Multiple Access

The upper sublayer is responsible for flow and error control is called Logical Link Control (LLC) layer.

The lower sublayer is responsible for multiple access resolution is called the Media Access Control (MAC) layer.

Random Access Traits

Attempt to ensure equal access to the medium for all stations

No scheduling or pollingSusceptible to collisions

Two or more stations trying to access the medium at the same, resulting in their frames interfering with each other.

Random Access Issues

When can a station access the medium?What should a station do if the medium is

occupied?What will determine the success or failure of

a given transmission?What should a station do if a collision occurs?

Random Access Protocols

AlohaSlotted AlohaCSMACSMA/CDCSMA/CA

Developed by the University of Hawaii to network the Hawaiian islands in 1970

Wireless LAN at 9600 bps Simple

Station sends frame as soon when it has a frame to send

It does not check the medium for availability It does not check for collisions If an acknowledgement is not received

within two propagation periods, it resends the frame

There are two versions of Aloha

PURE ALOHA

SLOTTED ALOHA

The basic idea on which pure aloha is based upon is : It let users

transmit whenever they have data to be sent. So there will be collisions ,of course, and the colliding frames will

be damaged. However due to feed back property of broadcasting a sender can always find out whether its frame was destroyed by listening to the channel. All the receivers acknowledge the packets which means that the packet is received and if there is no acknowledgement from the receiver then the transmission is assumed to be unsuccessful and the station then retransmits the packet after random amount of time.

The systems in which multiple users share a common

channel in a way that can lead to conflict are widely known as contention systems.

Vulnerable Time: the length of time in which there is a possibility of collision.

• In 1987, Robert’s publish a method called slotted aloha which doubled the capacity of pure aloha.

• Divide time up into discrete intervals, each corresponding to one packet.

• The stations can only transmit data in one of the time slots only.

• The vulnerable period is now reduced in half. If the frames collide they will overlap completely instead of partially.

• Aloha Protocol:– Whenever a station has data, it

transmits immediately– Receivers ACK all packets– No ACK = collision. Wait a random time

and retransmit

• The Slotted Aloha Protocol– Slotted Aloha - Aloha with an additional

constraint– Time is divided into discrete time

intervals (=slot)– A station can transmit only at the

beginning of a frame

Reduces the probability of collision by sensing the medium for the presence of a frame prior to transmitting “Listen before talking”

Collisions can still occur If a frame is sensed on the medium,

then a persistence strategy is employed

Vulnerability time = tprop

Persistence refers to the action a station takes if it senses a carrier on a CSMA system.

Three persistence strategies: Nonpersistent 1-Persistent p-Persistent

A station with a frame to send senses the line

If the line is idle, the frame is sent immediately

If the line is busy, the station waits a random amount of time before sensing the line again (backoff algorithm)

Probability of collision is reduced but the efficiency is decreased Medium has significant idle periods

even if stations have frames to send

A station with a frame to send senses the line

If the line is idle, the frame is sent immediately

If the line is busy, the station continues to sense the channel until the line is idle and then transmits

If more than one station is waiting a collision will occur

Relatively high collision rate

After a station senses an idle line it may or may not send the frame Sends with probability p Refrains with probability 1-p

A random number generator is used to determine whether the station sends

If the station refrains from sending, it waits one time slot before sensing again

Fewer collisions than 1-Persistent

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Carrier Sense Multiple Access with Collision Detection

When a frame is sent the line is monitored to determine if a collision occurs No collision: successful transmission Collision: the station sends a jam signal and

waits a random amount of time (backoff algorithm) before sensing the medium again

2 tprop is required before determining a transmission occurred without collision

Used in Ethernet

IEEE 802.3 standard specifies the truncated binary exponential backoff algorithm

A frame undergoing it’s nth retransmission attempt determines the delay by selecting an integer between the range of 0 and 2k-1 and multiplying by 2tprop, where k = min(n, 10)

Typically a maximum of 15 retransmissions are attempted before the frame is aborted

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CSMA with Collision Avoidance Collisions are completely avoided by

using appropriate delays, acknowledgements, and time-out values.

Backoff algorithm is also employed Method similar to that used in IEEE

802.11 (Wireless LAN)

Most common method is Token Passing A token is passed around a physical or

logical ring Only the station with the token may

transmit Other methods:

Reservation Systems Polling