http://www.eetimes.com/electronics-news/4137042/Understanding-the-3G-324M-Spec-Part-1

http://www.eetimes.com/design/communications-design/4136986/ContentItem_DBDBA109_FBEE_488C_BB53_2EC8CEAF4D2FUnderstanding the 3G-324M Spec: Part 1Eli Orr, Radvision

1/21/2003 5:15 AM EST

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 seamlessly stream 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 IP

The 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, in addition to voice, those delay-sensitive applications, such as PDA-based

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 variable transmission 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-324M

To 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.

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

The 3GPP standards body, which is responsible for developing the UMTS/WCDMA specifications,

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 been modified 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

H.245 Call Control Channel

3G-324M Adaptation Layers

H.245 Call Control Channel

Voice Channel—adaptive multi-rate (AMR) and G.723.1 Codecs

Video Channel—H.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 in Part 2.

Error Resilience Services and Concealment

The 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 levels—levels 0, 1, and 2—that 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 hence causing 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.

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 on AMR and MPEG-4 error resilience in Part 2).

H.223 Multiplexing/De-Multiplexing Protocol

When 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 opened—channel 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 modes—level 0, level 1, and level 2—which 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) synchronization flags ('01111110') are inserted

as a delimiter of mux-packet data units (PDUs). 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 defined according 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 CRC for 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 2

That 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 Author

Eli Orr is 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 eliorr@radvision.com.

Understanding the 3G-324M Spec: Part 2Eli Orr, Radvision

1/28/2003 3:32 AM EST

Understanding the 3G-324M Spec: Part 2The 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, low overhead of codecs, and no IP/UDP/RTP header overheads.

This is the second installment in our tutorial on the 3G-324M. In Part 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 well as 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 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 to support 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 the master 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 terminal supports, 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 channels for media

transmission. This procedure also includes parameter exchange for the use of this logical channel.

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

Mode request is provided to request the mode of operation from the receiver 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 measurement is provided to enable accurate quality characteristic

measurement.

Loopback testing is provided for use during development or in the field to assure proper

operation.

Miscellaneous call control commands and indications are provided to request the modes of

communication, flow control such as conference commands, 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.

Voice Channel—The AMR Codec

The 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, dynamically adjusts 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 Channel

The 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, 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 in more 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 support for 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 control mapping, since both protocols uses H.245 the mapping is trivial,

however the H.245 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 SDP messages 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 Orr is 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 eliorr@radvision.com.

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3G-324M Helps 3G Live Up To Its Potential As long as developers and service providers understand this protocol, 3G-324M could enable the differentiation of 3G services.

Broadband wireless is now a reality. Towers are converting to 3G throughout Europe and Asia. 3G-enabled phones are flying off the shelf and service providers are making a

substantial commitment to the new format. At the same time, consumers are happily embracing all of the new features and functionality that broadband wireless delivers.

Everyone wants to know what will drive adoption; give service providers and equipment vendors new revenue streams; and increase usage in both subscriptions and minutes used. Obviously, it will be something that leverages the broadband capabilities of the 3G network architecture—namely, mobile real-time multimedia communication.

At lower speeds, SMS and MMS work fine. Imagine if users were comfortable with non-real-time multimedia. 3G would be unnecessary, as 2G and 2.5G provide sufficient bandwidth—as long as the streams can be cached and reconfigured at the end point.

Obviously, the real applications for 3G are the real-time ones. They include video telephony (videoconferencing), video streaming, remote wireless surveillance, multimedia real-time gaming, video on demand, and more (FIG. 1). These "killer apps" will drive usage and increase service-provider revenue. Correspondingly, they also will raise equipment sales.

Originally, 3G was conceived as an "all-IP" solution by both the Third Generation Partnership Project (3GPP) and Third Generation Partnership Project 2 (3GPP2). In reality, however, 3G multimedia is not enabled by the IP protocol (SIP). The problem is that IP communications are sensitive to high bit error rates (BERs). Such high rates are found throughout the public cellular network.

Using IP as the underlying transport results in poor quality for conversational real-time communication. As an alternative, 3G real-time multimedia is now delivered over a circuit-switched protocol. Called 3G-324M, this protocol provides adequate quality for any sort of latency-sensitive applications.

