) that the frame will follow. A route is a sequence of bridge, LAN, bridge , LAN, ... A route is a sequence of bridge, LAN, bridge , LAN, ...

numbers.numbers. To find the best path to every LAN, broadcast a To find the best path to every LAN, broadcast a

discovery frame which is forwarded by every bridge discovery frame which is forwarded by every bridge so that it reaches every LAN. When the reply comes so that it reaches every LAN. When the reply comes back, the bridges record their identity in it, so that the back, the bridges record their identity in it, so that the sender can see the exact route taken and choose the sender can see the exact route taken and choose the best route.best route.


5858


5959


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Wireless LAN ProtocolsWireless LAN Protocols

Use a medium that has neither absolute nor readily Use a medium that has neither absolute nor readily observable boundaries.observable boundaries.

Are unprotected from outside signals.Are unprotected from outside signals. Communicate over a significantly less reliable medium Communicate over a significantly less reliable medium

than wired one.than wired one. Have dynamic topology.Have dynamic topology. Lack full connectivity (not every station can hear every Lack full connectivity (not every station can hear every

other station).other station). Have time - varying and asymmetric propagation Have time - varying and asymmetric propagation

properties. properties.

Wireless media:Wireless media:


6161

In Fig. 4.11(a), while A is transmitting to B, C, which is In Fig. 4.11(a), while A is transmitting to B, C, which is out of range of A, senses the medium idle and then out of range of A, senses the medium idle and then transmits. It will wipe out the frame from A to B.transmits. It will wipe out the frame from A to B.

Hidden station problem:

Exposed station problem:

In Fig. 4.11(b), while B is transmitting to A, C, which is In Fig. 4.11(b), while B is transmitting to A, C, which is within range of B, senses the medium busy and refrains within range of B, senses the medium busy and refrains from transmitting to D.from transmitting to D.


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Multiple Access with Collision AvoidanceMultiple Access with Collision Avoidance

Exchange of RTS (Request to Send) and CTS (Clear to Send) Exchange of RTS (Request to Send) and CTS (Clear to Send) frame prior to the actual Data frame to avoid collision.frame prior to the actual Data frame to avoid collision.

RTS/CTS includes the Duration/ID field for channel RTS/CTS includes the Duration/ID field for channel reservation.reservation.


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IEEE 802.11 Wireless LAN Medium Access ControlIEEE 802.11 Wireless LAN Medium Access Control

Use CSMA/CA (Carrier Sense Multiple Access with Collision Use CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)Avoidance)

A station wishing to transmit shall first sense the medium. If the A station wishing to transmit shall first sense the medium. If the medium is busy, the station shall defer until the end of the current medium is busy, the station shall defer until the end of the current transmission.transmission.

After deferrals, or prior to attempting to transmit again After deferrals, or prior to attempting to transmit again immediately after a successful transmission, the station shall immediately after a successful transmission, the station shall select a random back off interval and shall decrement the backoff select a random back off interval and shall decrement the backoff interval counter while the medium is idle.interval counter while the medium is idle.

If the medium is idle, after any deferrals or backoffs, prior to data If the medium is idle, after any deferrals or backoffs, prior to data transmission, RTS/CTS short frames are exchanged. transmission, RTS/CTS short frames are exchanged.

Distributed Coordination Function (DCF):


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Physical carrier sense: provided by the PHY.Physical carrier sense: provided by the PHY. Virtual carrier sense: provided by MAC. Use the Network Allocation Vector Virtual carrier sense: provided by MAC. Use the Network Allocation Vector

(NAV) which maintains a prediction of future traffic on the medium based (NAV) which maintains a prediction of future traffic on the medium based on duration information in RTS/CTS frames prior to the actual exchange of on duration information in RTS/CTS frames prior to the actual exchange of data.data.

Carrier Sense Mechanism:Carrier Sense Mechanism:

Point Coordination Function (PCF):Point Coordination Function (PCF): For contention free servicesFor contention free services Use polling with the point coordinator as the polling Use polling with the point coordinator as the polling

master to determine which station currently has the right to master to determine which station currently has the right to transmit.transmit.

To gain control of the medium by setting the NAV in To gain control of the medium by setting the NAV in stations.stations.

Point-coordinated traffic have priority access to the Point-coordinated traffic have priority access to the medium over stations under the DCF access method.medium over stations under the DCF access method.


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Cellular RadioCellular Radio

First generation cellular systemsFirst generation cellular systems AMPS (Advanced Mobile phone System) in US AMPS (Advanced Mobile phone System) in US TACS in UK etc.TACS in UK etc.

Second generation cellular systemsSecond generation cellular systems N-AMPS, TDMA, CDMA etc. N-AMPS, TDMA, CDMA etc. IS-54, GSM, IS-95(CDMA/FDMA), CT-2, CT-3, DECTIS-54, GSM, IS-95(CDMA/FDMA), CT-2, CT-3, DECT

Third generation systemsThird generation systems 1st and 2nd generation systems are designed to support voice 1st and 2nd generation systems are designed to support voice

communication with limited data communication capabilities.communication with limited data communication capabilities. The third generation systems offer services, mostly the wireless The third generation systems offer services, mostly the wireless

extensions of ISDN services.extensions of ISDN services. Personal Communication Systems.Personal Communication Systems.


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Global System for Mobile CommunicationsGlobal System for Mobile Communications


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GSM frameGSM frame


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Control Channels:Control Channels: Broadcast control channelBroadcast control channel Dedicated control channelDedicated control channel Common control channel: Common control channel:

paging channel, random access paging channel, random access channel, channel, access grant channel access grant channel

GSM frame (Cont.)GSM frame (Cont.)


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CDPD: Cellular Digital Packet DataCDPD: Cellular Digital Packet Data


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CDMA:Code Division Multiple AccessCDMA:Code Division Multiple Access


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Satellite CommunicationsSatellite Communications

Use FDM or TDM channel allocation Use FDM or TDM channel allocation schemes.schemes.

Static and dynamic channel allocation.Static and dynamic channel allocation. FDM with a common signaling channel for FDM with a common signaling channel for

dynamic channel reservation. The common dynamic channel reservation. The common signaling channel has a number of minislots signaling channel has a number of minislots for reservation.for reservation.


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TDM with Dynamic Slot AllocationTDM with Dynamic Slot Allocation


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C) All rights reserved by Professor Wen-Tsuen Chen1 IEEE 802.5 Token Ring ä Easy fault detection....

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Transcript of C) All rights reserved by Professor Wen-Tsuen Chen1 IEEE 802.5 Token Ring ä Easy fault detection....