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Module 5 VoIP Signaling Protocols. VoIP Call Signaling. H.323. The first call control standard for VoIP was H.323, which was adopted by the ITU-T in 1996 An umbrella standard that covers specification for transmitting audio, video and data across an IP network including the Internet

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h 323
  • The first call control standard for VoIP was H.323, which was adopted by the ITU-T in 1996
  • An umbrella standard that covers specification for transmitting audio, video and data across an IP network including the Internet
  • H.323 recommendations also included a standard called T.120 which is used by Microsoft’ Netmeeting
h 323 components and protoclss
H.323 Components and protoclss
  • Call Signaling: H.225
  • Media Control: H.245
  • Audio Codecs: G.711, G.722, G.723, G.728, G.729
  • Video Codecs: H.261, H.263
  • Data Sharing: T.120
  • Media Transport: RTP/RTCP
h 323 components
H.323 Components

H.323 Gateways

  • Provides interoperability between H.323 and non H.323 Switched Circuit Networks (such as PSTN). Gateways not needed unless interconnection with the SCN required. It translates between audio, video, and data transmission formats and communication systems and protocols.
  • H.323 Gatekeepers

Optional component of H.323 Perform Call processing, address translation, and distributed application manager functions

h 323 components1
H.323 Components
  • H.323 terminals: Are end devices and can either be PC or other stand-alone devices running multimedia applications.
  • Multi-point Control Units (MCU’s) : MCU’s provide support for conferences of 3 or more terminals. All terminals participating in a conference establish a connection with the MCU.

Note: Gatekeepers, Gateways and MCU’s are logically different components of a H.323 network but can be implemented on one device.

gatekeeper zones
Gatekeeper Zones
  • A zone is the collection of H.323 nodes such as gateways, terminals, and MCUs registered with the gatekeeper.
  • There can only be one active gatekeeper per zone.
h 323 gatekeeper functions mandatory
H.323 Gatekeeper Functions- Mandatory
  • Address Translation - Translates H.323 IDs (such as and E.164 numbers (standard telephone numbers) to endpoint IP addresses.
h 323 gatekeeper functions mandatory1
H.323 Gatekeeper Functions - Mandatory
  • Admission Control - Controls endpoint admission into the H.323 network. To achieve this, the gatekeeper uses the following:

- H.225 Registration, Admission, and Status (RAS) messages

- Admission Request (ARQ)

- Admission Confirm (ACF)

- Admission Reject (ARJ)

h 323 gatekeeper functions mandatory2
H.323 Gatekeeper Functions - Mandatory
  • Bandwidth Control - Consist of managing endpoint bandwidth requirements. To achieve this, the gatekeeper uses the following H.225 RAS messages:

- Bandwidth Request (BRQ)

- Bandwidth Confirm (BCF)

- Bandwidth Reject (BRJ)

  • Zone Management - The gatekeeper provides zone management for all registered endpoints in the zone. For example controlling the endpoint registration process.
optional gatekeeper functions
Optional Gatekeeper Functions
  • Call Authorization - With this option, the gatekeeper can restrict access to certain terminals or gateways and/or have time-of-day policies restrict access.
  • Call Management - With this option, the gatekeeper maintains active call information and uses it to indicate busy endpoints or redirect calls.
optional gatekeeper functions1
Optional Gatekeeper Functions
  • Call Control Signaling - With this option, the gatekeeper can route call-signaling messages between H.323 endpoints using the Gatekeeper-Routed Call Signaling (GKRCS) model. Alternatively, it allows endpoints to send H.225 call-signaling messages directly to each other.
gatekeeper routed call signaling vs direct endpoint signaling
Gatekeeper-Routed Call Signaling Vs Direct Endpoint Signaling

There are two types of gatekeeper call signaling methods:

