Call Agent

In a telecommunications system, a call agent is a Media Gateway Controller (MGC) when used in the context of Media Gateway Control Protocol (MGCP). It is concerned with the handling of specific services to users.^[1] MGCP is a server client protocol developed by Cisco, to make industry standard. MGCP is the only server-client voice communication protocol in existence. Other protocols SIP and H323 are called as peer-to-peer protocols. MGCP being an open standard, anyone can use it and create their own Call Agent; for example "Cisco Call Manager", other type of Call agent Examples are AVAYA IP EPABX, Nortel CS1000E etc., these are not using MGCP or SCCP but some proprietary signalling pattern between the call-agents and End-points.

A call Agent controls the signalling communication between phones, Media gateways like routers on which PRI lines Terminate, ex Cisco 28xx Series Routers (now new versions of Cisco Voice Routers are released 29xx and 39xx), Media Gateways like Analog Extensions (also called as FXS in Cisco devices context like Cisco VG224 and VG248, VG202 and VG204) and Analog Trunks (also called FXO in Cisco Device Context). Call Agent is responsible to register the end devices like phones and media gateways, which act as dump terminals. After one has dump extension registered to the call agent, when end device phone instrument handset (headphone or speaker) is picked up, the phone sends signal to call-agent and informs that the phone handset has been picked up what to do, Call Agent instructs the phone to give the dial tone. After the user listens to the dial tone, user enters the destination number to which they want to dial. The phone will send each digit immediately that is dialed by user one by one (i.e. the first digit will be sent first to call-agent immediately it is dialed then second and so on).

Call agent receives the digits dialed by the phone. Call agent identify that to which destination Extension the dialed (called) number belongs to. If number does not belong to any extension, call-agent checks for which trunk does the dialed number belongs to. After the destination is identified Call-agent sends the information to the destination extension or to the destination Trunk, with Details like calling Party number, Called party number, bandwidth to be used between the devices to send the voice. If the destination is an extension that is registered to the call agent, then the call-agent will instruct the phone instrument to send the ring out of the ring buzzers of the phone and simultaneously instruct the calling phone instrument to play the ringing tone. After the rings are played, and when the destination person answers the phone, Call-agent instructs the phones to send the voice traffic, called as "RTP Voice (Real Time Protocol) Traffic" directly to each other.

After this the end devices (so called dump terminals or client to the MGCP Server-client architecture) sends the Voice directly to each other. After the conversation the Call-Agent signals the phone to disconnect the call.

Thus from above we can understand that the Call-Agents are the Brains for the complete infrastructure and the destination end devices i.e. phone, PRI, CO (FXO), Extension (FXS) Media Gateway are under control of Call-Agent instructions for each activity behaving as dumb terminals.

Call agent handles

Basic features

a) Phone numbers switching logic
b) Call control
c) Agent/Endpoint registration

Advanced features

d) Bandwidth management (This is a function of Gatekeeper also)
e) Quality of Service (QOS) management (this is a function of gatekeeper also)
f) centralized configuration

Call Agents all of above features for all sites under the central controller. A central gateway controller includes both centralized configuration and maintenance of call control functionality. When new functionality needs to be added, only the controller needs to be updated.

Redundancy

For Current Critical voice industry it is evident to have a Highly stable and highly available setup available to sustain the uninterrupted Business, to achieve this Call-Agents come with Capability of Redundancy. Redundancy is provided in terms of

1. Hardware
2. Software

Hardware redundancy

To achieve high availability, Call agents are installed on multiple servers (hardware, Servers are provided with
a) Multiple Processors
b) Multiple hard disks
c) Multiple RAM (random access memory)
d) Multiple of Ethernet (LAN) ports, and these ports are Bundled at Server End (also called as Teaming)connected on Multiple LAN Switches to achieve Network Level Redundancy.
e)Multiple power Supply Inputs There are different type and Models of Server available in Market by well known Manufacturers Like, IBM, HP, etc. Note: compatibility of the Hardware Make,Model and Capacity with the software should be checked prior installing the software.

Software redundancy

Call Agents use a database to keep information about

• Phones : MAC Address, Extensions, Registration Status, IP Address, Calling Restriction
• Gateway: PRI, FXO, FXS Registration Status
• Trunks and Trunks Calling Patterns
• Extension Calling patterns
  etc.

To achieve redundancy, most of the call agents, when installed on multiple hardware, copy their database to the redundant hardware servers, such that if the primary hardware fails, the phones can get registered to the secondary hardware with software consisting of complete Database and the operations continue uninterrupted.

Note: Generally when the Primary Servers fails and Phones try to Register Secondary or Backup Call agents, Phone instruments take a complete reboot. But if there is an active call going on the phone do get reboot and re-register after the completion of call, as at that point of time phone will search for the Call-agent to send the disconnect signal as explained above.

References