Cisco IP Phone

This document describes a Cisco IP phone demo system that was temporarily in a test lab. The desktop phone instrument shown in the foreground uses a regular, 10BaseT, ethernet network to communicate with the other IP phones, laptops, or telco. The laptop in the background is running the call director, the PBX equivalent software, to establish calls between instruments, PCs, or the interface to the local telco. Once the call is established, the call director does not handle any of the phone-to-phone communications. The voice signal passes via IP packets between the phones.



The teleco interface box sitting under the laptop, provides analog and/or digital ports to the phone company and has an ethernet interface to the network to support calls to and from regular phones. The ethernet switch on the bottom is just used to provide an isolated ethernet network. As shown in the next photo, the back of each phone has a two-port, 10BaseT, ethernet hub. The next version will have a two-port 10/100BaseT switch. Thus the phone can go in upstream of the user's desktop or laptop system.



Each phone has a unique IP address that is normally assigned by a DHCP server. In the case of the demo system, a non-routed 10.x.x.x address was used on an isolated network.

ISSUE: IP Address Management

  • It would be impractical to drop these phones into populated, data networks that establish class-C sized subnets for different departments. A better solution would have the phones be dynamicly addressed and routed by NAT apart from the existing data network. This would also improve phone reliability by using dedicated network equipment with the savings being in the shared, wire infrastructure
  • Current phones use a power-brick (aka, ISDN phones) which means loss of facility power makes the units go dead.
  • Overhead is very low, <64 kbs, and requires sound. During the testing, the handset had to be hung over a cooling fan to see any appreciable load.
  • Attempts to saturate a shared-hub with dual-Sniffer blasting failed to impact phone operation.
  • Latency is ~125 ms. which is only noticable if in the same room using speaker phone.
  • Wide-area operation requires attention to quality of service between the sites.
  • Recommended solution for small offices, ~50-100 people, and networked offices with QoS network connections