We can consider the data link layer as two sub-layers. The upper sub-layer is responsible for data link control, and the lower sub-layer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sub-layer. Figure 4.1 shows these two sub-layers in the data link layer.

Data link layer divided into two functionality-oriented sub-layers

IEEE has actually made this division for LANs. The upper sub-layer that is responsible for flow and error control is called the logical link control (LLC) layer; the lower sub-layer that is mostly responsible for multiple access resolution is called the media access control (MAC) layer.

In any broadcast network, the key issue is how to determine who gets to use the channel when there is competition for it. The protocols used to determine who goes next on a multi-access channel belong to a sub-layer of the data link layer called the MAC (Medium Access Control) sub-layer.

The Channel Allocation Problem is how to allocate a single broadcast channel among competing users.

Static Channel Allocation in LANs and MANs

i. The traditional way of allocating a single channel, such as a telephone trunk, among multiple competing users is Frequency Division Multiplexing (FDM). If there are N users, the bandwidth is divided into N equal-sized portions each user being assigned one portion. Since each user has a private frequency band, there is no interference between users.

ii. When there are only a small and constant number of users, each of which has a heavy (buffered) load of traffic (e.g., carriers' switching offices), FDM is a simple and efficient allocation mechanism. However, when the number of senders is large and continuously varying or the traffic is bursty, FDM presents some problems.

iii. If the spectrum is cut up into N regions and fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted. If more than N users want to communicate, some of them will be denied permission for lack of bandwidth, even if some of the users who have been assigned a frequency band hardly ever transmit or receive anything.