Telephony design - in detail

Priority business requirement with today's enterprise companies is to reduce their capital and support costs while improving the effectiveness of their network infrastructure, security and management. Cisco has developed IP Telephony solutions, which address those requirements while allowing for migration from current PBX switches in a cost effective manner. Current PBX switches are both expensive to purchase and support. IP Telephony offers an alternative that replaces the company PBX with Call Manager software that will run on a Windows server. The current version of Call Manager has features such as unified messaging integration, call routing with fail over, easy migration from the current PBX infrastructure, and management that is integrated with CiscoWorks.

Since Call Manager was designed to replace your PBX, there are all of the telephony features that you would expect with any voice switch. It is important to distinguish between IP Telephony and Voice over IP, Voice over Frame Relay and Voice over ATM. The voice router modules interface with the existing PBX at each company office and don't use IP phones. IP Telephony will replace the PBX switch at each office with Call Manager for switching and routing of all voice traffic. As well the data circuits are utilized with Call Manager as with Voice over IP. There is a capital expenditure with the purchase of IP Phones which are designed to work with Call Manager software. As well there is a cost effective option called a Soft Phone, which runs as an application at the client desktop.

The employee requirements should be considered to determine which is most appropriate for the employee. As mentioned there is diversity (fail over) built into IP Telephony from a circuit, call routing and IP Phone perspective. The current PSTN circuit can be utilized for fail over should the IP WAN circuit be unavailable. The PSTN circuit is configured to transport voice traffic during periods when the IP WAN circuit is congested or when it is unavailable. Call routing diversity is provided with Call Manager clustering which designates a group of servers that continue processing calls when a server isn't available. The IP Phone can be plugged into a different campus switch port than the desktop or it can plug into the same port and assign separate vlans to data and voice traffic. Plugging the phone into a different switch than what the desktop uses will prevent both phone and switch being unavailable should a campus switch fail however that becomes very expensive.

Quality of service is important with any voice migration project since voice traffic is delay sensitive. Bandwidth must be allocated to reduce or eliminate jitter. IP phones can prioritize voice traffic before it arrives at the campus switch. The campus switch examines each packet and will allocate bandwidth for packets that are designated as higher priority.