C ALL A GENT OR C ALL C ONTROLLER )

The media gateway controller maintains the “call state” for the endpoints of every call on a media gateway. Its primary role is to provide the call logic and call control signaling for one or more media gateways. It may also:

f Manage network resources such as ports or bandwidth.

f Interact with the application server to provide a service or feature to a user.

f Interface with call routing and accounting functions f Participate in management tasks in a mobile

communi-cations environment.

The media gateway controller interfaces with the applications server or media server using SIP (session initiation protocol).

The softswitch or the application server uses what is known as third party call control to control the media server.

f ff f

f M^EDIA G^ATEWAY (^{A SUB}-COMPONENT OF A

MEDIA GATEWAY CONTROLLER)

As viewed by an IP network, a media gateway is an endpoint or a collection of endpoints. Its primary role is to transform me-dia from one transmission format to another, most often from

circuit to packet formats or from analog/ISDN circuit to packet.

It is always controlled by a media gateway controller.

The media gateway interfaces the IP network with an ac-cess endpoint or network trunk, or a collection of endpoints and/or trunks. As such, it serves at the gateway between the packet and the external networks such as the PSTN, cellular network, etc. For example, the media gateway can provide the gateway between an IP and circuit network (e.g. IP to PSTN), or between two packet networks (e.g. IP to 3G mobile networks or ATM) Its primary role is to transform media from one transmission format to another, most often between cir-cuits and packets, between ATM packets and IP packets, or between analog/ISDN circuits and packets.

f The media gateway always has a master/slave relationship with the media gateway controller

f It may perform media processing functions such as transcoding, packetization, echo cancellation, jitter buffer management, packet loss compensation, etc.

f It may perform media insertion functions such as call progress tone generation, DTMF (touchtone) generation, comfort noise generation, etc.

SIP – Session Initiation Protocol

A protocol is a set of rules governing the format of messages that are exchanged between computers and people. Session Initiation Protocol is a protocol for the establishment, modification and termination of conferencing and telephone sessions over IP-based net-works. SIP uses text based messages much like HTTP (Hyper Text Transfer Protocol used for moving docu-ments around the Internet.)

– Compliments of Newton’s Telecom Dictionary

f It may perform signaling and media event detections such as DTMF (touchtone) detection, on/off hook detection, voice activity detection, etc.

Many media gateways are controlled with media gateway control protocol (see box describing MGCP).

Note: An earlier meaning of “softswitch” referred to the me-dia gateway and controller. Now softswitch has come to refer to the interaction all of these servers mentioned here and their col-lective operation.

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The routing function provides call routing information to the media gateway controller, while the accounting function collects call accounting information for billing purposes. The accounting function can also have a broader role of authentication, authori-zation and accounting in remote access networks.

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The signaling gateway provides the gateway for signaling between a VoIP network and the PSTN, whether SS7/TDM or ATM based. It encapsulates and transports PSTN signaling pro-tocols over IP.

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The applications server is the application execution entity.

Its primary role is to provide the service logic and execution for one or more applications and/or services.

The applications server provides enhanced service logic for applications such as voice mail, a conference bridge or a pre-paid calling card service. Other applications include delivering services and information to an end user. It can access data net-works such as the Internet to gain access to information systems to provide services to the user.

For example in a voice mail application, the applications server instructs a media server to play the ring that the caller hears, prompts the caller to enter the digits (touchtone) and re-port the collected digits back to the applications server. The applications server verifies the passcode and if the caller is a user, it determines whether there are messages.

A Feature Server is a limited Applications Server. It uses a proprietary API (applications programming interface) while the Applications Server uses a standard protocol, SIP.

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A media server offers its resources to applications servers, feature servers and media gateway controllers. Its primary role is to operate as a server that handles requests from the applications server or the media gateway controller for performing media pro-cessing on packetized media streams. It supports multiple concurrent capabilities including:

f Digit detection and collection

f Streaming of tones and announcements f Algorithmic tone generation

f Recording of multimedia streams f Speech recognition

f Speech generation from text f Conference bridging

f Fax processing f Transcoding

f Voice activity detection and loudness reporting f Scripted combinations of the above

You may have a single media server supporting multiple ap-plications servers, with apap-plications such as voice mail, network announcements (“the number you have called is not in service at this time… “) and pre-paid calling cards.

A gateway takes the signal and digitizes it (if it is not already in a digital form); potentially compresses it, depending on the codec; packetizes it for transmission; and routes the packet to the destination. Note that all of these steps take time. This added time is called latency. Latency is one of the most important fac-tors contributing to the perceived quality of the voice transmission.

Low latency results in good quality, while long latency results in poor quality.

The gateway reverses the operation for packets coming from the packet network ending up on the Public Switched Telephone Network (PSTN). Both operations (coming and going) take place simultaneously at each end-point, allowing a full two-way conversation.

A transcoder takes a media stream in one codec (stands for coder/decoder) and converts it to another. Media gateways take voice signals from their analog form into digital signals and vice-versa.

A softswitch network may connect to the existing public switched telephone network (PSTN) in three different ways:

f 1. Physical media-path connections, through the circuits that connect media gateways to circuit switches.

f 2. Physical signaling-path connections, through the cir-cuits that connect signaling gateways to the SS7.

f 3. Logically, the softswitch may access network elements in the PSTN for routing, billing, emergency services and other value added services as needed by using the physical SS7 connection to exchange messages with other SS7-connected network elements.

The early softswitch applications were Internet offload (see box) and toll bypass (where long distance voice traffic is trans-ported as packets). More recent applications replicate the functions of today’s PBXs and the PSTN to which users are

already accustomed. Even newer applications that will be adopted depend upon the marketplace, but may include things such as web-based collaboration and unified messaging.

Media Gateway Control Protocol

In November, 1998 Level 3 Communications, Inc.

and Bellcore (now Telcordia Technologies) announced the merger of their respective technical specifications for a new protocol designed to bridge between the con-ventional circuit-switched PSTN (Public Switched Telephone Network) and the emerging IP (Internet Protocol) packet switched networks. The merged speci-fication, Media Gateway Control Protocol (MGCP) represents a combination of the Internet Protocol De-vice Control (IPDC) specification developed by a consortium formed by Level 3 and made up of leading communications hardware and software companies, and the Simple Gateway Control Protocol (SGCP) de-veloped by Bellcore and Cisco Systems. MGCP is a draft specification (not a standard accepted by IETF) for a physically decomposed gateway, i.e. a protocol converter, the components of which may be distrib-uted across multiple physically distinct devices. MGCP enables external control and management of data com-munications equipment operating at the edge of emerging multi-service packet networks – known as

“media gateways” – by software programs known as call agents or media gateway controllers. Examples of media gateway controllers include voice over IP gate-ways, voice over ATM (asynchronous transfer mode) gateways, modem banks, cable modems and set top boxes, soft PBXs and circuit cross-connects.

– Compliments Newton’s Telecom Dictionary

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This is a network separate from the one that is carrying all of the telephone traffic. That network is called SS7, Signaling Sys-tem 7. It acts as a traffic cop for PSTN network traffic, assigning highways and routing customer traffic. SS7 is what makes the long distance carriers able to set up coast-to-coast calls in less than one second.

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