Asynchronous Transfer Mode (ATM)

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Agenda Definition Basic characteristics Why asynchronous How to access the network Service Categories Connections Protocol Architecture pros & cons Troubleshooting

ATM – definition "A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".

ATM Characteristics ATM uses packet switching with fixed length packet of 53 bytes – CELLs. Each cell has a 5-byte header and 48-byte payload ATM uses Virtual Circuits ATM can run over any physical layer. Often runs over fiber optics using the SONET standard at speeds of 155.52 Mbps, 622 Mbps and higher

ATM Characteristics A Cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-sized block of information ATM uses asynchronous TDM – to multiplex cells coming from different channels Uses fixed slots (size of cells) Fills a slot with a cell from any input channel that has a cell – slot is empty if none of the channels has a cell to send

Asynchronous Transfer Mode (ATM) MUX Wasted bandwidth 4 3 2 1 4 3 2 1 4 3 2 1

ATM Access ATM is a cell-switched network. The user access devices, called the endpoints, are connected through a user-to-network interface (UNI) to the switches inside the network. The switches are connected through network-to-network interfaces (NNIs).

STDUCTURE Public UNI X X X X X X X X X Private ATM network Private UNI Private NNI Public UNI Public ATM network A X X X NNI Public UNI X B-ICI Public ATM network B X Public UNI X X

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

Virtual Paths Physical Link Virtual Channels

ATM Service Categories Real-time services Constant bit rate – uncompressed audio/video info Videoconferencing, TV, pay-per-view, VOD, etc. rt-Variable bit rate Non-real-time services nrt-VBR – high end system QoS, critical response time Bank transactions, airline reservations, etc. Unspecified bit rate – best-effort service e.g. text/image messaging, telecommuting Available bit rate – bursty apps requiring reliable end-to-end connection e.g. LAN, router-to-router reliability

ATM Protocol Architecture Management plane Plane management Control plane User plane Layer management Higher layers Higher layers ATM adaptation layer ATM layer Physical layer

adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analogous to Internet transport layer ATM layer: “network” layer cell switching, routing physical layer AAL user ATM layer Physical layer

Pros & cons higher transmission speeds quality of service bandwidth-on-demand application-independent Cost Complexity Availability

Troubleshooting Performing Basic Interface Checks Determining Network Connectivity Performing Loopback Tests Troubleshooting 155-Mbps and 622-Mbps Interfaces Troubleshooting T1 and E1 Interfaces Troubleshooting DS3 and E3 Interfaces Troubleshooting CBR T1 and CBR E1 Interfaces Troubleshooting 25-Mbps Interfaces

References: Asynchronous Transfer Mode (ATM) fundamentals, white paper from Internation Engineering Consortium ATM tutorial-fnc educational services Higher Layer Protocols:TCP/IP and ATM- Eytan Modiano,Massachusetts Institute of Technology ATM Switch Troubleshooting Guide-Cisco