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Understanding the 3G-324M Spec: Part

1

Eli Orr, Radvision

1/21/2003 5:15 AM EST

When discussing wireless multimedia services, there is quite a bit of space between

what people talk about and what's reality. For the past few years, many members of

the sector have painted a picture of an all IP wireless network that will seamlessl

Understanding the 3G-324M Spec: Part 1When discussing wireless multimedia services, there is quite a bit of space between

what people talk about and what's reality. For the past few years, many members of

the sector have painted a picture of an all IP wireless network that will seamlesslystream audio and video over IP links. In reality, however, the all IP network is far

from a reality. Today's IPv4 networks are not optimized to handle the delay sensitive

applications required on wireless links and do not provide sufficient address space to

handle tons of IP-enabled mobiles. At the same time, the high bit-error rates

associated with today's wireless links, make the delivery of IP packets difficult, to say

the least.

While the vision of an all IP wireless network has been pushed out, the promise of a

feature-rich, multimedia wireless experience has not. This is due to the emergence the

3G-324M standard, which supports the real-time streaming of wireless multimedia

services over existing circuit-switched wireless networks.

In this two-part series article, we'll provide a tutorial of the 3G-324M specification. In

Part 1, we'll provide an overview of the specification, look at the error resilience and

concealment techniques, discuss H.223 multiplexing/demultiplexing, and describe the

3G-324M adaptation layers. InPart 2, we'll examine H.245 support, voice

coding/decoding, and the video channel. We'll also provide insight into some real-life

implementation issues. Let's kick off our discussion with a look at the problem with

delivering multimedia over 3G links.

3G's Inability to Deliver Multimedia Over IPThe two main 3G standards bodies, 3GPP & 3GPP2, envision 3G as running entirely

over an IP-based communications network (the Internet). However, as stated above,

reality has pushed this vision quite a few years out and, with the current telecom

downturn, the length of time until 3G is entirely IP-based might be further

substantially extended.

Granted, there are many current IP-based applications that run well over mobile

devices today. However these are all non-delay sensitive services such as multimedia

messaging (MMS), MP-3 streaming (with buffering), wireless imaging (JPEG), and

other common Internet services such as e-mail, web surfing, and online chatting.

Unfortunately, today's IP network has severe limitations in its ability to support, inaddition to voice, those delay-sensitive applications, such as PDA-based
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videoconferencing and video on demand, which service providers are today looking

to roll out to customers.

The crux of the problem is that today's IP network (the Internet) is not sufficiently

robust for delay sensitive applications and, in fact, will not be so until service

providers move to IPv6 and SIP-based IP communications. IP, with its variabletransmission delays (many hops of routing processing and congestion delays) and IP

packet overheads to carry the codecs data can't deliver conversational multimedia

session.

Unfortunately IPv6, which will remedy the situation, and is at the crux of a total

migration to IP for 3G networks, will take years to be fully deployed and operate at

99.999% reliability. A network that includes a hybrid of IPv6 and IPv4 is not enough,

and fully IPv6 deployment is required. There are many hurdles in the process

including: IPv6 interoperability issues, protocol maturity, necessary OSS upgrades

throughout the Internet, and the development, acceptance and deployment of an IPv6

addressing scheme worldwide.

Enter 3G-324MTo solve the problems created by an all-IP wireless network, the wireless industry has

adopted the 3G-324M spec (Figure 1). 3G-324M supports the real-time streaming of

multimedia broadband wireless communications by routing traffic over the circuit

switched network, instead of the IP network. Being circuit-switched based, the

standard has all the hallmarks of a protocol ideal for streaming real-time multimedia,

including a fixed delay, low overhead of codecs, and no IP/UDP/RTP header

overheads.


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Figure 1: Diagram illustrating the key elements of the 3G-324M spec.

