IP Telephony signalling Summary of course scope

IP Telephony signalling Summary of course scope

IP Telephony signalling

Overview

H.323

Raimo Kantola-S-2006 Signaling Protocols 11 - 1

PABX

AN

Summary of course scope

H.323 or

SIP

CAS, R2

ISDN

IP

Control Part of an Exchange

Or

Call Processing

Server

IP

SIP or

ISUP

D ia m et er

HLR/

HSS

MAP

CCS7

ISUP

Raimo Kantola-S-2006 circuit

Megaco/MGCP/…

INAP

Media Gateway or Switching Fabric packets

Signaling Protocols

SCP

11 - 2

1

Next Generation Network (NGN) is the

ETSI effort to harmonize packet telephony

The network architecture is layered in a much more strict sense than in case of CSN

Services

Control

Switching

• IP Applications

• Virtual Home Environment

• Open Service Architecture

Network Specific

• call control

• session management

• mobility management

• Transcoding at the edge

• Switching

• Routing

Raimo Kantola-S-2006 Signaling Protocols 11 - 3

IP Telephony Signaling alternatives

In Terminals Intelligence In Network

SIP

- ascii based

- devil in details

- Adopted by 3G

- Bakeoffs drive vendor interoperability

H.323

- Inherits ISDN

- complex

- Many

- Widely used

- first working solution

SIGTRAN works on ISUP over STCP over IP

- many view this as an interim solution!

Megaco/H.248/MGCP

- newest

- seems to be quality spec.

- architecture holds promise

- Interoperability?

Raimo Kantola-S-2006 Signaling Protocols 11 - 4

2

H.323 is a key standard for packet based multimedia communication

H.323 over: LANs, Enterprise Area Networks, MANs, Intranets,

Internets include dial-up connections and PP-connections over

CSN/ISDN with PPP packet transport.

Example networks:

- Ethernet (IEEE 802.3)

- Fast Ethernet (IEEE 802.3u)

- FDDI

- Token ring (IEEE 802.5)

- ATM

MM includes:

- Audio (mandatory)

- Video (opt)

- Data (opt)

Communication = conference or two party call.

Raimo Kantola-S-2006 Signaling Protocols 11 - 5

H.323 is used in IP trunking

E.g. India E.g Finland

DSS1

GW

IP network

H.323

GW

PSTN

PSTN

IP network is most often an Intranet, not the public Internet.

Fall-back to PSTN may be used in case of overloaded IP -network.

Makes use of arbitrage due to the regulated high International PSTN tariffs.

Initially (1997-98) a promising business - now does not look too promising.

H.323 provides also comprehensive conferencing services!

Raimo Kantola-S-2006 Signaling Protocols 11 - 6

3

H.323 uses H.225.0, H.245 and

RTP

H.323 Terminal

RAS

Gatekeeper

G.711

and others

Raimo Kantola-S-2006

H.225.0

H.245

RTCP

RTP

UDP

IP

RAS

Gateway

ISDN/ Q.931

CCS#7 /ISUP

PCM-voice

RAS - Registration/Admission/Status

RTP - Real-Time Transport Protocol

RTCP - RTP Control Protocol

Note: this is an example configuration!

Signaling Protocols 11 - 7

H.323 supports many call modes

• Directly between two H.323 endpoints

(no GK)

• Between two H.323 endpoints using a GK

• Many conference types

– ad hoc multipoint conference (start with 2-party call - expand to conf)

– broadcast conference (one sender, many receivers)

– broadcast panel conference (mp conf + bc conf)

– centralized multipoint conference (terminals pp to MCU, MP sends to terminals)

– decentralized multipoint conference (no MCU - all to all coms)

– hybrid multipoint conference - centralized audio or video

– mixed multipoint conference (mix of decentralized + centralized modes)

Raimo Kantola-S-2006 Signaling Protocols 11 - 8

4

Mixed multipoint conference example

D

MCU

E

A B C

Multicast audio and video

Decentralized side

F

Unicast audio and video

Centralized side

Raimo Kantola-S-2006 Signaling Protocols 11 - 9

H.323 zone is controlled by a Gatekeeper

Zone

T1

GK

GW

T4 T5

T2 T3

R R

MCU

• Zone has at least one terminal, MCUs and GWs are optional.

• Zone has one and only one GK.

• Gatekeeper controls access to the network for Ts, GWs and

MCUs and provides

- address translation

GK control is not enforced, so this is an intranet solution, i.e. all parties engage in cooperation voluntarily.

- gateway location

- bandwidth management

Raimo Kantola-S-2006 Signaling Protocols 11 - 10

5

Means of Control over subscribers

• Dynamic non-global IP –addresses ! Internet has become only A –subscribers network. Push needs help from a server.

• NAT and Firewall can block access from the network to a user and even from the user to the network.

