3G Media Gateway Software for GSM Transcoder

© 2006 NOKIA 1 [email protected] 1.12.2006

3G Media Gateway Software for GSM

Transcoder

Author: Mikko Mäkinen

Supervisor: Professor Raimo Kantola

Instructor: Lic.Sc. (Tech) Jyri Suvanen


Contents of the presentation• Objectives of the thesis

• Speech transcoding in mobile networks

• Role of TRAU (Transcoder and Rate Adaptation Unit) and MGW (Media Gateway) in GSM and 3G networks

• Digital Signal Processing (DSP) Hardware and Software (HW/SW)

• Comparing TRAU and MGW software

• Applying MGW software for TRAU environment

• Performance of the new software

• Conclusions


Objectives of the thesis• Problem:

• Two separate network elements (TRAU and MGW) are handling basically the same task i.e. transcoding speech. However, their development has been relatively independent in Nokia up to this date.

• Methods:• Study the roles of the TRAU and the MGW unit in current and

future network architectures• Comparison of the MGW and the TRAU software• Apply the MGW software in the TRAU hardware environment

and implement needed software parts• Feasibility & performance study of the new software

• Thesis was carried out in Nokia Network’s Signal Processing System unit


Speech transcoding in mobile networks• Radio frequencies are scarce and expensive resource

• Speech encoding saves radio channels remarkably• Traditional PCM (64kbit/s) versus HR (5.6kbit/s), FR (13kbit/s),

EFR(12.2kbit/s) and AMR-NR(4.75-12.2kbit/s)

• Exploitation of the statistical characteristics of the human speech• Short term correlation – linear prediction• Long term correlation – fundamental period of voiced sounds

• Hybrid coders – combination of waveform coders and vocoders

• Compatible interconnections towards other wireless and wireline networks usually implemented with PCM


Role of TRAU and MGW (1/3)

MS – Mobile Station

BTS – Base Transceiver Station

BSC – Base Station Controller

TRAU – Transcoding and Rate Adaptation Unit

MSC – Mobile Switching Station

GMSC – Gateway MSC

VLR – Visitor Location Register

HLR – Home Location Register

AuC – Authentication Centre

EIR – Equipment Identity Register

PSTN – Public Switched Telephone Network

PLMN – Public Land Mobile Network

• TRAU is a part of the GSM access network

• Handling FR/HR/EFR/AMR-NB to PCM conversion

• Most of its new markets in developing countries (plus capacity updates in developed countries)


Role of TRAU and MGW (2/3)

• MGW is a part of the 3G core network (Release4)

• MGW handles only user plane traffic

• Placed behind TRAU if co-operated with the GSM/EDGE access network

• Handling data type conversion (IP/ATM/PCM) in addition to the media format conversion

• Under active product development (new features + capacity updates)

RAN – Radio Access Network

GERAN – GSM/EDGE RAN

UTRAN – UMTS Terrestrial RAN

CS – Circuit Switched

PS – Packet Switched

RNC – Radio Network Controller

SGSN – Serving GPRS Support Node

GGSN – Gateway GPRS Support Node

MGW – Media Gateway


Role of TRAU and MGW (3/3)• Problem: Duplicate transcoders in 3G core/ GSM access network

situation• Expensive for the operator and multiple lossy transcodings decrease

the speech quality

• Solution 1: Avoid tandem transcoding with end-to-end codec negotiation

• Tandem Free Operation (TFO) – pack encoded bits inside 64kbit/s PCM stream, no bandwidth savings

• Transcoder Free Operation (TrFO) – send encoded bits directly over an IP/ATM network, saves also bandwidth compared to 64kbit/s PCM

• Does not solve the problem of duplicate hardware units

• Solution 2: Remove separate TRAU-unit, implement Ater-interface in MGW

• Cost efficient• Dynamic allocation of 2G/3G transcoding capabilities• Unfortunately Ater-interface is proprietary and thus this solution

works only in limited situations (access and core network from the same manufacturer)


DSP HW/SW (1/2)• “Why to use specialized DSP hardware with processor specific

Assembly code when commercial off-the shelf processors have higher clock rates, wider range of third party compilers/development tools/software/etc. and support for higher (more abstracted) programming languages (C/C++/Java..) that are easily ported for new hardware environments?”

