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 0M2 = 1 1 0 1M3 = 0 0 1 0

M1 = 0M2 = 1 M3 = 0

1

0

0

Token Ring

• Stations are arranged in a ring• A token circulates in the ring

• 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

token address

1-bit delay

Node failures - sender fails

Corruption - Full empty 1 0 - empty Full 0 1

* cannot include parity/checksum

token New/old

Full, oldFull, new

Full, new

Empty, *

Master node

Empty, *

token New/old

Full, oldFull, new

Full, new

Full, new

Sender fails

Full, new

token New/old

Full, newFull, new

Full, new

Empty, old

Full, old

Corruption: empty full

Full, new

token New/old

Full, oldempty, old

empty, old

Empty, old

empty, old

Corruption: Full empty

token New/old

Full, oldFull, new

Full, new

Empty, new

empty, oldFull, old

Full, old