Information Technology Reference
In-Depth Information
not exclusively, applicable to application layer protocols. The following is an
example of a message definition by ASN.1 and the example message is the
OpenLogicalChannel message.
ASN.1 deals only with the syntax and semantics of message specifications.
The binary encoding of data structures is covered in other recommendations,
notably X.690 (basic encoding rules or BER) and X.691 [18] (packed encoding
rules or PER). BER allows data to be deciphered by systems that have general
knowledge of ASN.1 but do not know the details of the specification used
to form the data. In other words, the data types are encoded along with the
data values. PER is much more efficient since only data values are encoded
and the coding is designed with very little redundancy. This method can be
used when both the transmitter and the receiver expect data to adhere to a
known structure.
H.245 is implemented using the packed encoding rules. Since both sides
of a call know that messages will conform to the H.245 specification it is not
necessary to encode that specification into the messages. For decoding sim-
plicity, the aligned variant of PER is used. This forces fields that require eight
or more bits to be aligned on octet boundaries and to consume an integral
number of octets. Alignment is done by padding the data with zeros before
large fields.
H.223 is the ITU-T recommendation for data multiplexing protocol. As
shown in the framework figure of 3G-324M, it is the real sending and receiv-
ing interface for all the video/audio/data applications during the call session.
The recommendation defines the structures and routines for data transmis-
sion multiplexing. Therefore, it is the major focus of our investigation about
how to improve the multimedia data transmission efficiency of 3G mobile
networks.
3.2.3 H.223 Multiplexing and Demultiplexing
3G-324M defines three levels of H.223 transport in order to provide different
degrees of error resilience.
Level 0, or baseline H.223, provides support for synchronization and bit
stuffing. Level 0 allows 16 different multiplexing patterns to assemble media,
control, and data packets. Multiplexing patterns are negotiated between the
endpoints. The error resilience capabilities of Level 0 are limited. Bit errors
can break the HDLC protocol, can interfere with bit stuffing, and can be the
cause of flag emulations in the payload.
Level 1, defined in H.223 Annex A, provides a synchronization mecha-
nism that considerably improves performance over error-prone channels.
HDLC is replaced by a more robust framing and framing flag of larger
length.
Level 2, defined by H.223 Annex B, is a further enhancement of Level 1,
providing support for forward error correction (FEC) and including a larger
header, which describes the contents of the multiplexed PDU.
