Multiple Access Links and Protocols

Two types of Network Links

Error?

• Node has no method of distinguishing data -> collision
• Who, when, and for how long does a person talk at a cocktail party?

• Channel partitioning could be breaking down into time / subjects.

• The more rules there are, the easier it is to manage - the complexity increases as there are less rules, because there is higher chance of collisions.

• The channel we are using to transmit must be the same channel we are using to coordinate.
• Decentralized means there is no single point of failure.

1. Channel Partitioning Protocols

Time Division Multiple Access (TDMA):

Kinda like Round-Robin - not exactly the same because there could be empty time slots.

• The R/N is both the upper bound and the lower bound.
  - This is because if the 6 nodes are active within a second, each node will split the second into 1/6 second.
• Each node is using its internal clock to synchronize (coordinate) the data trasmission.

FDMA:

• The properties are the same as TDMA.
• In the diagram, each frequency bands are 4KHz. Equally allocated, and thus fair.
• Just like TDMA, idle nodes will make the allocated resources unused.

2. Taking-Turns Protocols

Very much like round-robin.

• Overhead?: there is no way to actually acheive the maximum bandwidth of R, because of the polling process.

• Token passing is an example of "taking-turns" protocols.
• The node holds onto the token only if it has frames to be sent.
  • The node passes the token if it does not have.
• When the node has the token, the node can only send the frame upto the maximum number of frames.

• Loss of token can be disastrous.

3. Random Access Protocols

• partition: wasted resources due to idle nodes
• Taking turns: needed master node or complicated protocol.

• If the two or more nodes win the chance to transmit the frame then there is a conflict.
• If none of the nodes get the chance to transmit, then it is empty (idle).
• It is like flipping a coin to decide which node gets to transmit.

• The efficiency is halved, because the chance of colliding is almost doubled due to the overlap in transmission.

Design Flaws in RAP

• Senses the channel. If the channel is busy, the transmission is deferred; otherwise if it is idle, the frame is transmitted.

• Node B starts transmitting packets at time t0.
  • However, when t1, D node listens to the channel and sees it is not busy because the packets are not fully transmitted yet due to the propagation delay.
  • Therefore node D starts transmitting as well, resulting in collision.

• When collision is detected, the transmission is aborted.

• The time delays of retransmission increases exponentially, and each node gets to pick one of the time delays randomly.

• If the size of the frame is too small, the collision may not be detected by the sending nodes, which causes no retransmission.
• The solution is to set the lower boundary (64 bytes) minimum.