What is ATM?
Before the discussion of wireless ATM can begin, the concept of ATM in general needs to be discussed. ATM, or by its more formal name Asynchronous Transfer Mode, is a basic packet-based networking system designed for the simultaneous transmissions of voice, video, and data. In the mid 1980s, the major telecommunication companies decided that they needed a new network to handle the surge of video and data, along with voice, traffic being sent over their existing networks. From this, the concept of ATM was born. From an increasing need to handle data traffic, which is inherently packet-based, as well as voice traffic, ATM was designed to work as a packet-switched network. In a packet-switched network, all traffic is broken into small pieces, which are easier to transmit than one large chunk of data. The problem with using this type of network design for ATM is that the old telephone network is circuit-switched, or in other words creates a physical direct connection between the source and destination during the transmission. ATM, therefore, is designed so that it can handle circuit-switched traffic on its packet-switched backbone. To accomplish this, ATM creates virtual circuit connections over the packet-based network between the source and the destination. These virtual circuit connections provision a set number of network resources dedicated to the connection between a specific source and destination, making it appear to the old telephone network that a circuit connection is established. This allows an ATM network to guarantee the same or greater quality of service for voice traffic as the old telephone network does, while at the same time providing a much greater level of service for data and video traffic than was previously available.2
The idea of breaking voice into packets created quite a problem for the standards process. The US telecom carriers wanted to set the packet size to 64 bytes, which is the size of one voice data packet. The European telecom carriers wanted to set the packet size to 32 bytes so that their transmission lines would not require echo cancellation equipment. Instead of making a technical compromise, a more radical solution was implemented. The two proposals were averaged together to get the packet size. The standard was set to 48 bytes with a 5-byte header, creating a 53-byte ATM cell, or packet. Since 48 is not a power of 2, it is completely out of character for normal data standards. The final data packet size of 53 bytes is technically imperfect, but is still useful for all of the types of traffic that would need to be transmitted over an ATM network. 3
In order for ATM networks to be useful to the telecoms, they needed to be able to transmit large amounts of data. ATM was defined to allow data rates ranging from 25 to 622 Mbps (megabits/sec). To achieve these data rates, ATM is designed to assume that no packets or a very minimal number of packets will be lost during...