The 3GPP standards body, which is responsible for developing the UMTS/WCDMAspecifications, defines very specifically the structure and implementation

requirements of the 3G-324M standard in two technical specs (TS): TS 26.112 for CS

call setup and TS 26.111 for 3G-324M initiation and operational procedures. The

3GPP2 standard body, which developed the cdma200 specs, has also approved a

technical spec for 3G-324M operation requirements over CDMA2000 networks called

"3GPP2 C.S0042 for Circuit-Switched Video Conferencing Services."

The 3G-324M standard is a derivative of ITU's H.324, which was developed for the

PSTN and the V.34 modem protocol. H.324 is a tedious protocol for the setup and

tear down of videoconferencing sessions over analog phone lines and has beenmodified for 3G wireless by leveraging the circuit switched network to support the

delivery of delay-sensitive applications (video streaming, videoconferencing) to 3G

end points today. The protocol does not use addressing but operates only after a

mature E.164 addressing method is used by the underlying protocol such as W-

CDMA to locate party and the call is being setup between the two call peers.

The standard uses several sub protocols and technologies to enable call control and

multimedia channels operation over a bit stream channel between two communication

parties:

Error Resilience Services and Concealment H.223 Multiplexing/Demultiplexing


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H.245 Call Control Channel 3G-324M Adaptation Layers H.245 Call Control Channel Voice Channeladaptive multi-rate (AMR) and G.723.1 Codecs Video ChannelH.263 and MPEG-4 Simple Profile Codecs

In this part, we'll examine error resilience and concealment, the adaptation layers, and

H.223 multiplexing/demultiplexing. The remaining topics will be covered inPart 2.

Error Resilience Services and ConcealmentThe 3G-324M operates in a wireless environment where high bit error rates (BER)

occur often during the call session. The base line H.223 defines the multiplexing

between the underlying bit stream, the call control, audio, video and data channels.

The problems with baseline H.223 can be summarized as follows:

1. Bit errors that break high-level data link controller (HDLC) bit stuffing2. Flag emulations in the payload3. Corrupted framing flags4. Errors in mux-packet header5. Errors in payload bits

To solve these problems, the mobile group in ITU-T introduced a hierarchical

structure into H.223 that features multiple levelslevels 0, 1, and 2that deliver

higher levels of error resilience.

In the baseline H.223, the HDLC protocol performs the framing of the multiplexed

packets. HDLC is commonly used in many fixed data networks. However, variable-

length HDLC is not considered robust to transmission errors. The main reason is the

transparency procedure, which is needed to provide uniqueness of the framing flags.

HDLC encoder provides the uniqueness by adding a stuffing "0"-bit after each five

contiguous "1"-bits in the payload. Due to this procedure, HDLC-decoder may lose

synchronization with data if transmission errors corrupt the structure of the

transparency procedure (problem 1).

Another problem with HDLC is that after some bit errors, flag emulations in the

payload are very probable, due to the shortness of the framing flag (problem 2). Flag

emulations destroy the multiplexed-packet structure and may split the multiplexed-

packets in incorrect positions. Bit errors can also corrupt the framing flags hencecausing concatenated or lost multiplexed-packets (problem 3).

The baseline H.223 from the original H.324 is called level 0, actually this level does

not provide any error resilience services. Level 1, defined in H.223 Annex A, replaces

HDLC by a more robust framing. In this level, the stuffing bits are removed and the

length of the flag is increased. The mobile ad-hoc group selected a 16-bit

pseudorandom noise (PN) sequence for the framing flag. As a result, the framing flag

is no longer a unique bit pattern, but the problem of flag emulations in the payload is

negligible if the probability of it is low enough. The drawback of the longer flag is

that it has a higher probability of being corrupted.


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Level 2, which is defined by H.223 Annex B, adds a multiplexed-packet header. The

framing is the same as in level 1. The role of the header is very important, since it

describes the contents of the multiplex packet. Errors in the header may cause mis-

delivery of layer may be unnecessary overhead most of the time in typical channel

conditions.

