Media Access Layer

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CIS 725

Media Access Layer

Medium Access Control Sublayer

• MAC sublayer resides between physical and data link layer

• Broadcast/multiacess channels

• N independent stations

- each station generates traffic independently

- if two transmit at the same time, both frames are garbled

Medium Access Control Sublayer

• MAC sublayer resides between physical and data link layer

• Broadcast/multiacess channels

• N independent stations

- each station generates traffic independently

- if two transmit at the same time, both frames are garbled

Static Allocation

• Frequency division multiplexing

• Time division multiplexing

Pure ALOHA

• A station transmits whenever it wants

• Sender detects collision and retransmits after random time

Pure ALOHA

In pure ALOHA, frames are transmitted at completely arbitrary times.

Pure ALOHA

Vulnerable period for the shaded frame.

Slotted Aloha

• Time is divided into slots

• Each station waits until beginning of next slot before transmitting

Pure ALOHA (3)

Throughput versus offered traffic for ALOHA systems.

Carrier Sensing

• Ability to detect if channel is busy

• CSMA Protocols (Carrier Sense Multi

Access)

• Messages must be long enough to detect collision

1-persistent CSMA

• Listen to the channel

• If busy then wait until channel is idle

• When idle, transmit frame

• If collision then start again after random time

Non-persistent CSMA

• Sense the channel

• If idle then transmit else start over again after random time

If collision then start again after random time

p-persistent CSMA

• Listen to the channel

• If idle then transmit with probability p else wait for random amount of time

Persistent and Nonpersistent

CSMA

Collision-free protocols

• Stations are numbered 0..N-1

- 1-bit contention slots are used to determine who wants to transmit

CAN (Controller area network) protocol

• Priority-based arbitration mechanism

• Message id = priority

• For each message, the id is first transmitted

Message with the lowest id wins

M1 = 0 1 0 0

M2 = 1 1 0 1

M3 = 0 0 1 0

M1 = 0

M2 = 1

1

M3 = 0 0 0

Token Ring

• Stations are arranged in a ring

• A token circulates in the ring

token address

• To send data, acquire the token; place data on the ring; when data comes back, insert token back

• Listen mode: copy input bit to output

1-bit delay

Node failures

- sender fails

Corruption

- Full  empty

1

0

- empty

Full

0

1

* cannot include parity/checksum

token New/old

Full, new

Empty, *

Full, new

Master node

Full, old

Empty, *

token New/old

Full, new

Full, new

Sender fails

Full, new

Full, old

Full, new

token New/old

Full, new

Full, new

Full, new

Empty, old

Full, new

Full, old

Corruption: empty

 full

token New/old empty, old empty, old

Corruption: Full

 empty

Full, old

Empty, old empty, old

token New/old

Full, new

Full, new

Full, old

Empty, new

Full, old empty, old

Full, old

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