Two basic types of networks: Two basic types of networks



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Two basic types of networks:

  • Two basic types of networks:

    • Switched networks: transmission lines, multiplexers, and switches; routing, hierarchical address for scalability.
    • Broadcast networks: a single shared medium, simpler, no routing, messages received by all stations, flat address; however, when users try to transmit messages into the medium, potential conflict, so MAC is needed to orchestrate the transmission from various users.
    • LAN is a typical broadcast network.

Both are to transfer user information despite transmission impairments

  • Both are to transfer user information despite transmission impairments

  • For peer-to-peer:

    • Main concern: loss, delay, resequencing
    • Using control frames to coordinate their actions
    • Delay-bandwidth is important
    • Involved only two peer processes
  • MAC:

    • Main concern: interference from users
    • Using some mechanisms to coordinate the access of channel
    • Delay-bandwidth is important
    • Need the coordination from all MAC entities, any one does not cooperate, the communication will not take place.


Introduction to broadcast networks

  • Introduction to broadcast networks

  • Overview of LANs: frame format & placement in OSI.

  • Random access: ALOHA & CSMA-CD (Carrier Sensing Multiple Access with Collision Detection ) i.e., Ethernet.

  • Scheduling: token-ring.

  • LAN standards (brief view)

  • LAN bridges: used to connect several LANs.





Satellite Channel

  • Satellite Channel













Shared medium is the only means for stations to communicate

  • Shared medium is the only means for stations to communicate

  • Some kind of MAC technique is needed

  • Like ARQs, which use ACK frame to coordinate the transmission and consume certain bandwidth, the MAC will need to transfer some coordination information which will consume certain bandwidth of shared medium.

  • Delay-bandwidth product plays a key role in the performance of MAC (as in ARQs).







Ethernet (CSMA-CD):

  • Ethernet (CSMA-CD):

    • Efficiency = 1/(1+6.44a) where a = tpropR/L.
  • Token-ring networks:

    • Efficiency = 1/(1+a’ ) where a’ = ring-latency in bits/L where ring-latency contains:
      • The sum of bit delays introduced at each ring adapter.
      • Delay-bandwidth product where delay is the time required for a bit to circulate around the ring.














Why random access?

  • Why random access?

    • Reaction time (i.e. 2 times of propagation delay) is very important for performance, e.g. in Stop-and-Wait, when reaction time is small (i.e. the ACK will arrive soon) the performance is very good, however, if reaction time is large, then performance is very bad.
    • Therefore, proceed the transmission without waiting for ACK and deal with collision/error after the fact, i.e. random access.
  • Three types of random accesses:

    • ALOHA, slotted ALOHA, and CSMA-CD


Basic idea:

  • Basic idea:

    • let users transmit whenever they have data to be sent.
    • When collision occurs, wait a random time ( why? ) and retransmit again.
  • Differences between regular errors &collision

    • Regular errors only affect a single station
    • Collision affects more than one
    • The retransmission may collide again
    • Even the first bit of a frame overlaps with the last bit of a frame almost finished, then two frames are totally destroyed.














Problem with ALOHAs: low throughput because the collision wastes transmission bandwidth.

  • Problem with ALOHAs: low throughput because the collision wastes transmission bandwidth.

  • Solution: avoid transmission that are certain to cause collision, that is CSMA. Any station listens to the medium, if there is some transmission going on the medium, it will postpone its transmission.





Based on how to do when medium is busy:

  • Based on how to do when medium is busy:

    • 1-persistent CSMA
    • Non-persistent CSMA
    • p-persistent CSMA












When the transmitting station detects a collision, it stops its transmission immediately, Not transmit the entire frame which is already in collision.

  • When the transmitting station detects a collision, it stops its transmission immediately, Not transmit the entire frame which is already in collision.

  • The time for transmitting station to detect a collision is 2tprop.

  • In detail: when a station wants to transmit a packet, it senses

  • channel, if it is busy, use one of above three algorithms (i.e., 1-persistent, non-persistent, and p-persistent schemes). The transmitter senses the channel during transmission. If a collision occurred and was sensed, transmitter stops its left transmission of the current frame; moreover, a short jamming signal is transmitted to ensure other stations that a collision has occurred and backoff algorithm is used to schedule a future re-sensing time.

  • The implication: frame time X >= 2tprop, , since X=L/R, which means that there is a minimum limitation for frame length.










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