The 3G-324M specification defines Annex A for low BER handling and B for

moderate BER handling as mandatory error levels resilience to be support. In addition

the mandatory AMR and recommended MPEG-4 codecs provide tools for error

resilience to minimize the quality degradation caused by bit errors. The most

challenging component in mobile videophone is the video codec. It is generally

known that compressed video is very sensitive to transmission errors.

Error resilience is essential for the mobile conversational multimedia communication

for error detection and concealment on the fly. These solutions do not reduce errors

like forward error correction (FEC) and automatic repeat request (ARQ), but can

reduce the quality damage on decoded video quality. (Note: We'll discuss more onAMR and MPEG-4 error resilience inPart 2).

H.223 Multiplexing/De-Multiplexing ProtocolWhen a 3G-324M protocol is initialized, after a circuit-switched channel is opened

between the two communicating parties, the H.223 multiplexing protocol is initiated

between parties in the network. Once the multiplexing protocol is initiated, the 3G-

324M spec calls for the synchronization of the multiplexing process between the

communicating parties in order to establish the call control (H.245) as the first logical

channel to be openedchannel 0.

The basic function of multiplexing protocol is to interleave multiple media streams

such as video, speech, user data, and control signals (H.245) into single stream so that

it can be sent over a transmission channel. 3G-324M uses ITU-T H.223 mobile

extensions of level 2 as its multiplex protocol.

H.223 has a flexible mapping scheme suitable for a variety of media and for a

variable frame length. In its mobile extension, it obtains synchronization and control

stronger against channel errors without losing its flexibility. There are 3 operation

modeslevel 0, level 1, and level 2which are chosen according to the degree of

error resiliency required in a 3G-324M system.

Multiplexing level 0 is identical with H.223 specification, which provides

multiplexing, and quality of service (QoS) function appropriate for each media data.

Level 0 is made up of an adaptation and a mux layer.

The mux layer assembles multiple media packets into single bitstream according to

the selected multiplex pattern out of up to 16 multiplex patterns. The mux pattern can

be defined arbitrarily through the session negotiation procedure. Header Information

is attached to control such a flexible multiplexing mechanism. It consists of 4-bit

multiplex code (MC), 1-bit packet marker (PM), and 3-bit parity header error control

(HEC). The 3-bit HEC field provides error detection capabilities over the MC field

using a 3-bit CRC. Eight-bit high-level data link controller (HDLC) synchronizationflags ('01111110') are inserted as a delimiter of mux-packet data units (PDUs).


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Stuffing '0' bit insertion after every 5 succeeding 1's is defined to prevent the flag

emulation inside the payload.

Multiplexing level 1 employs a 16-bit pseudorandom noise (PN) sequence instead of

8-bit HDLC synchronization flag to improve the mux-PDU synchronization over

error-prone channels. Stuffing is prohibited to enable octet-oriented flag searches.This modification improves the flag detection performance over error-prone channels

remarkably with a slight probability danger of flag emulation conditions in cases of

conflict.

Multiplexing level 2 adds mux-PDU payload length information and FEC in the

header to improve synchronization and error resilience. Furthermore, the multiplexing

level can add an optional header field, which includes MC/PM/HEC for the previous

frame, to improve error resilience against burst errors through time diversity effects.

3G-324M Adaptation Layers

Under the 3G-324M specification, three types of adaptation layers are definedaccording to the media type (video, speech or data): adaptation layers 1 (AL1), 2

(AL2), and 3 (AL3). Let's look at each in detail.

AL1 is designed primarily for the transfer of data or control information. AL1 does

not provide any error detection or correction capability. Therefore, the higher layer

should provide any necessary error control, possibly including a retransmission

procedure. Used for the mandatory H.245 call control that initiated immediately after

the bit stream Multiplexing is synchronized between communicating parties and used

for optional data channels. This AL assumes that the upper layer provides error

control.

