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!"
- 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
- Entire packets and transmission time wasted
To avoid(or reduce) time wasted, we have a CSMA/CD(collision detection).
CSMA/CD (collision detection)
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
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
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)
'Computer Science ๐ > Network Programming ๐ฐ๏ธ' ์นดํ ๊ณ ๋ฆฌ์ ๋ค๋ฅธ ๊ธ
MAC Protocols (2) (0) | 2023.05.04 |
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MAC protocol (1) (0) | 2023.05.04 |
TCP (2) (0) | 2023.05.02 |
TCP (1) (0) | 2023.04.29 |
UDP (0) | 2023.04.27 |