3G-324M enables conversational real-time multimedia over third-generation (3G) technology. The 3G real-time multimedia services based on 3G-324M started in Japan. They have now expanded to the U.K., Italy, Australia, and Spain. They're currently finding footholds in more and more countries. Correspondingly, the subscriber growth rate continues to grow per month in each of these markets. In addition, more and more 3G-324M-enabled mobile terminals (3G PDAs, smart phones, and feature phones) are becoming available.

Instead of operating over IP protocol, as most communication protocols do today, 3G-324M operates over a Time Division Multiplexing (TDM) circuit-switched (CS) channel (FIG. 2). That channel is opened by the baseband protocol between communicating peers. TDM has the benefit of a fixed-low-delay service, which saves the need for routing on every hop of the IP's communication path. For low-fixed-delay services in a high-bit-error-rate environment, such as conversational voice and video, it has been found to operate well in public cellular networks.

3G-324M may be a throwback to the circuit-switched world and not "next-generation IP." Unlike IP, however, it does work for conversation video calls in a cellular network. It also allows service providers and equipment developers to enable the broadband killer applications that were previously mentioned. Those applications are delivered as a hybrid of communication technologies based on IP and 3G-324M CS.

To put it simply: IP isn't ready to support real-time multimedia over wireless. Take video telephony, for example. It requires medium to high bandwidth, low delay (two-way), medium to high quality, and a continuous connection. To provide video that's acceptable to the mobile user, the wireless network must provide a certain quality of service (QoS).

Frame-delay variation, bit errors, and frame loss can have severe effects on the video quality.

Even in the fastest CDMA2000 EV-DO network, 3G video telephony for conversational multimedia communications over IP was found to be unsuitable. By "packaging" many bits into an IP packet that is carried over a public mobile network, this approach incurred a large rate of packet loss. Because IP needs to be processed for addressing in each hop on the network, each packet loss and the request to retransmit caused additional delays. In a real operational scenario of public cellular networks, the overall delay that's caused is unacceptable for conversational multimedia services.

Often, the IP packet has too many bits that cannot be recovered. The entire packet then needs to be retransmitted. As a result, the multimedia experience becomes unacceptable. Yet when a circuit-switch-based time-division-multiplexing session was opened between two communicating peers, it was found to operate well in the same bit-error-rate conditions. Of course, this session boasted suitable error detection and correction for the bits (H.223 Annexes A and B) and concealment for the codecs (MPEG-4, SP-L0, and GSM-AMR).

3G-324M's role as the video-telephony enabler to all 3G technologies is now becoming clear. Consider a transmission link with a BER of 10−5. It might be acceptable for non-real-time data transmission with some form of error correction. In a video stream, however, this error rate would cause a serious degradation in the quality of the received video. Frame-delay, frame-loss, and rate-control issues also have a significant impact on the quality of the video that's received. Simulation is needed to assess the picture quality under different propagation channels along with error-correction and/or concealment schemes.

To understand the role of 3G-324M today, it's important to know the technology's background. For conversational multimedia services in a 3G network, 3G-324M operates on an established circuit-switched channel between source and destination parties. Multipoint communication between more than two 3G-324M terminals is possible. It requires both a gateway-to-IP network and an H.323 multipoint control unit (MCU).

3GPP, which supports UMTS technology, originally defined 3G-324M as part of Release '99 in December 1999. In August of 2002, it approved 3G-324M usage for CDMA2000. In 2003, TD-SCDMA become a 3GPP standard. It adopted 3G-324M and began to operate in China as the formal 3G standard.

3G-324M is an addressless protocol. It doesn't include the call setup with the baseband. The call setup for the protocol is defined in the following specifications:

3GPP TS 24.008: Mobile radio interface Layer 3 specification 3GPP TS 27.001: General on Terminal Adaptation Functions (TAF) for Mobile

Stations (MSs) 3GPP TS 29.007: General requirements on interworking between the public-land

mobile network (PLMN) and the integrated-services digital network (ISDN) or public-switched telephone network (PSTN)

3GPP TS 23.108: Mobile radio interface Layer 3 specification core network protocols; Stage 2 (structured procedures)

3GPP defines UMTS/W-CDMA solution architectures. The organization's codec working group (TSG-SA/WG4) was responsible for the specification of the visual phone. It defined 3G-324M. As a baseline of the terminal specification, ITU-T H.324M was adopted.