AL2 is designed primarily for the transfer of digital audio. AL2 provides an 8-bit

cyclic redundancy check (CRC) for error-detection. AL2 also supports optional

sequence numbering, which may be used to detect missing and misdelivered AL-

PDUs. AL2 transfers variable-length AL SDUs of integral number of octets. Speech

error detection and sequence numbering mechanism are provided. The optional 8-bit

sequence numbering (SN) provides a capability for sequencing AL-PDUs. The

sequence number may be used by the AL2 receiving entity to detect missing and

misdelivered AL-PDUs.

AL3 is designed primarily for the transfer of digital video. AL3 includes a 16-bit CRCfor error-detection. AL3 also supports optional sequence numbering, which may be

used to detect missing and misdelivered AL-PDUs. AL3 transfers variable-length AL

SDUs and provides an optional retransmission procedure, designed primarily for

video. Video error detection, sequence numbering, and ARQ are provided.

On to Part 2That wraps up the first part of our discussion on the 3G-324M protocol. In Part 2,

we'll further the discussion by looking at the audio code, the video codec, and H.245

terminal control. To view Part 2 of the article,click here.

About the AuthorEli Orris a product manager in Radvision's Technology Business Unit. He has more


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than 18 years in computing systems, the last 10 years focused in the development of

IP-based communications systems and technologies. Eli can be reached at

[email protected].

Understanding the 3G-324M Spec: Part2

Eli Orr, Radvision

1/28/2003 3:32 AM EST

The age of an all-IP wireless network could be years away. However, the need for

delivering multimedia services over wireless links is at an all-time high. To

bridge this gap, the 3GPP and 3GPP2 standards bodies are converging on the 3G-

324M specific Understanding the 3G-324M Spec: Part 2

The age of an all-IP wireless network could be years away. However, the need for

delivering multimedia services over wireless links is at an all-time high. To

bridge this gap, the 3GPP and 3GPP2 standards bodies are converging on the 3G-

324M specification for supporting video transmissions over wireless lines.

3G-324M supports the real-time streaming of multimedia broadband wireless

communications by routing traffic over the circuit switched network, instead of the IP

network. Being circuit-switched based, the standard has all the hallmarks of a

protocol ideal for streaming real-time multimedia, including a fixed delay, lowoverhead of codecs, and no IP/UDP/RTP header overheads.

This is the second installment in our tutorial on the 3G-324M. InPart 1, we explored

error resilience and concealment techniques, H.223 multiplexing/demultiplexing, and

the 3G-324M adaptation layers. In Part 2, we'll examine H.245 support, voice

coding/decoding, and the video channel. We'll also provide insight into some real-life

implementation issues. Let's kick off our discussion with a look at the problem with

delivering multimedia over 3G links.

H.245 Terminal Control Protocol

3G-324M uses H.245 as a terminal control protocol. H.245 is used by H.323 as wellas by H.324 for PSTN and by H.310 for ATM. Currently (Q4/2002) ITU H.245

version 9 is ratified by the SG16 of ITU.

The oldest version of H.245 that can be supported in a 3G-324M implementation is

version 3. However it is highly recommended to support higher version such as 6 or 7

which support by most implantations today or even upper for richer set of call control

services of commands and indications. H.245 is fully backward compatible so that

higher version can operates with lower version of H.245 and vise versa.

Since 3G-324M rides on a channel opened between two communicating parties it

does not need any addressing such as in H.323. With this fact, it is expected that the
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gateway (e.g. between 3G-324M, H.320, H.323 and SIP) will provide the

interoperability between different networks can be realized rather easily.

Since H.323 is not needed, H.245 requires the numbered simple retransmission

protocol (NSRP) and control channel segmentation and reassembly layer (CCSRL)

sublayer support to ensure reliable operation. H.245 requires mobile terminals tosupport NSRP and SRP modes. If both terminals start the session in level 0, H/245-

enabled systems must operate in the SRP. If the terminals start a session at level 2,

NSRP mode is employed. CCSRL, on the other hand, is used for carrying H.245 large

packets required for operation.

