DATA COMMUNICATION PRINCIPLES Medium Access Control MAC Layer

DATA COMMUNICATION PRINCIPLES : Medium Access Control (MAC) Layer for IEEE Wireless LANs

Medium Access Control (MAC) Layer for IEEE Wireless LANs The wireless LAN architecture is similar to a broadcast type fixed LAN. In both cases, there is a direct connection among all stations, requiring only the physical and data link layers. All the stations in the network receive every packet of data transmitted. However, only the intended recipient is expected to actually process the packet and respond to it. This may be accomplished either by establishing a relation of ‘trust’ among the stations, or by making it physically impossible for an intruder to interject the network traffic. In fixed LANs, it is possible to keep the intruders away rather easily, as the transmission medium can be physically monitored. However, in WLAN such is not the case. The wireless terminal does not have to be used visibly, it could be in a pocket, a bag, or even outside a room. Similarly, power in a fixed LAN is easily provided from the mains. Such is not the case with a wireless terminal. The power source is a part of wireless terminal and, therefore, must be used judiciously. These and many other wireless-specific factors make the wireless MAC quite complex. However, the common functions of the WLAN and fixed LAN MAC are not less complex. For example, in both cases, there is a chance that two or more stations try to broadcast their packets simultaneously, resulting in mixing of packets before arriving anywhere. This mixing due to simultaneous transmission is termed as a packet collision. One simple way to avoid such collision is to regulate transmission of packets.

This has been successfully done using polling in mainframe computing systems. In a typical mainframe computer, a large number of terminals are connected to a single processing system. The computer operating system polls the terminals in some specified order asking if they have any data to send. This avoids the occurrence of any collision. Another mechanism to prevent collisions is to use a special packet, called a token, to be captured before a station can transmit a data packet. In these token passing mechanisms, if a token is unavailable, it is assumed that some station is using it. Once the station with token has finished transmission of its data, it releases the token on the network medium. The token is then free for the next specified station. There are many other mechanisms that might prevent collisions. But, all such mechanisms suffer from two potential problems: number one, they utilize too much network bandwidth in regulating the transmission; number two, they require a controlling station.

In a mainframe computer, the mainframe provided a good point of control. However, in LANs, where all user terminals are same, or similar, implementing control can easily lead into maintenance situations. Due to these reasons, controlled access mechanisms in LANs have not been very popular. Contrary to the controlled access mechanisms, the random access, or contention-based access mechanisms are used more often in wireless and fixed LANs. This necessitates a discussion on such medium access techniques before we describe the salient features of the IEEE WLAN MAC sublayer.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press