Echo Cancellation in 3G Media Gateway - Aalto · Echo Cancellation in 3G Media Gateway Espoo, 14th of ... (MSS) and Media Gateway (MGW). ... / Media gateway is a border element between

Echo Cancellation in 3G Media GatewayEcho Cancellation in 3G Media Gateway

Espoo, 14th of May, 2002Espoo, 14th of May, 2002

Author:Author: Baris EfeBaris EfeSupervisor:Supervisor: Timo KorhonenTimo Korhonen

Echo Cancellation in 3G Echo Cancellation in 3G Media GatewayMedia Gateway

Master's Thesis Presentation




AgendaAgenda/ Background

• 3G Network and Media Gateway • H.248• Echo Cancellation

/ Echo Canceller Implementation/ Channel Monitor




Media GatewayMedia Gateway/ Media Gateway (MGW) is a new network element introduced with 3GPP Release 4

(march 2001) and Release 5 (march 2002) network standards.

/ In release 4 and 5 networks, a horizontally layered network approach is adopted. User traffic and network control traffic are separated.

/ This separation allows using different packet or cell switching technologies i.e. TCP/IP and ATM to carry the user traffic instead of circuit switched TDM lines.

/ Traditional MSC has evolved into two separate network elements: MSC Server (MSS) and Media Gateway (MGW). The MSC server is responsible for call control and mobility management functions and the media gateway handles the transport and media conversion between core network and external network elements.




Media Gateway in Release 4 NetworkMedia Gateway in Release 4 Network

/ Media gateway is a border element between different kinds of signalling and user plane interfaces. It terminates bearer channels from the circuit switched network and media streams from the backbone network (RTP streams in an IP network or AAL2/ATM connections in an ATM backbone) and does theconversion between these terminations. Speech transcoding etc.

/ Media gateway forwards the SS7 signaling to MSC server with SIGTRAN method or vice versa. MSC server controls media gateway with H.248 protocol.




Media Gateway in Release 5 NetworkMedia Gateway in Release 5 Network

/ In Release 5 network the role of media gateway is minor, it is used to interconnect the 3G network to PSTN and other PLMN networks. Media gateway is controlled by Gateway Control Server and Connection Processing Server with H.248 protocol.




H.248 ProtocolH.248 Protocol

/ H.248 is the official industry protocol developed for interfacing media gateways with the media gateway controllers. It is also known as Media Gateway Control Protocol (MEGACO).

/ H.248 is an IP based protocol and it can be used on UDP and TCP transport protocols. H.248 allows an authenticated server to instruct a MGW to connect streams to packet and cell switching networks.

/ There are two basic concepts in H.248 protocol: terminations and contexts. Terminations represent media streams entering or leaving the MGW. A context is an association between a number of terminations and it describes the topology (the call scenario) and the media mixing and/or connection parameters involved in the association.




Echo Sources: Electric EchoEcho Sources: Electric Echo

/ In traditional analog PSTN networks, the last connection between the telephone and local exchange is 2-wire connection (both transmission directions are on the same copper pair and the connections between switches in the core network are 4-wire connections (each transmission direction is transmitted separately on different pairs). In order to connect 2- and 4-wire sections together, devices called 2-to-4-wire hybrids are used. Hybrid is a kind of transformer which is located in the local exchange.

/ There would be a perfect 2-wire/4-wire conversion if the impedances of local line ZTW and balancing circuit ZB in the hybrid were equal. However, this is not possible as the impedance of local 2-wire lines varies according to the length of connection. Therefore there is practically always an impedance mismatch which causes part of the signal energy to be reflected back. This reflection is called electric echo due to its generation mechanism.




Echo Sources: Acoustic EchoEcho Sources: Acoustic Echo

/ Another source of echo in the telecommunication systems is the acoustic echo. The acoustic echo is generated due to the poor isolation between the earpiece and the mouthpiece of the mobile terminals and between the speaker and the microphone of the hands free phones.

/ The level ratio of incoming and reflected signals is called Echo Return Loss (ERL) and it is expressed in dB values. Typical ERL values for electric echo in nation-wide networks vary between 6...30 dB. Acoustic echo is not as prevalent as hybrid echoes. Typically, acoustic echo loss varies between 20-50 dB for the handsets.

/ The common form of echo in PSTN is the composite echo, which is the case that both acoustic and electric echo are present in the system.




/ For echo to be noticeable, the human ear must detect some delay between the source signal (in this case, the spoken word) and the echo signal.

/ If the round trip delay (twice of the one way transmission delay) is shorter than 20 ms, echo effects are imperceptible. When the round trip delay is about 30 ms, echo is perceived as an annoying effect. The degree of annoyance of the echo depends both on the amount of delay as well as on the level difference between the voice and echo signals. 0

10

20

30

40

50

60dB

T

5 10 20 30 50 100 200 300T1208560-96/d01

ms

Acceptable (*)

TELR

Limiting case

Talker Echo Loudness RatingMean one-way transmission timeThe “Acceptable” curve is equivalent to the curve with “1%” probability of encountering objectionable echo.