In addition to providing NSRP and CCSRL support, the H.245 control protocol

provides following functionalities and services:

Master-slave determination is provided to determine which terminal is themaster at the beginning of the session. Due to the fact that H.245 is

symmetric control protocol, it is necessary to decide the master terminal,

which has the right to decide the conditions in case of the conflict.

Capability exchange is provided to exchange the capabilities both terminalsupports, such as optional modes of multiplexing, type of audio/video

codecs, data sharing mode and its related parameters, and/or other

additional optional features.

Logical channel signaling is provided to open/close the logical channelsfor media transmission. This procedure also includes parameter exchange

for the use of this logical channel.

Multiplex table initialization/modification is provided to add/delete themultiplex table entries.

Mode requestis provided to request the mode of operation from thereceiver side to the transmitter side. In H.245, the choice of codecs and its

parameters are decided at the transmitter side considering decoder's

capability, so if the receiver side has a preference within its capability,

this procedure is used.

Round-trip delay measurementis provided to enable accurate qualitycharacteristic measurement.

Loopback testing is provided for use during development or in the field toassure proper operation.

Miscellaneous call control commands and indications are provided torequest the modes of communication, flow control such as conferencecommands, jitter indication and skew, or to indicate the conditions of the

terminal, to the other side.

H.245 uses the abstract syntax notation 1 (ASN.1) to define each message parameters

that provides readability and extensibility effectively. To encode these ASN.1

messages into binary, the packed encoding rule (PER) is used to realize the very

bandwidth effective message transmission. As mentioned before after the

multiplexing level synchronization between communicating parties is completed the

first logical channel opened (channel 0) is H.245 call control with the CCRL and

NSRP to assure that the H.245 channel will be highly reliable and can use large

packets during operation.


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Voice ChannelThe AMR CodecThe 3G-324M specifications define the AMR codec as mandatory. 3G-324M also

recommends the use of G.723.1, which is used by many H.323 terminals today.

The AMR codec was originally developed and standardized by the ETSI for GSM

cellular systems. The AMR codec, rolling out in networks and terminals, dynamicallyadjusts the amount of bits allocated to voice coding and error control, providing the

best possible voice quality at each instance based on radio conditions. AMR

significantly enhances the effectiveness of frequency hopping and tighter reuse

patterns by allowing a greater percentage of radio channels to be in use

simultaneously, resulting in an additional capacity gain of about 150%.

AMR was chosen by 3GPP as the mandatory codec for 3G cellular systems. The

AMR codec includes eight narrowband codec modes: 12.2, 10.2, 7.95, 7.4, 6.7, 5.9,

5.5 and 4.75 kbit/s. It also supports comfort noise (CN) for a discontinuous

transmission (DTX) operational mode.

Besides the adaptation of rate, the AMR codec also supports unequal bit-error

detection and protection (UED/UEP). The UEP/UED mechanisms allow more speech

over a lossy network by sorting the bits into perceptually more and less sensitive

classes. A frame is only declared damaged and not delivered if there are bit errors

found in the most sensitive bits. On the other hand, acceptable speech quality results

if the speech frame is delivered with bit errors in the less sensitive bits, based on

human aural perception. An important characteristic for high BER environment such

as wireless network is AMR's robustness for packet loss, through redundancy and bit

errors, sensitivity sorting. Another benefit of AMR is the adaptive rate adaptation for

switching smoothly between codec modes on the fly.

The Video ChannelThe 3G-324M standard calls out the H.263 codec as mandatory and MPEG-4 as

recommended codec for video processing. However, MPEG-4 is the 3G-324M

standard de-facto used by all major supporting vendors. Resiliency and high

efficiency make MPEG-4 codec particularly well suited for 3G-324M.

H.263 is a legacy codec that is used by many existing H.323 wire lined devices.

