MAC Protocols (3)

Today, I learned about the CSMA collisions, "Taking turns" MAC protocols and Cable Internet.

 

Compared to ALOHA, CSMA is more polite protocol algorithm. 

 

CSMA: Carrier Sense Multiple access

CSMA: listen before transmit:

• If channel sensed idle: transmit entire frame.
• If channel sensed busy, defer transmission.
• In human analogy, "don't interrupt others!"

CSMA collision

• Collision can still occur:
  • Due to propagation delay, two nodes may not hear each other's transmission
• When collision occurs:
  • Entire packets and transmission time wasted
    • Distance & Propagation delay play role in determining collision probability

 

To avoid(or reduce) time wasted, we have a CSMA/CD(collision detection).

 

CSMA/CD (collision detection)

CSMA/CD

 

CSMA/CD: carrier sensing, deferral as in CSMA. Also, 

• While transmitting data, sense channel to detect collisions
• Collisions could be detected within a short time. Once detected, stop data transmission
• Colliding transmissions aborted: reducing channel wastage

Collision detection:

• Easy in wired LANs: measure signal strengths, compare transmitted, received signals
• Difficult in wireless LANs: received signal strength overwhelmed by local transmission strength
• Meaning hard for transmitter device to transmit and detect together

In human analogy: the polite conversationalist. 

 

"Taking turns" MAC protocols

1. Polling:

• Master node "invites" slave nodes to transmit in turn
• Typically used with "dumb" salve devices
• Concerns:
  • Polling overhead
  • Latency
  • Single point of failure (master)
  • Depending on the number of users, the bandwidth is fairly distributed

Polling

 

2. Ring (token passing):

• Control token passed from one node to next sequentially.
• Token message
• Concerns:
  • Token overhead
  • Latency
  • Single point of failure(token)
  • If single user on the Ring, the bandwith is a little bit less than a whole bandwitdth

Ring

 

Cable Internet

Cable Internet

• Multiple 40Mbps downstream(broadcast) channels
  • Single CMTS transmits into channels
• Multiple 30Mbps upstream channels
  • Multiple access: all users contend for certain upstream channel time slots (others assigned)