• H.323 – ties subscribers to a zone. SIP registration enables moving points of attachment.

Raimo Kantola-S-2006 Signaling Protocols 11 - 11

H.323 terminal equipment

Video I/O eq

Audio I/O eq

User data app

T.120, etc

System control

User Interface

Raimo Kantola-S-2006

Video codec

H.261, H263

Audio codec

G.711, G.722

G.723,G.728

G.729

Receive path delay

H.225.0

Layer

System control

H.245 Cntrl

Call Cntrl,

H.225.0

RAS Cntrl

H.225.0

Scope of Rec. H.323

Signaling Protocols

LAN

Interface

11 - 12

6

Audio and Video coding

• Audio: G.711 is compulsory (PCM –coding). In practice much more efficient coding methods are used based on negotiation.

• Video: H.261 is compulsory. H.263 (from 1995) is mentioned in H.323. Some H.323v4 products claim support of H.264 (= MPEG4 part 10) video coding.

– H.264 coding is 2 to 4 times as efficient as MPEG2!

– H.264 can provide TV Quality streaming at less than

1Mbps and DVD quality at about 8 Mbit/s according to http://www.pixeltools.com/

– H.264 is meant for both IP based broadcast/multicast and videoconferencing.

Raimo Kantola-S-2006 Signaling Protocols 11 - 13

H.323 supports many parallel addressing methods

• H.323 entity shall have at least one Network Address (e.g.

IP address)

• TSAP (Transport Service Access Point) identifiers allow multiplexing several channels sharing one Network

Address - map to TCP/UDP port numbers (source port, destination port).

• An endpoint may have one or many Alias addresses - may represent the Ep or a Conference that the Ep is hosting.

Include: E.164 numbers, H.323 IDs (e.g. John Smith), email addresses. Aliases are unique in a zone.

Raimo Kantola-S-2006 Signaling Protocols 11 - 14

7

Gateway translates between transmission formats, communication procedures and media formats

• Example: H.225.0 to and from H.221 (transm.f)

• H.245 to and from H.242 (comm procedure)

• Media format: Audio, video, data

• Represents characteristics of network endpoint to

SCN endpoint and an IP destination to and EP connected to the IP network. May also work as an

MCU

• Can also do call set-up and clearing

Raimo Kantola-S-2006 Signaling Protocols 11 - 15

GK provides call control services, when present, shall do:

• Address translation (e.g. alias to transport address using DNS + E.164 to transport address)

– uses the translation table produced from registration messages

• Admission control: ARQ/ACF/ARJ of H.225.0

– based on call authorization, bandwidth, other criteria

• Zone management

ARQ/ACF/ARJ – Admission Request/Confirm/Reject

Raimo Kantola-S-2006 Signaling Protocols 11 - 16

8

GK may optionally do

• Call control signalling. May also direct the endpoints to set up the call signalling channel between themselves in a peer to peer fashion

• Call Authorization using H.225.0 signalling

• Bandwidth management controls the number of simultaneous calls in the zone

• Call management - keep list of calls -> busy conditions

• GK management, Directory service etc -

Raimo Kantola-S-2006 Signaling Protocols 11 - 17

Endpoint can discover a

Gatekeeper automatically

Well-known GK discovery MC address

Ep

GRQ (GK request)

GCF/GRJ (GK conf/reject)

[transport address of GK’s RAS Ch, alternateGK, cryptoinfo]

• Automatic discovery eases maintenance of individual terminals

• Terminals may also have the GK id configured

Raimo Kantola-S-2006 Signaling Protocols 11 - 18

9

RAS signalling function

Endpoint

RAS [uses H.225.0 msgs]

GK

• Performs

– Registration of endpoints, Admission of calls, Bandwidth changes for calls

– Status

– Disengage of endpoints.

• Uses RAS signalling channel =/= call signalling channel and H.245 control channel. GKs have a well def. TSAP id for RAS sig. channel

• Endpoint=H.323 terminal or GW or MCU (is callable)

Raimo Kantola-S-2006 Signaling Protocols 11 - 19

Ep

Endpoints register using GK’s RAS Channel

Transport Address prior to any calls are made

GK

RRQ(Registration rq([alias, transport address, …]

RCF[access token]/RRJ

• Security policy may require that registration has time-to-live and has to be repeated from time to time.

• Endpoint or GK may un-register using the URQ message.

• The GK maintains an alias to Network Address translation table.

• Access token may be used later in call setup

Raimo Kantola-S-2006 Signaling Protocols 11 - 20

10

Ep

Call Admission sets the upper limit for the aggregate bitrate of the call

ARQ (admission rq) [Requested

Σ call bandwidth:: payload only]

GK

ACF [may reduce BW, use direct or GK sig]/ARJ

[ transport address of call signaling channel]

Call

BRQ (Bw Change rq)

11 - 21 Raimo Kantola-S-2006 Signaling Protocols

Call signalling uses H.225.0

Ep|GK

Endpoint

H.225.0 call signalling

• Call signalling= call setup, request changes in Bw of a call, get status of Ep, disconnect call

• Call signalling is largely inherited from ISDN

• Call Signalling Channel is opened prior to H.245 procedures and prior to any other logical channels between endpoints. Eps have a well known TSAP id for the Call Sig. channel and a well-known

Discovery Multicast address.

