4.1 THE CHANNEL ALLOCATION PROBLEM

• In the literature, broadcast channels are sometimes referred to as

  multiaccess channels or random access channels.
• The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.
• Technically, the MAC sublayer is the bottom part of the data link layer, so logically we should have studied it before examining all the point-to-point protocols in Chap. 3.
• The central theme of this chapter is how to allocate a single broadcast channel among competing users. The channel might be a portion of the wireless spectrum in a geographic region, or a single wire or optical fiber to which multiple nodes are connected.
• The conventional way of allocating a single channel, such as a telephone trunk, among multiple competing users is to chop up its capacity by using one of the multiplexing schemes we described in Sec. 2.4.4, such as FDM (Frequency Division Multiplexing).
• Independent Traffic. The model consists of N independent stations

  (e.g., computers, telephones), each with a program or user that generates

  frames for transmission. The expected number of frames generated

  in an interval of length 􀀶t is 􀁨 􀀶t, where 􀁨 is a constant (the arrival

  rate of new frames). Once a frame has been generated, the station

  is blocked and
• Single Channel. A single channel is available for all communication.

  All stations can transmit on it and all can receive from it. The stations

  are assumed to be equally capable, though protocols may assign them

  different roles (e.g., priorities).
• Observable Collisions. If two frames are transmitted simultaneously,

  they overlap in time and the resulting signal is garbled. This

  ev ent is called a collision. All stations can detect that a collision has

  occurred. A collided frame must be transmitted again later. No errors

  other than those generated by collisions occur.
• 4. Continuous or Slotted Time. Time may be assumed continuous, in

  which case frame transmission can begin at any instant. Alternatively,

  time may be slotted or divided into discrete intervals (called

  slots). Frame transmissions must then begin at the start of a slot. A

  slot may contain 0, 1, or more frames, corresponding to an idle slot, a

  successful transmission, or a collision, respectively.
• Carrier Sense or No Carrier Sense. With the carrier sense assumption,

  stations can tell if the channel is in use before trying to use it.

  No station will attempt to use the channel while it is sensed as busy.

  If there is no carrier sense, stations cannot sense the channel before

  trying to use it. They just go ahead and transmit. Only later can they

  determine whether the transmission was successful.
• Recent research confirms that the pattern still holds (Fontugne et al., 2017).

  Nonetheless, Poisson models, as they are frequently called, are commonly used, in part, because they are mathematically tractable. They help us analyze protocols to understand roughly how performance changes over an operating range and how it compares with other designs.
• Similarly, a network may have carrier sensing or not. Wired networks will

  generally have carrier sense. Wireless networks cannot always use it effectively because not every station may be within radio range of every other station