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