ATM Option System Reference - Comtech EF Data

ATM Option
1.2 Principles of Operation
1.2.1 ATM Protocols and AAL Service Classes
All ATM cells look alike, no matter which AAL service class is used to generate their
payload. There are four ATM protocols that handle these service classes: AAL1, AAL2,
AAL3/4 and AAL5. The SDM-9360, SDM-9380 and SDM-9585 support AAL1 and
AAL5 only. The SDM-9230 supports AAL5 only. Refer to the section “ATM Support on
the NetPerformer” on page 1-13 for details on the current product offering.
• AAL1: for use with constant bit-rate traffic that requires constant timing between
source and destination in a connection-oriented mode (a predefined path based on
virtual circuits).
- This protocol is used for AAL Service Class A
- One octet is reserved for timing information and detection of lost or out-ofsequence
cells, leaving an actual payload size of 45 to 47 octets
- Example applications are uncompressed voice, DS1 circuits and constant bitrate
video
- Not available on the SDM-9230.
• AAL2: for use with variable rate traffic that requires constant timing between
source and destination in a connection-oriented mode.
- This protocol is used for AAL Service Class B
- It includes a payload header with the sequence number and information type,
a trailer indicating the number of data octets in the cell, and a CRC to check
the payload contents
- Example applications are packetized voice or video.
• AAL3/4: for use with variable rate traffic that does not require timing between
source and destination, that is, traffic that is sensitive to loss but not to delay.
- This protocol is used for AAL Service Class C (connection-oriented) or Class
D (connectionless)
- The Convergence Sublayer takes user data up to 65,535 octets, adds a header
with a begin tag and buffer allocation information, and a trailer with an end of
data tag and data length indicator. It may also add a pad to ensure that the data
can be cut into 44-octet payloads
- The SAR Sublayer takes this large packet and breaks it up into 44-octet payloads,
adds payload headers and trailers that include sequence numbering,
segment type, multiplexing and length information as well as a payload CRC
- Example applications are Frame Relay and X.25 traffic (Class C), LAN and
Switched Multi-megabit Data Service (SMDS) traffic (Class D)
• AAL5: for use with variable bit-rate traffic in a connection-oriented mode. Also
known as Simple and Efficient Adaptation Layer (SEAL).
1-4 Memotec Inc.