Post on 15-Apr-2017
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