Media Gateway Control Protocol

(MGCP)

PTCL Training & Development

MGCP

• Used between SoftSwitch and IAD(integrated access device)/AG (access gateway)/TMG (trunk media gateway)/UMG (universal media gateway)

• The Softswitch (MGC) Controls Media Gateways by means of MGCP

• is a signaling and call control protocol used within Voice over IP (VoIP) systems that typically interoperates with the public switched telephone network (PSTN)

• In essence MGCP is a Master (softswitch) and Slave(Media Gateways) Protocol

• MGCP uses SDP for media transporting

Media Gateway Control Protocol (MGCP)

MGCP Call Flow Scenario

SG

SS

RTP/UDP/IP

phone phone

LE LE

TMG/UMG 1 TMG/UMG 2

SG: To establish call b/w PSTN & SS

IAM

IAM

ACM

MDCXCRCX

ACM

CRCX

RESP

LEGENDS: IAM- Initial Address MessageACM- Address Completion MessageCRCX- Create ConnectionMDCX – Modify connection (add video)RESP – Response to connectionANM = Answer Message

Media Stream

MGCP Messages…….

Command Direction of Flow and Command itself

Endpoint Configuration MGC to MG EPCX

Create Connection MGC to MG CRCX

Modify Connection MGC to MG MDCX

Delete Connection MGC to MG DLCX

Notification Request MGC to MG RQNT

Notify MG to MGC NTFY

Audit Endpoint MGC to MG AUEP

Audit connection MGC to MG AUCX

Restart In Progress MG to MGC RSIP

OFF Hook

ACM

ANM

IP NEWTORK

RESP

RESP

MDCX

OFF hook

Ringing

IAM

MGCP

CRCX

MGCP

IAM

ACM

ANM

CRCX

RESP

Phone

MG2LEX1 MGC MG1

Phone

LEX2MGC

MGCP Call Flow Scenario

• MEGACO was designed after some limitations in MGCP like MGCP could handle low density

• ITU-T developed MDCP whereas IETF had developed MGCP

• A compromise between the two originated in the form of MEGACO-Protocol or MEGACOP

• ITU-T (SG-16) also developed H.248 in parallel to MEGACO

• Finally ITU-T & IETF agreed upon that both MEGACO & H.248 shall be identical

MEGACO/H.248, point of origin…

• MEGACO defines the rules/protocols for the Sofswitch to control MGWs

• Typically MEGACO/H.248 provides support for media streams

• It provides support for VoIP & multimedia support for PSTN or VoIP within the IP network

• MEGACO provides base architecture for controlling MGWs –how MGWs can be controlled by SS (MGC)

• MEGACO/H.248 works for a distributed architecture

MEGACO/H.248, functions

• MEGACO provides:• Total control over Media Gateways

MGs• Call admission and billing• Signaling interface to PSTN• Translation for H.323 or SIP protocols

The Softswitch (MGController) instructs MGWs like:

• To sense off-hook condition• To apply dial-tone & collect dialed

digits• To ADD any call connection and its

context (coders required)• To add UDP port numbers etc

MEGACO/H.248, workings..

• Two terminologies are used:– Termination and Context• Termination:– A termination is a logical entity that

transmit/receive media stream or control streams

– For example: Endpoint is a termination• Context:-- It is the association between two

Terminations (Endpoints)For example: between two Endpoints (Local

Call) between two RTP

ports (Transit Call)

MGCP and MEGACO base model

• MEGACO Terminations and Context

MGCP and MEGACO base model

Termination Termination

Termination

Termination

Termination

Termination

X

X

SimpleContextVoice Call

MultimediaContext(Conference Call)

NullContextNo Connection

Context

Context

• MGCP terms basic constructs as Endpoints & ConnectionsEndpoints: source/sink of data

Connections: an association between two Endpoints

• MGACO terms basic constructs as Terminations & ContextsTermination: an entity within a MGW that

source/sinks dataContext: an association between two Terminations

Possible Contexts: Origin & Dest in same MGW (local call)

Between two diff MGW (transit call)

MGCP and MEGACO base model

• Sofswitch MGC creates new Context inside MG by ADD

• MGC adds, subtracts Terminations to a Context

• MGW NOTIFIES MGC of events (off-hook, on-hook) inside MG

• MGC audits (checks) Context & Termination between MGWs

• MGC uses Service Change command (adding video/data) during a call to inform MGW

MEGACO and SoftSwitch!

• MEGACO/H.248 define aspects of signaling as Packages:– Commonly defined functions like:

• Connectivity, Transfer and Hold etc• Examples of Packages are:

• Transactions = Number• Contexts = Number, $ • Termination = Identity or $• Package Data = off-hook, on-hook,

port #• Events, Signals, statistics = ring back

tones, tone itself, time to ring

MEGACO Packages….

Messages Description of Messages

Add Addition of a Termination to a context. First Add message creates new context

Modify Modification of features of termination, dial tone

Subtract Removal of a termination fro a context (disconnection)

Move Removal of a termination from context and addition of this termination to another context (Conference Call)

Audit Value Requests Information about features, events, signalsWhen SS learns features like PRI/BRI or any other

Audit Capabilities Requests Information about all possible features, events, signals allowed for this termination

Notify Notification on Events (off-hook, on-hook)

Service Change Notification of changes of termination like dead, bad

MEGACO Messages