MPEG-4 is much more flexible and offers advanced error detection and correction

services, which are a big value add when delivering video over a wireless network.

Let's look at the error detection and correction services in more detail.

When supported, 3G-324M says that MPEG-4 visual codecs shall support simple

profile 1 level 0. MPEG-4 visual (ISO/IEC 14496-2) is a generic video codec. One of

its target areas is mobile communications.

Error resiliency and high efficiency make the MPEG-4 visual codec particularly well

suited for 3G-324M. MPEG-4 visual is organized into profiles. Within a profile,

various levels are defined. Profiles define subsets of tool sets. Levels are related to

computational complexity. Among these profiles, the simple visual profile provides

error resilience (through data partitioning, RVLC, resynchronisation marker, and

header extension code) and low complexity. MPEG-4 allows various input formats,


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including general formats such as QCIF and CIF. It is also baseline compatible with

H.263.

As stated above, error resilience is achieved through resynchronization, byte

alignment, data partitioning the reversible variable length code (RVLC), adaptive

intra refresh (AIR), and error detection and concealment. Let's look at each of these inmore detail.

1. Resyncrhonization: Under the MPEG-4 spec, a resynchronization marker can

reduce the error propagation caused by the nature of variable length code (VLC) into

single frame. In MPEG-4, the resynchronization marker is inserted at the top of a new

group of blocks GOB with the header information (multiplexed block number

[MBN], quantization parameters) and optional HEC, so that decoding can be done

independently. It is a good idea to place the resynchronization marker prior to

important objects like people to improve error resilience with minimum increase of

overhead.

2. Byte alignment: Bit-stuffing for the byte alignment gives additional error detection

capability through its violation check.

3. Data partitioning: A new synchronization code named motion marker separates the

motion vector (MV) and discrete cosine transform (DCT) field to prevent from inter-

field error propagation, thus allowing effective error concealment to be performed.

When errors are detected solely in the DCT field, that multiplexed block (MB) will be

reconstructed using correct MV. This results in natural motion better than simple MB

replacement of the previous frame.

4. RVLC: The RVLC enables forward and backward decoding without significant

impact on coding efficiency. This feature localizes error propagation ideally into

single MB.

5. AIR: Different from the conventional cyclic intra refresh, AIR employs motion-

weighted intra refresh, which results in better perceptual quality with quick recovery

in corrupted objects.

6. Error detection and concealment: Errors can be detected through exception or

violation in the decoding process, and then concealment will be applied. The

functionality is included for mobile application. The endpoint of H.324 can supportfor MPEG-4 audio, so that MPEG-4 audio could be used for H.324 mobile phone

terminal.

Integrating 3G-324M With Other Multimedia While 3G-324M is a straightforward

protocol to implement in end devices and media servers, designers will face

challenges making 3G-324M with other protocols such as H.323 and SIP. Let's look

at this issue in more detail.

H.323 is based on Q.931 for call setup and H.245 for call control. 3GPP defines

TS.26.112 for call setup procedure in UMTS. The interworking device shall map

theTS-26.112 call setup into Q.931 H.323 calls and vise versa. For call controlmapping, since both protocols uses H.245 the mapping is trivial, however the H.245


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in 3G-324M is addressless. Thus a transcoding function may also be required to

ensure that 3G-324M works with various H.323 devices supporting codecs such as

H.261 and H.263.

SIP is based of session description protocol (SDP) for both call setup and call control.

Hence both TS 26.112 and 3G-324M H.245 call control should be mapped into SDPmessages and vise versa. Again a transcoding functions may be required to ensure

that 3G-324M systems work with SIP-based systems.

Editor's Note: To view Part 1 of this article,click here.

About the Author

Eli Orris a product manager in Radvision's Technology Business Unit. He has more

than 18 years in computing systems, the last 10 years focused in the development of

IP-based communications systems and technologies. Eli can be reached at

[email protected].
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