The Medium Access Control Sublayer

Some networks have a single channel that is used for all communication. In these networks, the key design issue is the allocation of this channel among the competing stations wishing to use it. Numerous channel allocation algorithms have been devised. A summary of some of the more important channel allocation methods is given in Fig. 4-52.

Figure 4-52. Channel allocation methods and systems for a common channel.

The simplest allocation schemes are FDM and TDM. These are efficient when the number of stations is small and fixed and the traffic is continuous. Both are widely used under these circumstances, for example, for dividing up the bandwidth on telephone trunks.

When the number of stations is large and variable or the traffic is fairly bursty, FDM and TDM are poor choices. The ALOHA protocol, with and without slotting, has been proposed as an alternative. ALOHA and its many variants and derivatives have been widely discussed, analyzed, and used in real systems.

When the state of the channel can be sensed, stations can avoid starting a transmission while another station is transmitting. This technique, carrier sensing, has led to a variety of protocols that can be used on LANs and MANs.

A class of protocols that eliminates contention altogether, or at least reduce it considerably, is well known. Binary countdown completely eliminates contention. The tree walk protocol reduces it by dynamically dividing the stations into two disjoint groups, one of which is permitted to transmit and one of which is not. It tries to make the division in such a way that only one station that is ready to send is permitted to do so.

Wireless LANs have their own problems and solutions. The biggest problem is caused by hidden stations, so CSMA does not work. One class of solutions, typified by MACA and MACAW, attempts to stimulate transmissions around the destination, to make CSMA work better. Frequency hopping spread spectrum and direct sequence spread spectrum are also used. IEEE 802.11 combines CSMA and MACAW to produce CSMA/CA.

Ethernet is the dominant form of local area networking. It uses CSMA/CD for channel allocation. Older versions used a cable that snaked from machine to machine, but now twisted pairs to hubs and switches are most common. Speeds have risen from 10 Mbps to 1 Gbps and are still rising.

Wireless LANs are becoming common, with 802.11 dominating the field. Its physical layer allows five different transmission modes, including infrared, various spread spectrum schemes, and a multichannel FDM system. It can operate with a base station in each cell, but it can also operate without one. The protocol is a variant of MACAW, with virtual carrier sensing.

Wireless MANs are starting to appear. These are broadband systems that use radio to replace the last mile on telephone connections. Traditional narrowband modulation techniques are used. Quality of service is important, with the 802.16 standard defining four classes (constant bit rate, two variable bit rate, and one best efforts).

The Bluetooth system is also wireless but aimed more at the desktop, for connecting headsets and other peripherals to computers without wires. It is also intended to connect peripherals, such as fax machines, to mobile telephones. Like 801.11, it uses frequency hopping spread spectrum in the ISM band. Due to the expected noise level of many environments and need for real-time interaction, elaborate forward error correction is built into its various protocols.

With so many different LANs, a way is needed to interconnect them all. Bridges and switches are used for this purpose. The spanning tree algorithm is used to build plug-and-play bridges. A new development in the LAN interconnection world is the VLAN, which separates the logical topology of the LANs from their physical topology. A new format for Ethernet frames (802.1Q) has been introduced to ease the introduction of VLANs into organizations.