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The methodology used when designing the Enterprise Edge is called prepare, plan, design, implement, operate, and optimize (PPDIOO). Some keys to PPDIOO are the processes of analyzing network requirements, characterizing the existing network, and designing the topology:

• Analyzing the network requirements includes reviewing the types of applications, the traffic volume, and the traffic patterns in the network.
• Characterizing the existing network reviews the technologies used and the locations of hosts, servers, network equipment, and other end nodes.
• Designing the topology is based on the availability of technology, the projected traffic usage, network performance, constraints, reliability, and implementation planning.

New network designs should be flexible and adaptable to future technologies and should not limit the customer's options going forward. Voice over IP (VoIP) is an example of a technology that network designs should be able to support if the customer decides to move to a converged network. The customer should not have to undergo major hardware and software upgrades to implement these types of technologies. Another important consideration is the design's cost-effectiveness throughout the design and implementation stages. For example, the support and management of the network should be an important factor.

Response Time

Response time measures the time between the client user request and the response from the server host. The end user will accept a certain level of delay in response time and still be satisfied. However, there is a limit to how long the user will wait. This amount of time can be measured and serves as a basis for future application response times. Users perceive the network communication in terms of how quickly the server returns the requested information and/or how fast the screen updates. Some applications, such as a request for an HTML web page, require short response times. On the other hand, a large FTP transfer may take a while, but this is generally acceptable.

Throughput

In network communications, throughput is the measure of data transferred from one host to another in a given amount of time. Bandwidth-intensive applications have more of an impact on a network's throughput than interactive traffic such as a Telnet session. Most high-throughput applications usually involve some type of file-transfer activity.

Reliability

Reliability is the measure of a given application's availability to its users. Some organizations require rock-solid application reliability; this has a higher price than most other applications. For example, financial and security exchange commissions require nearly 100 percent uptime for their applications. These types of networks are built with a high amount of physical and logical redundancy. It is important to ascertain the level of reliability needed for a network that is being designed. Reliability goes further than availability by measuring not only whether the service is there but whether it is performing as it should.

Bandwidth Considerations

Table 5-4 compares a number of different WAN technologies, along with the speeds and media types associated with them.

Table 5-4. Physical Bandwidth Comparison

Bandwidth	Less Than 2 Mbps	2 Mbps to 45 Mbps	45 Mbps to 100 Mbps	100 Mbps to 10 Gbps
Copper	Serial, ISDN, Frame Relay, TDM, DSL	Frame Relay, Ethernet, DSL, cable, T3	Fast Ethernet	Gigabit Ethernet
Fiber	—	Ethernet	FastEthernet, ATM	Gigabit Ethernet, 10Gigabit Ethernet, ATM, SONET/SDH, POS, dark fiber
Wireless	802.11b	802.11b, wireless WAN (varies)	802.11a/g	802.11n

The WAN designer must engineer the network with enough bandwidth to support the needs of the users and applications that will use the network. How much bandwidth a network needs depends on the services and applications that will require network bandwidth. For example, more bandwidth is needed for VoIP traffic than interactive SSH traffic. A large number of graphics or CAD drawings require an extensive amount of bandwidth compared to simple text-based information being transferred on the network, such as HTML files.

When designing bandwidth for the WAN, remember that implementation and recurring costs are always important factors. QoS techniques become increasingly important when delay-sensitive traffic such as VoIP is using the limited bandwidth available on the WAN.

LAN bandwidth, on the other hand, is inexpensive and plentiful. To provide connectivity on the LAN, you typically need to be concerned only with hardware and implementation costs.

Window Size

The window size defines the upper limit of frames that can be transmitted without getting a return acknowledgment. Transport protocols such as TCP rely on acknowledgments to provide connection-oriented reliable transport of data segments. For example, if the TCP window size is set to 8192, the source stops sending data after 8192 bytes if no acknowledgment has been received from the destination host. In some cases the window size might need to be modified because of unacceptable delay for larger WAN links. If the window size is not adjusted to coincide with the delay factor, retransmissions can occur, which affects throughput significantly. It is recommended that you adjust the window size to achieve better connectivity conditions.

Data Compression

Compression reduces the packet to a smaller size that can be transmitted and then decompressed on the other side of the WAN link. More CPU or hardware time is required to compress and decompress the data, but in return this saves bandwidth and reduces delay on the WAN link.

Compression is available in both software and hardware. Hardware data compression aids the main CPU by offloading the compression and decompression tasks by using the hardware CPU instead. The hardware compression modules can be installed in an available slot on a modular router.