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Asynchronous Transfer Mode (ATM)
Issues Driving LAN Changes
• Traffic Integration
– Voice, video and data traffic
– Multimedia became the ‘buzz word’
• One-way batch Web traffic
• Two-way batch voice messages
• One-way interactive Mbone broadcasts
• Two-way interactive video conferencing
• Quality of Service guarantees (e.g. limited jitter, non-blocking streams)
• LAN Interoperability
• Mobile and Wireless nodes
ATM
• ATM standard (defined by CCITT) is widely accepted by common carriers as mode of operation for communication – particularly BISDN.
 ATM is a form of cell switching using small fixed-sized packets
ATM Conceptual Model Four Assumptions
1. ATM network will be organized as a hierarchy.
User’s equipment connects to networks via a UNI (User-Network Interface).
Connections between provided networks are made through NNI (Network-Network Interface).
2. ATM will be connection-oriented.
A connection (an ATM channel) must be established before any cells are sent.
ATM Connections
• two levels of ATM connections:
virtual path connections
virtual channel connections
• indicated by two fields in the cell header:
virtual path identifier VPI
virtual channel identifier VCI
3. Vast majority of ATM networks will run on optical fiber networks with extremely low error rates.
4. ATM must support low cost attachments.
• This decision lead to a significant decision – to prohibit cell reordering in ATM networks.
è ATM switch design is more difficult.