  • Direct Endpoint Signaling - With this method, call setup messages are directed to the terminating gateway or endpoint.
  • Gatekeeper-Routed Call Signaling (GKRCS) - With this method, the call setup messages are directed through the gatekeeper.
  • Note: Cisco IOS gatekeepers are Direct Endpoint signaling based and do not support GKRCS.
h 323 signaling1
H.323 Signaling

Three main areas of control:

  • Registration Admission and Status (RAS) signaling
  • Call Control Signaling
  • Media Control and Transport signaling

Signaling can occur between:

- Endpoints and Gatekeepers

- Gatekeepers

- Endpoints directly, without gatekeepers (In this case, no RAS messages exchanged).

ras signaling
RAS Signaling
  • RAS is the signaling protocol used between gateways and gatekeepers. The RAS channel is opened before any other channel and is independent of the call setup and media transport channels.
  • Gatekeeper Discovery: Automatic or manual. The Gatekeeper discovery multicast address is, UDP ports 1719 (gatekeeper registration and status port) and 1718 (gatekeeper discovery port).
gatekeeper autodiscovery ras messages
Gatekeeper Autodiscovery RAS Messages
  • Gatekeeper Request (GRQ): A multicast message sent by an endpoint looking for the gatekeeper
  • Gatekeeper Confirm (GCF): The reply to an endpoint GRQ indicating the transport address of the gatekeeper’s channel
  • Gatekeeper reject (GRJ): Advises the endpoint that the gatekeeper does not want to accept it’s registration. Usually due to a configuration error on the gateway or gatekeeper

Occurs after the discovery process but before you can place any calls

  • Registration Request (RRQ)
  • Registration Confirm (RCF)
  • Registration Reject (RRJ)
  • Unregister Request (URQ)
  • Unregister Confirm (UCF)
  • Unregister Reject (URJ)
endpoint location
Endpoint Location
  • Location Request (LRQ) : Sent to request the endpoint or gatekeeper contact information for one or more E.164 addresses
  • Location Confirmation (LCF): Sent by gatekeeper to the endpoint indicating location address
  • Location Reject (LRJ): Sent by gatekeepers that receive an LRQ for which the requested endpoint is not registered or has unavailable resources.

Admissions messages between endpoints and gatekeepers provide the basis for call admissions and bandwidth control

  • Admissions Request (ARQ): An attempt by an endpoint to initiate a call
  • Admissions Confirmation (ACF): An authorization by the gatekeeper to admit the call
  • Admissions Reject (ARJ): Denies the endpoint’s request to gain access to the network for this particular call
status information
Status Information

The GK can use the RAS channel to obtain status info. Typical polling period for status messages is 10 seconds

  • Information Request (IRQ)
  • Information Request Response (IRR)
  • Status Enquiry: GK typically uses this message to verify call state
bandwidth control
Bandwidth Control

Bandwidth is initially managed through the ARQ/ACF/ARJ but bandwidth can change during the call

  • Bandwidth Request (BRQ): Sent by an endpoint to the gatekeeper requesting an increase or decrease in call bandwidth
  • Bandwidth Confirmation (BCF): Sent by the gatekeeper confirming acceptance of the bandwidth change request
  • Bandwidth Reject (BRJ): Sent by the gatekeeper rejecting the bandwidth change request if the requested bandwidth is not available
h 225 call control setup signaling
H.225 Call Control (Setup) Signaling
  • H.225 call control signaling is used to setup connections between H.323 endpoints. The ITU H.225 recommendation specifies the use and support of Q.931 signaling messages.
  • A reliable (TCP) call control channel is created across an IP network on TCP port 1720.
  • Setup, Call Proceeding, Alerting, Connect, Release Complete, Facility Messages
  • Setup message sent on the well known H.225 port 1720
  • The connect message can contain the transport UDP/IP address for H.245 signaling
h 245 media control and transport
H.245 Media Control and Transport
  • H.245 handles end-to-end control messages between H.323 entities.
  • H.245 procedures establish logical channels for transmission of audio, video, data, and control channel information.
  • A reliable control channel is created over IP using the dynamic assigned TCP port in the final call signaling message
  • The exchange of capabilities, the opening and closing of logical channels, preference modes, and message control takes place over this control channel. H.245 control also enables separate transmit and receive capability exchange as well as function negotiation, such as determining which codec to use
  • Fast Connect is another method besides H.245 to establish media channels between endpoints.
  • Fast Connect enables media connection establishment for basic point to point calls with one round-trip message exchange. These procedures dictate that the calling endpoint include the faststart element in the initial setup message. Preferred by some vendors.
h 323 products
H.323 Products
  • In Cisco AVVID IP Telephony, H.323 Gateways and Gatekeepers are implemented in Cisco IOS gateways and routers.
session initiation protocol sip
Session Initiation Protocol (SIP)

