While VoIP-Voice Over Internet Protocol comes in many forms, the four major standards-based systems are:
1) H.248 MEGACO-MEdia GAteway and COntrol,
2) H.323,
3) MGCP-Media Gateway Control Protocol and
4) SIP-Session Initiation Protocol.
H.323 and H.248 are ITU-International Telecommunications Union (www.itu.it) standards. IETF-Internet Engineering Task Force (www.ietf.org) originated SIP as an internet technology based on PC-Personal Computers and other intelligent devices. MGCP is a voice protocol designed to interoperate between circuit-switched and packet-switched networks. MGCP separates the signaling and call control from the media gateway. H.248 originated from the PSTN-Public Switched Telephone network using SS7-Signaling System 7 with dumb devices.
1) H.248 is an ITU standard also known as MEGACO-Media Control Gateway. H.248 is a master/slave communications protocol with two basic constructs called Terminations - media connections called physical (time slots) and ephemeral (IP flow) and Contexts - associations between terminations which can be added or deleted. Shown here is the call flow for H.248.
H.248 uses SS7-Signaling System 7 as the primary signaling system for call routing. SS7 is a packet-switched network used to control the circuit-switched PSTN-Public Switched Telephone network. The SS7 network is tightly control with NO public access. The A-F links pass SS7-Signaling System 7 packets from SSP-Service Switching Points via STP-Signal Transfer Points to SCP-Service Control Points. SSP-Service-Signal Switching Points are the local Class 5 End Central Offices. Signal Transfer Points can contain some database functions but the SCP-Service Control Point is the primary database lookup service.
The SS7 network provides many unique features such as *69 automatic callback, 800 toll free service, look ahead call routing to route calls to any call center or agent in the world and emerging services such as CRBT-Color Ring Back Tones (shown here) tones, music and other information services.
2) H.323 is an ITU-International Telecommunications Unions standard and the logical progression from circuit-switched to packet-switched telephone call processing including video services. H.323 networks consist of Call Processing servers, MG-Media Gateways and GateKeepers. Call Processing servers provide routing and communications (connections) to MG and end-user devices (phones called terminals). MG or Gateways provide H.323 call termination and interface with non-H.323 networks such as PSTN-Public Switched Telephone Network (circuit-switched long distance networks). Used in larger networks, optional GK-GateKeepers or Gatekeepers provide central call administration and control, bandwidth administration and signaling.
1- At bootup/login, H.323 terminals (phones) registers with GateKeeper.
2 - When Sender goes off-hook and dials number, request is sent to GateKeeper.
3 - GateKeeper (optional) authorizes call to be completed and tracks bandwidth.
4 - Sender send Q.931 call setup message to receiver.
5 - Receiver is notified of setup message by ringing.
6 - Sender initiates CODEC (media stream) compatibility exchange with receiver.
7 - Optional RSVP-Reservation request is sent.
8 - Sender/Receiver opens a RTP-Real-Time Protocol session.
9 - Upon on-hook, RTP session is terminated.
Here are some technical notes regarding protocols.
1 - When a call is initiated a TCP-Transmission Control Protocol session is created for H.225.0 Messages such as, RAS-Registration-Admission-Status, RIP-Request In Progress, Bandwidth change and other functions.
2 - A second TCP session is created for H.245 Channel Usage Messages such as Master Slave Determination messages, Terminal set capability messages, Open/Closed Logical channel signaling messages and other functions.
3 - To provide QoS-Quality of Service, an RSVP-ReSerVation Protocol packet is sent.
3) MGCP-Media Gateway Controller Protocol is a device control protocol developed by IETF-Internet Engineering Task Force designed to control devices such as MG-Media Gateways and IAD-Integrated Access Devices. The difference between MGCP and other multimedia control protocol systems is that MGCP allows the endpoints in the network to control the communication session. MGCP is a protocol that operates between a MG-Media Gateway and a MGC-Media Gateway Controller known as Call Agents or Soft Switches. This process allows the Media Gateway Controller to control the Media Gateway.
These devices use text format (MIME means Multipurpose Internet Mail Extensions) messages to set up, manage, and terminate multimedia communication sessions (Layer 5) in a communications system. The text is formatted according to SDP-Session Description Protocol and placed in a SAP-Session Announcement Protocol packet.
That is, telephone signaling messages are sent via UDP-User Datagram Protocol packets with a SAP header and a text payload (telephone number, email, protocol, connection information, bandwidth, etc.). Once signaling is completed, RTP-Real Time Protocol packets are sent via UDP between the callers.
4) SIP-Session Initiation Protocol is an IETF-Internet Engineering Task Force signaling protocol for internet conferencing, telephony, presence, events notification (emergency calling) and instant messaging. Designed around internet applications such as HTTP-HyperText Transfer Protocol, SIP is more multi-media focused than just for voice applications. Shown here is the call flow for SIP.
This tutorial is part of SIP Essentials 2.0c available in the onsite and online courses. The online version is $299 for SIP 2.0c and for $499 as part of OCS-101 Office Communications Server online version per person or less with discounts. For more information go to http://www.techtionary.com or please call Tom Cross at 303-594-1694 or cross@gocross.com Discounts are also available to members of the SIP Forum. For a complete detailed course outline go to: http://www.techtionary.com/ocs/sip-essentials.htm
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