ITU H.324 was defined for visual PSTN phone terminals. Initially, it was developed for PSTN with the V.34 modem protocol. Its mobile extension is defined as H.324M (originality called H.324 with mandatory support of Annex C). H.324M was realized with the improvement of error resiliency to the multiplexing protocol, which is defined in Annex A/B/C to H.223.

The major sub-protocols and procedures of 3G-324M are:

Error-resilience services H.223 multiplexing/de-multiplexing protocol ITU-T H.245 call control Optional codecs to be used: MPEG-4 Simple Profile, H.263 for video, and adaptive

multi-rate (AMR) for audio

For mobile conversational multimedia communication, error resilience is essential for error detection and concealment on the fly. H.223 provides Annexes A, B, C, and D for such services. Annexes A and B define the handling of light to moderate BER levels. These annexes were made mandatory by 3GPP. They're commonly used by vendors today.

In addition, MPEG-4 video provides tools for error resilience. It thereby minimizes the video-quality degradation that is caused by errors. These solutions don't reduce errors like forward error correction (FEC) or automatic repeat request (ARQ). But they can reduce the damage on decoded video quality.

For instance, 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 this codec particularly well suited for 3G-324M.

In contrast, MPEG-4 Visual is organized into profiles. Within a profile, various levels are defined. The profiles define subsets of toolsets. The levels are related to computational complexity. Among these profiles, Simple Visual Profile provides low complexity and error resiliency (through data partitioning, reversible variable-length coding (RVLC), a resynchronization marker, and header extension code). MPEG-4 allows various input formats including general formats like quarter common intermediate format (QCIF) and common intermediate format (CIF). It also is baseline compatible with H.263. Details on the MPEG-4 error-resilience services follow:

The resynchronization marker can reduce the error propagation caused by the nature of variable-length code (VLC) into a single frame. In MPEG-4, the resynchronization marker is inserted at the top of a new group of block (GOB) with the header information (macro-block, or MB, number and quantization parameters) and optional header extension code (HEC). Decoding can then be done independently. It's a good idea to place the resynchronization marker before important objects like people. This approach will improve error resilience with a minimum increase of overhead.

Byte alignment: Bit stuffing for the byte alignment provides additional error-detection capability through its violation check.

Data partitioning: A new synchronization code, which is named motion marker, separates the motion-vector (MV) and discrete-cosine-transform (DCT) fields. In this way, it prevents inter-field error propagation. Effective error concealment can therefore be

performed. When errors are detected solely in the DCT field, the MB will be reconstructed using correct MV. Compared to the simple MB replacement of the previous frame, this approach results in better natural motion.

Reversible variable-length code (RVLC): The RVLC is designed to enable forward and backward decoding without significantly impacting coding efficiency. Ideally, this feature localizes error propagation into a single macro block.

Adaptive intra refresh (AIR): In contrast to the conventional cyclic version, AIR employs motion-weighted intra refresh. It results in better perceptual quality along with the quick recovery of corrupted objects.

Error detection and concealment: Errors can be detected by exceptions or violations in the decoding process. Concealment will then be applied. This functionality is included for mobile applications. The code point of H.324 can support MPEG-4 audio, thereby making it usable for an H.324 mobile-phone terminal.

A 3G-324M protocol is initialized after a circuit-switched channel is opened between two communicating parties. The H.223 multiplexing protocol is the first to be established between those parties. After initiating this protocol, the multiplexing process must be synchronized between the communicating parties. It's also important to establish the call control (H.245) as the first logical channel to be opened (channel 0).

The basic function of the multiplexing protocol is to interleave multiple media streams into a single stream. Such media streams could include video, speech, user data, and control signals (H.245). That single stream can then be sent over a transmission channel. 3G-324M uses the ITU-T H.223 mobile extensions of Level 2 as its multiplex protocol.

H.223 has a flexible mapping scheme that's suitable for a variety of media and a variable frame length. In its mobile extension, it flaunts stronger synchronization and control against channel errors without losing its flexibility. Three operation modes exist from Level 0 to Level 3. They are categorized according to their degree of error resiliency.

Multiplexing Level 0 is identical to the H.223 specification. It provides multiplexing and QoS functions that are appropriate for each media data. Two layers, which are known as the adaptation and multiplexer (MUX) layers, realize these features (FIG. 3). Three types of adaptation layers are defined according to their media type (video, speech, or data):

Adaptation Layer (AL) 1: User data and control signals (H.245). This AL assumes that the upper layer provides error control.

