ATM Characteristics

When ATM was developed in the early 1990s, its speed provided a key advantage over T-1 and T-3 services, which are based on time division multiplexing. It was also faster than routers available at that time. ATM's speed is due to its fixed-size cells, switching in hardware and asynchronous technology, which does not depend on timing. Rather, cells are forwarded based on priority and arrival time.

Fixed-Sized Cells - Less Processing: ATM packages data into discrete groups called cells. These cells are of a fixed size. Handling fixed-sized cells requires less processing than older routers with variable-sized packets. The ATM switch does not have to look for bits telling it when the cell is over. Each cell is 53 bytes long. Five of the 53 bytes contain header information. This includes bits that identify the type of information contained in the cell (for example, voice, data , or video) so that the cell can be prioritized. The remaining 48 bytes are the "payload" - user data such as voice, video, or sales proposals.

Switching in Hardware - Less Address Lookup: ATM cells are switched in hardware. This means that an ATM switch does not have to look up each cell's address in software. Rather, an ATM switch sets up a route through the network when it sees the first cell of a transmission. It puts this information into its hardware and sends each cell with the same header routing information down the virtual path previously established. For example, all cells with XXX in the header use route 234. Using the same path for each cell makes ATM a connection-oriented service.

Asynchronous Switching - Improving Network Utilization: With asynchronous switching, every bit of network capacity is available for every cell. This is different than synchronous multiplexing technology such as T-1/E-1 and T-3/E-3. With T-3 multiplexing, every one of the 672 input transmissions is assigned a time slot. If device A has nothing to send, its slot is sent through the network empty. ATM has no synchronous requirements. It statistically multiplexes cells onto the network path based on quality-of-service information in the header. With ATM, network capacity is not wasted forwarding empty cells.


Source: The Essential Guide to Telecommunications, 4th Edition by Annabel Z. Dodd