Calculating Atm Cell Rates - Ericsson HL950 Administrator's Manual

4.2.4

Calculating ATM Cell Rates

In the following subsections you can learn more about how to calculate ATM cell rates.
4.2.4.1 Understanding ATM Cell Format with AAL1
CES uses the CBR service class and AAL1 to emulate a constant bit rate connection, such as E1, T1 or
Nx64 V.35/V36/X.21. ITU-T Recommendation I.363.1 defines AAL1.
An ATM cell using AAL1 at the AAL sublayer "robs" a byte from the 48-byte payload field of the cell
for an AAL1 header. This robbed byte consists of two subfields — the sequence number (SN) field
and the sequence number protection (SNP) field. In turn, each subfield consists of its own subfields
that provide time stamps, sequence numbers and other bits to adapt ATM's asynchronous nature to the
synchronous layer-1. The ATM network uses these bits to help resolve problems with cell delay
variation, cell misinsertion, and cell loss.
AAL1 transfers data in two modes:
Structured - Maps one or more E1 or T1 digital signal level 0 (DS-0) time slots to an ATM
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permanent virtual circuit (PVC). Each DS-0 time slot or channel represents a single Nx64
circuit that can transmit CBR data at a rate of 64 kbps. For example, many video codecs
operate at Nx64 kbps rates. Structured mode allows you to configure each video codec to have
a subset of the E1 bandwidth.
Unstructured - Maps the entire E1 or T1 bandwidth or all time slots or Nx64 kbps
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V.35/V36/X.21 to an ATM PVC.
Both modes use the AAL1 header byte. In addition, depending on the block size, structured mode also
robs another byte for use as a pointer byte, which is discussed in the next section.
The following diagrams illustrate the difference between unstructured and structured AAL1 cells.
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Figure 14
Unstructured Protocol Data Unit (PDU) Format
Figure 15
Structured Protocol Data Unit (PDU) Format
Multi Service Edge Device HL950
Administrator's Guide
EN/LZT 108 5995 R3
June 2003