Information Technology Reference
In-Depth Information
Level 3, defined by H. Annex C and D, is a further enhancement of Level 2,
providing the most robust set of mobile extensions to the basic specification.
This annex specifies both a new adaptation layer and a multiplexing layer
and provides several enhancements including error detection and correc-
tion, sequence numbering, automatic repeat request, and several retransmis-
sion schemes.
In H.223 every level comprises an adaptation, a multiplexing, and a demul-
tiplexing layer. AL1, AL2, and AL3 are the three defined adaptation layers.
AL1 is designed for data transfer and is typically used to transport user data
and H.245 control messages. It relies on upper layers for error control and
handling. This is shown in Figure 3.2.
AL2 provides an 8-bit cyclic redundancy check (CRC) and optional
sequence numbering to allow loss detection. AL2 has the capability to handle
AL service data units (SDUs) of variable length and is the preferred adapta-
tion layer for the transport of audio data. AL3 is designed primarily for video
and includes a 16-bit CRC and optional sequence numbering.
3.2.4 Summary
3GPP adopted H.324M with some mandatory requirements to form 3G-324M
protocol stack for low bit rate multimedia communication in 3G circuit
switched network. In this protocol stack, the H.245 recommendation is used
for call control and signaling and the H.223 recommendation is used for
multiplexing. These protocols work together closely to provide reliable ser-
vice over an error prone wireless network.
Application layer
Audio I/O
Video I/O
H.245
control
Data
protocol
LAPM
Audio codec
Video codec
AL-SDU
AL-SDU
AL-SDU
AL-SDU
Adaptation layer
MUX-SDU
MUX-SDU
MUX-SDU
MUX-SDU
Multiplex layer
Figure 3.2
General structure of the H.223 module.
