Medium Access Control: Static Allocation

Introduction

Point-to-point data link layer types need to select one of the framing techniques described previously and optionally add retransmission algorithms, such as those explained for the transport layer to provide a reliable service.

LAN Organizations

A LAN is composed of several hosts that are attached to the same shared physical medium. And LAN can be organized in a few different ways. We’ll focus on four main ones:

1. A bus-shaped network where all hosts are attached to the same physical cable.

2. A ring-shaped network where all hosts are attached to an upstream and a downstream node so that the entire network forms a ring.

3. A star-shaped network where all hosts are attached to the same device.

4. A wireless network where all hosts can send and receive frames using radio signals.

Collisions

The common problem among all of these network organizations is how to efficiently share access to the local area network. If two devices send a frame at the same time, the two electrical, wireless, or optical signals that correspond to these frames will appear at the same time on the transmission medium, and a receiver will not be able to decode either frame. Such simultaneous transmissions are called collisions. A collision may involve frames transmitted by two or more devices attached to the Local Area Network. Collisions are the main cause of errors in wired Local Area Networks. They also reduce the throughput of the network, which is problematic.

Medium Access Control Algorithms

All Local Area Network technologies hence rely on Medium Access Control algorithms to regulate the transmissions to either minimize or avoid collisions.

There are two broad families of Medium Access Control algorithms: deterministic and optimistic.