Multiple Access Protocols, Slides of Computer Networks

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Multiple Access

Multiple Access

AGENDA

INTORDUCTION

TYPES OF NETWORK LINK

CHANNEL PARITIONING PROTOCOLS

RANDOM ACCESS

Slotted ALOHA

ALOHA CSMA

TAKING-TURNS PROTOCOLS

LOCAL AREA NETWORKS

BROADCAST-LINK

• It can have multiple sending and receiving nodes all connected to the same, single shared broadcast channel. - Examples: Ethernet, Wireless LAN Multiple Access Problem: How to coordinate the access of multiple sending and receiving to a shared broadcast channel.

On the basis of functionality oriented sublayers :

DATA LINK LA# DataYER Link Control - for data link control

# Multiple Access Resolution - for resolving access to shared media

Note : if the link is dedicated , we do not require the lower sublayer.

• Dedicated – line / channel reserved exclusively for one type of communication.

Multiple Access

Multiple access protocol

Multiple access protocol is used to coordinate access to the link. Nodes can regulate their transmission onto the shared broadcast channel by using Multiple access protocol. It is used both wired and wireless local area network and satellite network. All nodes are capable of transmitting frame, more than two nodes can transmit frames at the same time. If so, the transmitted frames collide at all of the receivers.

When there is a collision, none of the receiving nodes can make any sense of any of the frames that were transmitted; In a sense, the signals of the colliding frames become inextricably tangled together. Thus, all frames involved in the collision are lost, and the broad cast channel as wasted during the collision interval. To rectify this problem Multiple access protocol was implemented

Classification of MAP

Multiple Access

Protocol

Channel-Partitioning

protocols

Taking-turns

Protocols

Random Access

Protocol

Definition

Multiple Access (protocols) allows several terminals

connected to the same multi-point transmission medium to

transmit over it and to share its capacity.

It allows several data streams or signals to share the

same communication channel or physical medium.

Multiple Access

1. ALOHA and its variants

Multiple Access

• (^) One of the earliest multiple access scheme
• (^) Just sent it!
• (^) Wait for ack
• (^) If no ack, try again after a random waiting time
  • (^) No back off

Pure ALOHA

• (^) CONCEPT - Each station sends a frame, when it has frame to send.
• (^) Since only one channel to share, there is a possibility of collision between frames

of different stations.

• (^) Frames in Pure ALOHA network :

Multiple Access

Vulnerable time for Pure ALOHA protocol

Multiple Access

Slotted ALOHA

• (^) To improve the efficiency of Pure ALOHA.
• (^) CONCEPT - Divide the time into slots of Tfr and force the station to send only at the beginning of the time slot.

Multiple Access

2. Carrier Sense Multiple Access

• (^) To minimize the chance of collision and increase the performance.
• (^) CONCEPT - Each Station listen before sending i.e. “Listen before Talk”.
• (^) It reduce collisions but cannot eliminate it.
• (^) Space/Time model of collision in CSMA :

Multiple Access

Vulnerable time in CSMA

Multiple Access