ATM Overview
Table of Contents
Introduction
Asynchronous Transfer Mode (ATM) is a protocol used with SONET/SDH backbones.
ATM Overview
- ATM uses asynchronous time-division multiplexing.
- Data is encoded into fixed-size cells 53 bytes. Fixed size was utilized to reduce jitter or variance in delay processing different size packets.
- Connection oriented model using virtual circuit between two nodes vs IP which is connectionless.
- Virtual circuits can be permanent or switched.
- Permanent - configured manually
- Switched - per call basis established using signaling and end after they are not needed any more.
- ATM AAL are Adaptation Layers. These connect the cell based services to digital bit-stream services. There are 5 layers: AAL1, AAL2, AAL3, AAL4, AAL5.
- Most implementations relay on AAL1, AAL2 and AAL5.
- For more info on AAL see ATM-AAL
ATM Cell Format
- Each cell is 53 bytes. Out of the 53 bytes 5 bytes are for the header and 48 bytes for the payload.
- There are two ATM cell formats UNI and NNI:
- User-Network Interface (UNI) -
- Network-Network Interface (NNI) -
- Fields:
- Generic Flow Control (GFC) - Used to identify multiple stations that share a single ATM interface. Field is typically not used and has the default value of 0.
- Virtual Path Identifier (VPI) - Along with VCI, identifies the destination of a cell to be switched over ATM switches towards its destination. Virtual Path is a bundle of virtual channels. VPI has a local significance on each link, just like in Frame Relay with DLCIs.
- Virtual Channel Identifier (VCI) - Along with VPI, identifies the destination of a cell to be switched over ATM switches towards its destination. Virtual Path is a bundle of virtual channels. VCI has a local significance on each link, just like in Frame Relay with DLCIs.
- Payload Type (PT) -
- Cell Loss Priority (CLP) - Used for congestion control. If there is extreme congestion CLP field set to 1 should be discarted, while CLP field set to 0 preferred and not discarded.
- Header Error Control (HEC) - Calculated checksum, but only for the header (first 4 bytes). HEC can correct a single bit error.
ATM Interfaces
- ATM Switches have two types of interfaces: UNI and NNI.
- UNI connects to end points such as hosts or routers.
- NNI connects to other ATM switches.