Adaptation Layer 2: Speech. Error detection and a sequence-numbering mechanism are provided.

Adaptation Layer 3: Video. This layer offers error detection, sequence numbering, and ARQ.

The multiplexer layer assembles multiple media packets into a single bit stream according to the selected multiplex pattern. That pattern is chosen out of up to 16 multiplex patterns. The MUX pattern can be defined arbitrarily through the session negotiation procedure.

Header information is attached in order to control such a flexible multiplexing mechanism. It consists of a 4-b multiplex code (MC), 1-b packet marker (PM), and 3-b parity (HEC).

As a delimiter of multiplexer-protocol data units (MUX-PDUs), 8-b HDLC synchronization flags ('01111110') are inserted. To prevent the flag emulation inside the payload, stuffing is defined ('0' bit insertion after every five succeeding '1s').

In Multiplexing Level 1, a 16-b PN sequence is used instead of an 8-b HDLC synchronization flag. It thereby improves the MUX-PDU synchronization over error-prone channels. Stuffing is prohibited to enable an octet-oriented flag search. This modification remarkably improves the flag-detection performance over error-prone channels. But in the case of conflict, there is a slight probability of flag emulation conditions. This multiplexing level is described in H.223 Annex A to overcome light error-prone channel for detection and concealment services.

In Multiplexing Level 2, MUX-PDU payload length information and FEC for the header is added over the Level 1 modification. As a result, it promises much better synchronization and error resilience. An optional header field, which includes MC/PM/HEC for the previous frame, can be applied to improve error resilience against burst errors through time-diversity effects. This multiplexing level is described in H.223 Annex B. Its goal is to overcome moderate error-prone channels for detection and concealment services. All Freedom of Mobile Access (FOMA) devices, which are approved by NTT-DoCoMo, support Multiplexing Level 2. This multiplexing level is the standard de-facto choice today.

H.245 TERMINAL CONTROL3G-324M uses ITU-T Recommendation H.245 as the multimedia-communications-control method. H.245 is used in a variety of applications, such as B-ISDN (within H.320), local-area networks (within H.323), and mobile communications. It has a wide variety of communications-control functions. Assuming it's used in an error-free environment, H.245 enables reliable control using in-channel request-response messaging.

Currently, ITU H.245 version 10 is ratified by the SG16 of ITU. A few vendors, such as RADVISION, support this version in their 3G-324M protocol toolkit. The minimal version to be supported is version 3. Support for the higher versions enables a richer set of call-control services. The rising video-codec protocol, H.264, requires advanced H.245 support with generic capabilities exchange.

Because 3G-324M rides on a channel that was opened between two communicating parties, it doesn't need any addressing like H.323. The gateway (e.g., between 3G-324M, H.320, H.323, and SIP) is expected to provide the interoperability between different networks. This gateway can be realized rather easily.

3G-324M for H.245 operation requires Numbered Simple Retransmission Protocol (NSRP) and Control Channel Segmentation and Reassembly Layer (CCSRL) sun-layers support. NSRP is defined in H.324/Annex A. Essentially, mobile terminals shall support the NSRP and the SRP mode. If both terminals start the session in Level 0, the SRP mode shall be used. If Multiplexing Level 2 is used, both terminals shall start with NSRP mode.

The CCSRL sublayer is used for carrying the large H.245 packets that are required for operation. H.245 provides the following functions: master-slave determination, capability exchange, logical channel management, multiplex table management, mode-change request, and miscellaneous commands and indications. The explanation for each function follows:

The master-slave determination figures out which terminal is the master at the beginning of the session. Due to the fact that H.245 is a symmetric control protocol, it's

necessary to determine the master terminal. That terminal has the right to decide the conditions in case of conflict.

The capability exchange exchanges the capabilities that both terminals support. These capabilities include the mode of multiplexing; type of audio/video codecs; data-sharing mode and its related parameters; and/or other optional features.

Logical channel signaling opens/closes the logical channels for media transmission. This procedure also includes parameter exchange for the use of this logical channel.

Multiplex-table initialization/modification adds/deletes the multiplex-table entries.

The mode request requests the mode of operation from the receiver side to the transmitter side. In H.245, the choice of codecs and parameters is decided at the transmitter side. This choice takes into account the decoder's capability. If the receiver side has a preference within its capability, this procedure is used.