• DSP processors are superior in certain application fields (price competitive and computationally demanding functions) due to their special characteristics

• Separate multiply-accumulate unit (e.g. for vector products)• Multiple memory fetches within single clock cycle• Fixed point arithmetic• Special instruction sets• Minimal and power consumption effective design

• Skilled Assembly programmer can beat modern C-compilers anytime in small, algorithm intensive tasks

• Tenfold margin is not extraordinary


DSP HW/SW (2/2)• However, modern transcoding units have new requirements for

DSP SW• Size of the software has explode• Dynamic functionality, lots of control oriented tasks• Shorter time-to-market cycles• Rapid hardware updates to increase capacity

► It is not possible to design each new hardware from scratch and reprogram the whole software for the new hardware

►Use of common and modular hardware units►Separation of the hardware dependent software from others►Separation of the control oriented and the algorithm intensive

pieces of software►Use of higher languages (mostly C) whenever its possible►Use of Assembly only when really needed!


TRAU and MGW software comparison• The development of these units have been relatively independent

due to their distinct functions and different historical background• However, today the functionality set of TRAU is almost an inclusive

subset of the corresponding MGW functionality set. Moreover, the MGW software offers several additional services such as the wideband AMR codec.

• The whole TRAU software was originally written in Assembly although in later releases some C-code was added -> simple and effective but nowadays unmanageable, error prone and stretched to its limits

• Not too optimized anymore, as the current version is only translated version of the previous release (optimized for a different processor)

• The MGW software has a more modern layered architecture (operating system, drivers, platform, application) making it flexible, modular and portable

• Even more detailed separation in application part, making it possible to for example outsource the development of standard speech codecs


Applying MGW software for TRAU environment (1/2)• Two similar products but both have distinct markets and

hardware environments, although both units use the same DSP currently (Texas Instruments’ TNETV3010)

• However, Nokia could enhance its software development and maintenance processes by reusing the more modern MGW software in TRAU unit. This is possible, as also the Assembly code is portable between units due to the same processors.

• Minimum target: same functionality and similar capacity

• Possible problems in external interfaces:• MGW SW has no support for direct serial port traffic that TRAU

uses for all user traffic• Operation & Maintenance (O&M) interface technically very

different• MGW’s dynamic resource control vs. TRAU’s static speech

coding channels


Applying MGW software for TRAU environment (2/2)

• Only few new modules

• However, lots of little changes and adjusting -> fiddly and time consuming project

• Only basic call through functionality, O&M interface ignored at this stage

• Simplified picture of the new Software


Performance of the new Software

• Clear performance advantage compared to the old TRAU SW

• Greatest difference in actual speech codecs (translated versus native code)

• Possibility to even double the capacity!

Average MIPS consumption of an EFR speech coding channel

0

10

20

30

40

50

60

70

Platform Serial PortForwarder

Others Speech codec Total

Module

MIP

S

First channel (withfixed structures)Additional channel

Old TRAU SW


Conclusions• Separate GSM transcoder unit have markets for years, although

larger investments should be reconsidered thoroughly

• The MGW software has clear performance advantage -> more handled channels per unit or less needed DSP hardware capacity

• Only minor changes needed for the new software -> possible to develop in parallel with the main MGW software -> only one software to maintain

• More flexible software for new features or other updates. AMR-WB is a possible newcomer for GSM networks in the future.

• Final product decisions not made yet, but used approach seems to be very promising

• Further study:• Implementation of the O&M interface if green light shown• Future of the AMR-WB codec (in GSM networks)


• Thank You!

• Questions/Comments?

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3G Media Gateway Software for GSM Transcoder

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