Alternative to H.323:

  • Session Initiation Protocol (SIP): IETF standard based on RFC 2543, which is obsoleted by RFC 3261
  • Application layer signaling-control protocol used to establish, maintain, and terminate multimedia sessions
  • SIP invitations can establish sessions and carry session descriptions
  • SIP supports unicast and multicast sessions as well as point-to-point and multipoint calls
  • SIP can operate in conjunction with other signaling protocols such as H.323
sip elements
SIP Elements

SIP is a peer-to-peer protocol as each node can be a client and server. Two main components: User agents and network servers

  • User Agents (UA): Client end-system applications that contain both a user-agent client (UAC) and a user-agent Server (UAS), otherwise known as client and server
  • Network Servers: Proxy Server, Redirect Server, and Registrar Server
proxy redirect servers registrar servers
Proxy, Redirect Servers, Registrar Servers
  • Acts on behalf of other clients and contains both client and server functions
  • A proxy server interprets and can re-write request headers before passing them on to the other servers
  • Rewriting the headers identifies the proxy as the initiator of the request and ensures that replies follow the same path back to the proxy instead of the client
  • Redirect Server: Accepts SIP requests and sends a redirect response back to the client containing the address of the next server. Redirect Servers do not accept calls, nor do they process or forward SIP requests
  • Registrar Server: Keeps a registration database of the SIP endpoints on the network
  • Also called SIP Universal Resource Locators (URLs)
  • Similar to email addresses identified by user@host
  • The user portion can be a user name or telephone number, and the host portion can be a domain name or network address
  • E.g


locating a server
Locating a Server
  • A client can send a SIP request either directly, to a locally configured proxy server, or to the IP address and port of the corresponding SIP URL
  • Sending the request to the proxy server is easy but sending it to the SIP URL is a bit complicated. Look at the textbook section Chapter 11.
sip message requests
SIP Message Requests

Six kinds of message requests:

  • Invite: Indicates that the user or service is invited to participate in a session
  • ACK: Represent the final confirmation from the end system and conclude the transaction initiated by the INVITE command.
  • OPTIONS: Enables you to query and collect user agents and network server capabilities
  • BYE: This method is used by calling and called parties to release a call.
  • CANCEL: This request enables user agents and network servers to cancel any in progress request
  • REGISTER: used by clients to register location information with SIP Servers
message responses
Message Responses
  • SIP message responses are based upon the receipt and interpretation of a corresponding request
  • Some common responses: 100 – Trying, 180 – Ringing, 200 – OK, 301 – Moved Permanently, 305 – Use Proxy, 400- Bad Request, 404 – Not found, 500 – Internal Server Error, 600 – Busy everywhere
  • Many more in Chapter 11. Please go through.
sip future
SIP Future
  • Increase in adoption among vendors and service providers
  • 3GPP has chosen SIP as the standard for multimedia communication
  • Closely tied with IMS (Internet Multimedia System), which is the 3GPP standard for merging the Internet and Cellular Worlds
  • Will likely not completely replace H.323
  • SIP and H.323 likely to co-exist in future for a while
additional references
Additional References
  • for latest developments in SIP technology