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Out-of-Band Signaling
The traditional PSTN uses in-band signaling for call control, meaning that some part of the bandwidth used to carry the voice samples is used for control information. Out-of-band signaling enables ATM to establish call paths for the voice samples using special signaling-only circuits that handle signaling for a large number of calls (see Figure 2).
The actual path for each set of voice samples (i.e., telephone call) is not set up until it is needed, which is generally after the called party answers the phone. The path is, in fact, a virtual one consisting of the stream of packets that is built when there is voice-sample data to transmit. This reduces the amount of bandwidth required to service a call, whereas TDM's full bandwidth is always present in its physical path during a call. The ATM packets do not need to go through the same physical path to reach their destination, so the switching equipment can also handle calls more reliably.
Figure 2. Out-of-Band Signaling: ATM's 53-Byte Payload and Signaling Packets Take Different Paths
Packetized Transmission
ATM was designed with the capabilities of modern digital hardware in mind. It uses the same size packet (53 bytes: 5 header, 48 payload) to transport all types of command and payload information, simplifying the low-level hardware design and allowing it to operate at very high speeds.
The operating part of ATM is the routing and management of the packets that provide transport services. Using a packet-based approach allows networks to implement high-reliability redundant paths for payloads, providing customers with the data integrity they need to run their businesses competitively. In the event of a path failure, the packets can simply be rerouted to their destinations using a different path without dropping any calls in-progress.
What Is Asynchronous about It?
ATM presents the packets that make up control information or voice samples to the network whenever there is enough bandwidth available to handle them. Thus, the transfer is asynchronously relative to the original source's viewpoint. The ordering of packets is guaranteed by ATM, otherwise the voice samples would produce some rather interesting sounds at the end points of the call. Figure 3 illustrates that the packets are asynchronous but ordered. There is a secured virtual path from source to destination before data flows. This contrasts a packet network, which has no secured path from source to destination.
Figure 3. VToA's Flexible Packet Routing and Signal Regeneration
All-Digital Transmission
Another advantage of ATM is its inherent all-digital transmission format. The fixed-length packets can be processed extremely fast and be sent using any available path to their destination. ATM is typically deployed using fiber-optic cable as the physical medium and can currently achieve speeds of up to 622 Mbps, over 400 times faster than TDM technology.
