Multiple Access

Data link layer divided into two functionality-oriented sublayers

LOGIC LINK CONTROL-Error and flow control MAC LAYER-Multiple access resolution

Taxonomy of multiple-access protocols discussed in this chapter

RANDOM ACCESS

In random access or contention methods

>No scheduling of time station

>No rule that which station should send next

Protocol used in this method-ALOHA

Carrier Sense Multiple Access

Carrier Sense Multiple Access with Collision Detection

Carrier Sense Multiple Access with Collision Avoidance

>1972-for Radio >Pure/Slotted >Globally used >Acknowledgement >Back off time TB

>Vulnerable time-Time in which possibility of collision

ALOHA

12.6

Frames in a pure ALOHA network

>Carrier sense multiple access >Vulnerable time-Time need to propagate from one to

another >Senses the medium >Method- 1.Persistent I 2. Non Persistent- Random amount of time 3.P-Persiustance- change of slot duration> max

propagation time

CSMA

12.8

Figure 12.10 Behavior of three persistence methods

>Carrier sense multiple access/collision Detection >Arguments algorithm to handle for collision >Monitors

CSMA/CD

12.10

Figure 12.16 CSMA/CA

>Carrier sense multiple access/collision avoidance >Two signal >For wireless –because collision cannot be detected >Three strategy-Interframe Space(short ifs high priority) Contention window Acknowledgement

CSMA/CA

Flow diagram for CSMA/CA

12.13

CONTROLLED ACCESS

In controlled access, the stations consult one another to find which station has the right to

send. A station cannot send unless it has been authorized by other stations.

Reservation

Polling

Token Passing

Three popular controlled-access methods

Reservation access method

Select and poll functions in polling access method

>Primary device

>Link is available

>Select-selection of primary

>Poll-primary device verify and allow to transmit

Logical ring and physical topology in token-passing access method

CHANNELIZATION

Channelization is a multiple-access method in which the available bandwidth of a link is

shared in time, frequency, or through code, between different stations.

Frequency-Division Multiple Access (FDMA)-Guard band

Time-Division Multiple Access (TDMA)-Access method of DLL to control PL

Code-Division Multiple Access (CDMA)-One chanel all transmission, simuntaniously

Protocol