share a physical link; the link bandwidth is
share a physical link; the link bandwidth is shared dynamically, giving much more efficient use of the link. This is called opportunistic bandwidth. Because there is now a queue of data awaiting available bandwidth, quality of service becomes an issue. The advantage is that the toll rates can be lower than the reserved bandwidth of a voice circuit. ATM Asynchronous Transfer Mode (ATM) was developed for high-speed packet transport over optical networks like SONET. ATM uses small fixed-size data cells rather than the large packets of frame relay. Each cell is 53 bytes long, with a payload of 48 bytes. These small cells are more suited to voice and multimedia traffic, where low latencies are demanded. Both permanent and switched (PVC and SVC) virtual circuits can be set up. The cell header carries the virtual channel and virtual path identifier that are used to identify connections within the network. ATM is linked to other network layers by the ATM Adaptation Layer (AAL). The packets are given one of five categories of priority by traffic class: 1. Constant bit rate 2. Real-time variable bit rate 3. Non-real-time bit rate 4. Available bit rate 5. Unspecified bandwidth or best effort 28 The Technology of Video and Audio Streaming Table 2.6 Synchronous Digital Hierarchies Data rate SDH SONET Signal Capacity Signal Capacity 51.84 Mbit/s STM-0 21 E1 STS-1, OC-1 28 DS1 or 1 DS3 155 Mbit/s STM-1 63 E1 STS-3, OC-3 84 DS1 or 1 E4 or 3 DS3 622 Mbit/s STM-4 252 E1 STS-12, OC-12 336 DS1 or 4 E4 or 12 DS3 2.48 Gbit/s STM-16 1008 E1 STS-48, OC-48 1344 DS1 or 16 E4 or 48 DS3
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