Notes On Chapter Fourteen -- The IEEE MAC Sub-Layer

• 14.1 Introduction
  
  
  • Multi-access Protocols
  • Static and Dynamic Channel Allocation
  
  
• 14.2 A Taxonomy of Mechanisms for Multi-Access
  
  
  • Three approaches by which computers can coordinate access to a shared medium are:
    • Controlled Access Protocols - for example:
      • Reservation
      • Polling
      • Token Passing
    • Random Access Protocols - for example:
      • ALOHA
      • CSMA/CD
      • CSMA/CA
    • Channelization Protocols - for example:
      • FDMA
      • TDMA
      • CDMA
  
  
• 14.3 Static and Dynamic Channel Allocation
  
  
  • An FDM system can assign a channel for the exclusive use of a communicating pair of computers.
  • In some cases a static assignment will suffice.
  • In many cases the pairs communicating will vary over time, so channels need to be allocated and deallocated dynamically.
  
  
• 14.4 Channelization Protocols
  
  
  • Examples:
    • FDMA - Frequency Division Multi-Access - for example:
      • 'New station' uses reserved control channel to make a request to a 'central controller'
      • Central controller chooses an unused channel and assigns it to the station
      
      
    • TDMA - Time Division Multi-Access - for example:
      • A station is assigned a time slot during which it is allowed to send
      • This could be a round-robin scheme -- so, for example, the station might be allowed to send every 3rd packet among a set of 25 stations.
      
      
    • CDMA - Code Division Multi-Access
      • All senders may send at the same time - they send "mutually orthogonal" data which is multiplexed by summing. This is comparable to interleaving at the bit level.
  
  
• 14.5 Controlled Access Protocols
  
  
  • 14.5.1 Polling
    • Centralized controller polls 'stations',giving them an opportunity to send one packet
    • selection may be simple round robin or some priority scheme.
    • the controller sends a polling message to a station and the station responds, either by sending a packet, or 'passing'.
    
    
  • 14.5.2 Reservation
    • Stations send messages in rounds.
    • Typically for a period of time, stations send messages to a central controller saying whether they will have a need to send a message in the next round.
    • Then the controller transmits a list of stations that will be transmitting in the next round.
    • Stations then use the list to determine when to send during the next round.
    • Variation: there is a separate channel for controller to use to gather reservations.
    • This technique is often used with satellite transmission.
    
    
  • 14.5.3 Token Passing
    • There is a special control message called the token.
    • The token is passed from station to station according to some preassigned 'circular order'.
    • When a station has the token, it is allowed to transmit a message.
    • The station passes the token immediately, or right after sending its message.
  
  
• 14.6 Random Access Protocols
  
  
  • 14.6.1 ALOHA
    • A protocol used in an early data network in Hawaii.
    • The medium was radio frequency
    • ALOHA pioneered 'random access techniques'
    • A master central station with a powerful transmitter is surrounded by 'slave stations' with weaker transmitters.
    • Transmissions of the master can reach all slaves, and transmissions of all slaves can reach the master.
    • Transmissions of most slaves can't reach all the other slaves.
    • The network employs two different frequencies - an outbound frequency for the master to send messages to slaves, and an inbound frequency for slaves to send messages to the master.
    • A slave that wants to send a packet to another slave, transmits it to the master on the inbound frequency. The master then resends the packet on the outbound frequency. Both the intended receiver and the sender get a copy of the packet sent by the master. The copy that is received by the original sender serves as an acknowledgment that the packet arrived safely.
    • Two slaves might send a packet at the same time. If so they interfere with each other (collide), are unintelligible to the master, and are NOT resent by the master on the outbound frequency.
    • If a slave sends a packet, and does not soon receive a copy of the packet back on the outbound frequency, then the slave will retransmit the packet, after a random delay.
    • Because retransmission delays are randomly chosen, there is a low probability that the two senders that collided before will collide again.
    • If the network is heavily utilized collisions can happen frequently despite these measures.
    
    
  • 14.6.2 CSMA/CD
    • The original Ethernet employs a single long cable to which all computers connect and transmit.
    • Ethernet Carrier Sense Multi-Access with Collision Detection (CSMA/CD) is similar to ALOHA but utilizes several innovations:
      • The "two-frequency idea" was dropped.
      • An NIC "carrier senses" and refrain from attempting to send while a transmission is in progress (this improves utilization).
      • Stations monitor the ether while sending, and abort transmission if a collision is detected.
      • After detecting a collision, an NIC utilizes a binary exponential backoff strategy:
        • wait a random time (bounded by some constant d) and attempts to retransmit.
        • If a second collision occurs, abort again and picks another random delay, this time bounded by 2d.
        • If collisions keep recurring while trying to send the same packet, keep doubling the maximum delay until success (or after trying some large number of times, give up.)
    
    
  • 14.6.3 CSMA/CA
    • One of the things that allows CSMA/CD to work on an Ethernet is the fact that all NIC's are able to receive each other's transmissions.
    • This is not true in the typical wireless LAN.
    • C1 --------- C2 ------------ C3
      Suppose (C1,C2) and (C2,C3) are in range, but (C1,C3) are not. Suppose further that C1 and C3 send at the same time, intending that C2 be the receiver. The packets will collide at C2, but the collision will not be detected by C1 or C3.
    • There is an alternative called Carrier Sense Multiple Access with Collision Avoidance.
    • When C3 wants to send to C2 it sends a very brief message announcing that fact.
    • Upon receiving that message C2 sends out a brief message saying it is about to receive a packet from C3.
    • Every computer within range of either C3 or C2 receives one or both of these short messages, and refrains from sending for a long enough time so that C3 can send its packet to C2 without interference.
    • If a control message from C3 collides at C2 with a control message from C1, then much like with ALOHA, C2 will NOT reply and C1 and C3 will apply a random backoff scheme until one of them succeeds in getting a control message through to C2.