Basics of Cisco CallManager Express

Basics of Cisco CallManager Express

Cisco CallManager Express runs on Cisco routers and provides collaboration services to the network. Complete understanding and correct configuration is very crucial for successful operation of the system. IP phones are used via Cisco switch to handle calls.

On switching power on, the IP phone boots up and registers with the Cisco CallManager Express. On configuring, the CallManager Express assigns an extension to each IP phone. The CallManager Express router and IP phones use Skinny Client Control Protocol (SCCP) to communicate.

Given below diagram illustrates IP phones connection with CallManager Express router.

Cisco CallManager Express

The SCCP protocol is used by the CallManager Express router to control all calls of the linked IP phones. The SCCP protocol is only used between CME and IP phones.

Another protocol named as Real-time Transport Protocol (RTP) is used to carry the audio stream on set up of call. It is a common protocol that carries time-sensitive traffic like voice and real-time video inside the IP packets.

When the call session between two IP phones ends, a signal will is sent from each IP phone to CME to update new status.

VoIP traffic needs higher bandwidth and fast processing devices for high quality of voice and video data. In case of excessive traffic on the network there will be distortion, voice cutting and jittering being the most common VoIP problems faced by networks.

To avoid such problems, it is recommended to isolate the VoIP traffic from the data traffic by the creating a separate VLAN marked as the ‘Voice VLAN’ in your network. Cisco switches come with built-in mechanisms that automatically identifies and prioritize VoIP traffic. Cisco devices ensure that VoIP packets are higher prioritized than data packets.

Given below image explains how a Cisco CME router is connected to the Internet and local PSTN network along with a Cisco voice-enabled switch with a couple of workstations and IP phones.

Cisco CallManager Express-1

In this diagram, a Cisco CME router connects to the local network and some IP phones are then connected to the network and workstations. All Cisco IP phones are equipped with a RJ-45 port allowing a workstation to connect directly to the IP phone.

This scenario has the advantage of reducing the requirement of switch ports up to 50%. The link between the switch and the IP phone is set as a ‘Trunk’ link but the link between the workstation and IP phone is set as an ‘Access’ link. Trunk links allow traffic of all VLANs to pass through them but the Access links allow only specific VLAN traffic.

The Cisco CallManager Express router connects to the core switch via a trunk link, and holds access to both Data and Voice VLANs. Voice traffic is routed to the PSTN network via the Voice VLAN, whereas Workstation traffic is routed to the Internet via the Data VLAN.


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