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11

H.323 Call Signalling Channel Routing

• ACF has the Transport Address of the Call Signalling Channel

• The address is either a GK address or an Endpoint address.

Gatekeeper Routed Direct Endpoint

Call Signalling Call Signalling

GK cloud GK cloud

1 2 3 8

4 5 6 7

1 2

Ep 1 Ep 2 Ep 1

3

7

RAS Signalling Channel msgs

Call Signalling Channel msgs

Raimo Kantola-S-2006 Signaling Protocols

1,5 - ARQ 3,4 - Setup

2,6 - ACF 7,8 - Connect

5 6

Ep 2

11 - 23

H.245 Control Channel Routing

The goal of call signalling is the setup/release of H.245

Control Channel!

Direct H.245 Control Channel GK routed H.245 Control

GK cloud

GK cloud

MC

1 2 3 8

4 5 6 7

1 2 3 8

Ep 1

9 10

4 5 6 7

Ep 2 Ep 1

9

Ep 2

RAS Signalling Channel msgs

Call Signalling Channel msgs

H.245 Control Channel

1,5 - ARQ 3,4 - Setup

2,6 - ACF 7,8 - Connect

9,10 - H.245 Channel

Raimo Kantola-S-2006 Signaling Protocols 11 - 24

12

H.245 carries end-to-end control messages between H.323 entities

Terminal

H.245 Control Channel

Terminal|GW|MC|GK

• Master/slave determination for conflict resolution

• Capability Exchange (e.g. what codecs are supported)

• Logical Channel Signalling (binds media type, algorithm etc. to ports)

• Bidirectional Logical Channel Signalling

• Close Logical Channel Signalling

• Mode Request (conference modes)

• Round Trip Delay Determination

• Maintenance Loop Signalling

• H.323 also uses flowControlCommand of H.245 to limit bandwidth

Raimo Kantola-S-2006 Signaling Protocols 11 - 25

Sample H.245 Logical Ch Signalling for two way RTP+RTCP communications setup

A B

OpenLogicalChannel[reverse RTCPch=x]

OpenLogicalChannelAck [RTPch, forward RTCPch=y]

OpenLogicalChannel [RTPch, reverse RTCPch=y]

OpenLogicalChannelAck[reverse RTCPch=x]

• In IP networks a logical channel corresponds to an IP port number

• Uses H.245 Control Channel

Raimo Kantola-S-2006 Signaling Protocols 11 - 26

13

H.323 Call identification uses

Call reference value - between two H.323 entities on a signalling channel (one for call signalling and another for RAS channel

• Call ID - a globally unique non-zero value created by the calling endpoint passed in all

H.225 messages

• Conference ID (CID) - in all sub-calls of a conference

Raimo Kantola-S-2006 Signaling Protocols 11 - 27

Both endpoints registered - Direct/GK-routed call signalling

RAS

Ep1 GK1 GK2 Ep2

ARQ

ACF

Setup

Call Proceeding

ARQ

ARJ

RAS

Facility

Release Complete

RAS

DRQ

DCF

ARQ

ACF

Disengage bw, mode

Setup

Call Proceeding

Alerting

Connect

Setup

Call Proceeding

ARQ

RAS

ACF

Alerting

Connect

Raimo Kantola-S-2006 Signaling Protocols 11 - 28

14

Gateway decomposition

DSS1 or ISUP

Media Gateway

Control

IP based signaling (e.g

H.323)

H.248 = Megaco

PCM voice Media

Gateway

RTP + RTCP flow

MG - Trunk gateway, residential gateway etc.

Many MGs can be controlled by one MGC, MGCs can be a mated pair --> higher availability performance.

Raimo Kantola-S-2006 Signaling Protocols 11 - 29

H.323 summary

• H.323 inherits call signaling from ISDN

• H.323 has many conference modes and many signalling and call routing options

• Call setup delay is reduced by using the Fast Connect

Procedure: packs all setup info from both H.225.0 and H.245 into fastStart element in setup and connect (call proceeding, alerting) messages

• Versions 1, 2, 3 and 4 are available! Version 4 products are available. Supports HTTP based 3 rd party service control. ITU-T has version 5 of H.323 (quick browsing did not reveal anything major new stuff …)

• In conferencing applications over IP H.323 is still the leader.

• Version Interoperability and Vendor interoperability are issues!

• More info e.g. in http://www.h323forum.org/

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15

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