The round-trip-delay measurement enables an accurate quality-characteristic measurement.

The loop-back test is useful for device test during development or in the field. It helps to assure proper operation.

Miscellaneous call-control commands and indications request the modes of communication, flow control like conference commands, and jitter indication and skew. Or they can indicate the conditions of the terminal to the other side.

To provide these functions, H.245 defines the messages to be used and the procedures for handling those messages. Using Abstract Syntax Notation 1 (ASN.1), H.245 defines each message parameter that effectively provides readability and extensibility. To encode these ASN.1 messages into binary, it utilizes the Packed Encoding Rule (PER). It thereby realizes very bandwidth-effective message transmission. After the multiplexing-level synchronization between the communicating parties is completed, the first logical channel opened (channel 0) is H.245 call control. It has CCRL and NSRP, which ensure that the H.245 channel will be highly reliable. It also will be able to use large packets during operation.

3G-324M-ENABLED DEVICESIn order to operate 3G-324M services, handheld devices, base stations, gateways, and servers must all support this protocol. The handheld-device category includes all types of handheld devices, including 3G PDAs, smart phones, and feature phones. Those devices are the clients of the service, which can operate video-call initiation and receiving and video on demand (VOD). The service also can operate data-entry services while a call session is active, such as dual-tone multiple-frequency (DTMF) signals for an e-commerce transaction.

The base station is the network device of the operator. It authenticates the served client's handheld devices. The base station also initiates a bridge between the handheld device and a backbone packet-switched and circuit-switched network (with E1/T1 ports interfacing the backbone). The base station should comply with the following 3G-324M-related call-setup specifications:

3GPP TS 24.008: mobile radio interface Layer 3 specification 3GPP TS 27.001: General on Terminal Adaptation Functions (TAF) for mobile

stations (MSs)

3GPP TS 29.007: General requirements on interworking between the public-land mobile network and the integrated-services digital or public-switched telephone networks

3GPP TS 23.108: Mobile radio interface Layer 3 specification core network protocols; Stage 2 (structured procedures)

The gateway also plays a vital role in 3G-324M's success. It bridges the 3G-324M circuit-switched and IP network (H.323 or SIP) signaling. The gateway translates the call-setup mentioned above into Q.931 (H.323) or SDP (SIP) messages. The call control of the H.245 over CS is transformed into H.245 (H.323) over IP or to SDP (SIP). The codecs are transformed as well. If there are no common codecs in the capability-exchange phase (H.245 or DSP capability-exchange procedure), a transconding is performed.

Lastly, the servers enable add-on services like auditing, video mail, and video on demand. They should have 3G-324M if they're interfaced directly with the base station's CS ports. The servers also may support DTMF to enable user controls like record, playback, or menu operation (OSD).

3G video telephony is paving its way toward the mass market month after month. Every day, mobile-device quality is enhanced, coverage improves, and there are more countries and 3G subscribers to call. The problems of carrying video-telephony communication over IP in a public 3G network won't be solved in the near future. Yet the 3G-324M solution does offer hope (FIG. 4). It is the only working 3G protocol with a rapidly growing number of subscribers. Most importantly, it boasts acceptance by all 3G technology camps including W-CDMA, TD-SCDMA, and CDMA200. Across the globe, equipment developers and service providers are embracing this protocol for their 3G solutions.

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1.  Background

Digital Video Trends Cellular Network Evolution Mobile Device Evolution

2. Cellular Video Applications

Video Download and Streaming Video Telephony and Messaging Video Surveillance and Monitoring

3. Technical Aspects of Cellular Video

Audio and Video Codecs (GSM-AMR, AAC, MPEG-4, H.264, etc.)

Protocols and File Formats (RTP, RTSP, SIP, 3GP, etc.)

End-to-end Cellular Video Standards (3GPP PSS, 3GPP PSCS, 3G-324M)

Video-enabled Handset Architecture

 

 

4. The Cellular Video Market

Value Chain and Revenue Models Content Issues Technology Vendors Case Studies and Industry Forecasts

5. Mobile Video Roadmap

Broadcast in Cellular Networks (MBMS, BCMCS)

Mobile Broadcast TV Networks (DVB-H, T-DMB, ISDB-T)

Alternative Mobile Video Delivery (PC/STB download, place shifting)

Future Outlook

6. Conclusion

7. Glossary