Voice Gateway Functions on a Cisco Router
The Cisco family of voice gateways, including integrated services routers (ISR), provide connectivity between analog interfaces, digital interfaces, and IP Telephony devices. Examples of analog interfaces are FXS and FXO. Examples of analog devices are analog phones, fax machines, and modems. T1/E1 and BRI are examples of digital interfaces. A PBX is usually connected to a gateway using T1/E1 interfaces, even though using an E&M interface is also possible. You can set up a gateway connection to the PSTN CO switch over a T1/E1 or an E&M connection. You can configure a gateway T1/E1 for CCS, where one channel is dedicated to signaling such as ISDN Q.931 or QSIG, and the rest of the channels are available for data or digital voice signals. You can also configure a gateway T1/E1 as CAS. When configured for CAS, a T1 interface can have all 24 channels available for data/digital voice, but each channel loses a few bits to signaling; for this reason, CAS is also referred to as robbed bit signaling (RBS). A gateway can have one or more LAN and WAN interfaces, such as Fast Ethernet, synchronous Serial interface, and ATM.
Gateways convert analog signals to digital and digital signals to analog. They might also be able to handle several different types of codecs. These capabilities depend on the DSPs installed in that gateway and its IOS feature set. DSPs also allow gateways to provide transcoding and conferencing services. Cisco IOS routers (gateways) support the most common VoIP gateway signaling protocols, namely H.323, SIP, and MGCP.
SRST is a useful IOS feature on gateways at remote sites with no CallManager servers. The IP phones at these types of sites communicate with and receive services from CallManager servers at another branch, such as a central branch. If the IP connectivity between the central and remote branch is lost, the IP phones at the remote branch are dysfunctional, unless the gateway of the remote site has the SRST feature. With SRST, the IP phones at the remote site survive, can call among themselves, and have limited features such as hold and transfer. However, the gateway with SRST has to route all other calls to the PSTN.
The IOS on certain Cisco routers and switches has the Cisco Unified CallManager Express feature. This feature allows the gateway to act as a complete CA (CallManager) for the IP phones at a branch. This is not disaster recovery, but a permanent solution or option for smaller branches.
In addition to the features listed, the Cisco gateways offer fax relay, modem relay, and DTMF relay services. Other features such as Hot Standby Routing Protocol (HSRP), Virtual Router Redundancy Protocol (VRRP), and Gateway Load Balancing Protocol (GLBP) provide fault tolerance and load sharing among redundant gateways.
Continue reading here: Cisco Unified Call Manager Functions
Was this article helpful?