MAC Protocols (2)

Continue to previous review.

 

An ideal multiple access protocol

Given: broadcast channel of rate R bps

Desiderata:

1. When one node wants to transmit, it can send at rate R.
2. When M nodes want to transmit, each can send at average rate R/M
3. The protocol is fully decentralized:
   1. No special node to coordinate transmissions
   2. No synchronization of clocks, time slots
   3. No master node that represents a single point of failure
   4. The protocol is simple, so that inexpensive to implement

 

MAC protocols: 3 classes

1. Channel Partitioning (TDMA, FDMA, CDMA)
   1. Divide channel into smaller "pieces" (in terms of time slots, frequency, code, etc.)
   2. Allocate each piece to a node for exclusive use
2. Random Access
   1. Channel is not divided, allow collisions
   2. "Recover" from collisions
3. Taking Turns
   1. Nodes take turns, but nodes with more to send can take longer turns

 

TDMA: Time division multiple access

• Access to channel in "rounds" (I slot in time frame)
• Each station gets fixed length slot (length = pkt trans time) in each round
• Unused slots go idle
• Example: 6-station LAN, 1, 3, 4 have pkt, slots, 2, 5, 6 idle
• For example, blue slots belong to station 

 

FDMA: Frequency division multiple access

• Channel spectrum divided into frequency bands
• Each station assigned fixed frequency band
• Unused transmission time in frequency bands go idle
• Example: 6-station LAN, 1, 3, 4 have pkt, frequency bands 2, 5, 6 idle.
• Example: User I always can use the blue band.

 

Three classes of MAC protocols

• Channel partitioning protocols
  • Time Division, Frequency Division, Code Division
• Random access protocols
  • ALOHA, S-ALOHA, CSMA, CSMA/CD, CSMA/CA
  • Carrier sensing: easy in some technologies (wire), hard in others (wireless)
  • CSMA/CD used in Ethernet
  • CSMA/CA used in 802.11
• Taking turns protocols
  • Polling from central site
  • FDDI: token ring

 

Random access protocols

• When node has packet to send
  • Transmit at full channel data rate R
  • Not a prior coordination among nodes
• If two or more transmitting nodes -> "Collision"
• Random access MAC protocol specifies:
  • How to detect collisions
  • How to recover from collisions (for example, via delayed retransmissions)
• Examples of Random access MAC protocols:
  • Slotted ALOHA, ALOHA
  • CSMA, CSMA/CD, CSMA/CA

 

Slotted ALOHA

Assumption:

• All data-frames same size
• Time divided into equal size slots (time to transmit I data-frame)
• Nodes start to transmit only at beginning of slot
• Nodes are synchronized
• If 2 or more nodes transmit in the same slot, all nodes detect collision

Operation:

• When a node obtains fresh frame, transmits in next slot
  • If no collision: successful transmission!
  • If collision: node retransmits frame in each subsequent slot with some random probability p until success.

 

Here is pros and cos for ALOHA.