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 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 = M2 = M3 = M1 = 0 M2 = 1 M3 =

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 tokenaddress 1-bit delay

Node failures - sender fails Corruption - Full  empty 1  0 - empty  Full 0  1 * cannot include parity/checksum

tokenNew/old Full, old Full, new Empty, * Master node Empty, *

tokenNew/old Full, old Full, new Sender fails Full, new

tokenNew/old Full, new Empty, old Full, old Corruption: empty  full Full, new

tokenNew/old Full, old empty, old Empty, old empty, old Corruption: Full  empty

tokenNew/old Full, old Full, new Empty, new empty, oldFull, old