Case Study: Your Turn to Put the Pieces of the Puzzle Together


You designed a telephony network for the XYZ Company in the case study in Chapter 1, "Touring the History Museum of Telephony." This design was based on traditional PBX and key system technologies. Then, in Chapter 2, "Making Waves: Turning Your Voice into Zeros and Ones," you calculated the bandwidth required to interconnect the XYZ Company's headquarters with two of its remote offices.

Based on your previous recommendations, the XYZ Company has decided to interconnect its PBXs and key system over the IP WAN, as opposed to using the T1 and fractional T1 connections specified in Chapter 1's case study. Therefore, your goal in this case study is to create a design that places a router at each XYZ Company location and to specify the router interfaces used to connect remote routers. Also, each XYZ Company location needs connections to its local CO as follows:

  • Headquarters48 connections to local CO

  • Remote Office 124 connections to local CO

  • Remote Office 25 connections to local CO

In addition to the required voice bandwidth you calculated in Chapter 2's case study, the links in this converged network solution also need to transport data. The data bandwidth requirements are as follows:

  • Link from HQ to Remote Office 1: 768 kbps for data

  • Link from HQ to Remote Office 2: 128 kbps for data

Demonstrate in your design how the router at each XYZ Company location connects into its local CO. This VoIP design lays the foundation for eventually replacing the PBXs and key system units with IP telephony components such as the Cisco CallManager, as discussed in the next chapter.

Use the following area to sketch your design.

Design for XYZ Company:

Suggested Solution

Although multiple solutions exist for the design scenario presented, the following is a suggested solution that meets the design criteria. The router located at the XYZ Company HQ needs WAN connectivity with each remote office. Based on the bandwidth requirements calculated in Chapter 2's case study, we have the following voice bandwidth requirements for each of these WAN links:

  • Link from HQ to Remote Office 1: 2026.08 kbps for voice

  • Link from HQ to Remote Office 2: 337.68 kbps for voice

In addition to voice, these intersite links need to carry data. The data requirements specified in the case study were:

  • Link from HQ to Remote Office 1: 768 kbps for data

  • Link from HQ to Remote Office 2: 128 kbps for data

To accommodate both voice and data, we sum the required bandwidth (that is, the voice bandwidth plus the data bandwidth) for these links to calculate each link's total required bandwidth:

  • Link from HQ to Remote Office 1: 2794.08 kbps total

  • Link from HQ to Remote Office 2: 465.68 kbps total

Although we could select from various WAN technologies to interconnect these sites (for example, Frame Relay, Point-to-Point Protocol [PPP], or Asynchronous Transfer Mode [ATM]), this suggested solution uses PPP links. The headquarters connects to Remote Office 1 using two T1s. Two T1s actually provide more bandwidth than the required 2.794 Mbps. However, these two T1s allow room for future growth. In this design, the two T1s are combined into a single logical link, using Multilink PPP (MLPPP).

The headquarters connects to Remote Office 2 using a fractional T1, running at 512 kbps. Again, the allocated bandwidth is a little more than the required 465.68 kbps, thus allowing for future growth.

The routers at each site should also connect to their local CO. As a best practice, once we exceed eight connections, we should choose digital interfaces instead of analog interfaces, due to cost considerations. Therefore, this suggested design selected the following CO connections for the XYZ Company routers:

  • Headquarters Router: Two T1 connections

  • Remote Office 1 Router: One T1 connection

  • Remote Office 2 Router: Five FXO connections

These suggested design solutions result in the design shown in Figure 3-19.

Figure 3-19. XYZ Company's Suggested Solution





Voice over IP First-Step
Voice over IP First-Step
ISBN: 1587201569
EAN: 2147483647
Year: 2005
Pages: 138
Authors: Kevin Wallace

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