[Network]数据链路层

EDC

CRC

d bits data(D) and r bits extension(R)

r+1 bits (G) a generator

divide to check if it is the same as former R

MAC

Channel partition Protocols

Random Access Protocols

• slotted ALOHA: L/R sec - sized slot
  • How it detect a collision
  • The maximum efficiency of A system of slotted ALOHA: 1/e - N nodes is going to transmit a frame. The conclusion here indicates that the utilization rate is low so that the throughput of this system is low though the slot size is big.  ----There is N-sized slots sequence. The utilization rate is this. from a perspective of continuity, for one slot, the utilization rate is also this big.
• ALOHA：start with a a time frame (The probability of transmitting within every time frame is the same), each node must detect if there is node transmitting.
• CSMA: listen before talk and stop if someone begins talking
  • The propagation delay of a broadcast channel is a crucial role for a end to sense a transmission in the network.
  • CSMA/CD: Stop when detecting collisions
    • Sense if the channel is busy (signal energy) & Sense while transmitting (detect and wait to retransmit)
    • How to choose the backoff time: time short vs. nodes small, time long vs. nodes large
      • Binary exponential backoff (Ethernet): If detecting collision wait K*512 bits time before back to sense-and-transmit-when-idle cycle
    • My idea: If the line is busy, the node will continue to wait until the line is idle. But when the node begins transmitting it does not mean that other nodes will sense the transmitting immediately (Although the ideal rate is almost speed of light). So other nodes tends to determine that the line is idle. And other nodes begins to transmitting. These activity and some delay cause the collisions. In the meanwhile, the node transmitting frames must keeps sensing the signal in the network where there may be some nodes begins to transmit without sensing the collisions.
    • Waste: sense collisions during the transmitting.
    • Efficiency

Taking - Turns Protocol