TELRT(*)

FIGURE 1/G.131Talker echo tolerance curves

Required Echo Attenuation

T Mean one way transmission delay(*)The "Acceptable" curve is equivalent to the curve with %1 probability of encountering objectionable echo

Talker Echo Tolerance [G.131]

Delay and EchoDelay and Echo




Release 4 Release 4 -- PSTN Call ScenarioPSTN Call Scenario

/ The total one way delay in UTRAN is expected to be between 50-85 ms [3GPP 25.853]

/ The Core Network Delay is very much dependent on implementation. Delay in core network is due to jitter buffering, signal processing and transmission delays. Core network delay is increasing with the number of media gateways involved in a call.

/ For an average size country the one-way delay in PSTN networks is expected to be less than 18 ms [G.114].




Release 5 Release 5 -- PSTN Call ScenarioPSTN Call Scenario

/ In Release 5 networks, the total delays will be more than Release 4 networks since there will be more packet switched elements in the system.




/ The Signal Processing Platform provides DSP services to the computation intensive applications in the 3G network elements.

/ The Signal Processing Resource Manager (SPRM) allocates and manages the signal processing resources of the DSP unit.

/ The Signal Processing Platform Services provides supplementary services such as: supervision, diagnostics, media connection establishment and DSP code loading.

/ The DSP platform provides services to the DSP applications with the help of DSP RTOS. DSP RTOS provides multichannel media handling capability by providing multitasking, scheduling, memory management and fault management functions.

Signal Processing Platform

Network Element Platform

DSP Platform

Network Element ApplicationDSP Application

DSP ApplicationFramework

DSP Algorithms

DSP Algorithm Interface

DSP Application


DSP Algorithms

DSP Algorithm Interface

DSP Application


Signal ProcessingResource Manager

Signal ProcessingPlatform Services

DSP Algorithms

Network Element Platform Interface

DSP Application Interface DSP Service Interface

DSP Platform Interface DSP Resource Interface

DSP Algoritm Interface

Signal Processing PlatformSignal Processing Platform




Echo Canceller ImplementationEcho Canceller Implementation

Echo Canceller Module

/ ECDM consists of three functions: echo canceller initialisation function, processing function and the releasing function. These functions interact with the echo canceller algorithm and control the operations of the algorithm.

/ From user traffic point of view, ECDM requires two data inputs; receive in and send in and provides one data output; send out.




PSTN TerminationPSTN Termination/ In a 3G media gateway, H.248 contexts and terminations can be implemented with

DSP software. Each H.248 termination application implements the required protocol stacks and data processing mechanisms for the interface it is connected. The different terminations for a 3G MGW can be for example; Iu termination, Nbtermination, PSTN termination, A termination and IP termination.

/ Echo Canceller Software is implemented as a part of PSTN termination application. H.248 includes commands for echo cancellation. A PSTN termination application handles all connections to/from a media gateway towards the PSTN networks using circuit-switched TDM connections with 64 kbit/s A-law or µ-law PCM speech [G.711].




Echo CancellerEcho Canceller/ An echo canceller monitors speech from the non-cancelled end that passes through

its receive-in path and uses this information to compute an estimate of the echo that is then subtracted from its send-path. Thus, the echo is eliminated (or cancelled), and only the near-end speech is sent to the far end.

/ The adaptive filter of the echo canceller assumes that the echo path is linear and time invariant. Therefore, it can only attenuate the linear component of the echo signal. However, in practice, the echo path has significant levels of non-linear distortion due to for example, quantization distortion, companding, acoustic echo non-linearities etc.

/ These non-linear effects often prevent echo cancelers from achieving the necessary echo return loss by using adaptive filtering techniques alone. To remove non-linear effects, non-linear processors (NLP), cascaded to the adaptive filters, are used in the echo cancellers.

/ While the NLP is attenuating the residual echo signal, it clips the background noise, which can not be attenuated by the adaptive filter. This is perceived by the listeners as such that the connection is disconnected or the other side voice signal is muted. This is a disturbing effect for the non-cancelled side if the noise level is high in the cancelled side. An artificial noise (called the comfort noise) is inserted by the echo canceller to the portions of signal where the NLP is operational.




Channel MonitorChannel Monitor

/ The echo canceller module can also be used to provide information on the nature or relative quality of a speech or voice-band data signal present on a voice circuit as a channel monitor. Channel Monitor functionality is integrated into the echo canceller. The channel monitoring parameters are:

/ · Echo return loss• Echo return loss is the attenuation of a signal from the receive out port to the

send in port of the echo canceller




Channel Monitor (Cont.)Channel Monitor (Cont.)

/ · Echo path delay• Echo path delay is the delay of the echo signal from the receive out port to the

send in port of the echo canceller./ · Pure delay/ · Echo dispersion time/ · Echo return loss enhancement

• Echo return loss enhancement (ERLE) is the attenuation of the echo signal while it passes through the echo canceller when the non linear processing is not performed on the residual echo signal




Channel Montior (Cont)Channel Montior (Cont)




Questions?

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Echo Cancellation in 3G Media Gateway - Aalto · Echo Cancellation in 3G Media Gateway Espoo, 14th of ... (MSS) and Media Gateway (MGW). ... / Media gateway is a border element between