Application Requirements of WAN Design

Just as application requirements drive the Enterprise Campus design (as illustrated in Chapter 4, "Designing Basic Campus and Data Center Networks"), they also affect the Enterprise Edge WAN design. Application availability is a key user requirement; the chief components of application availability are response time, throughput, packet loss, and reliability. Table 5-2 analyzes these components, which are discussed in the following sections.

Table 5-2 Application Requirements on the WAN

Requirement

Data File Transfer

Data Interactive Application

Real-Time Voice

Real-Time Video

Response time

Reasonable

Within a second

150 ms of one-way delay with low jitter

Minimum delay and jitter

Throughput

High

Low

Low

High

Packet loss tolerance

Medium

Low

Low

Medium

Downtime (high reliability has low downtime)

Reasonable

Low

Low

Minimum

^ Zero downtime for mission-critical applications ^

Response Time

KEY POINT

Response time is the time between a user request (such as the entry of a command or keystroke) and the host system's command execution or response delivery.

Users accept response times up to some limit, at which point user satisfaction declines. Applications for which fast response time is considered critical include interactive online services, such as point-of-sale machines.

NOTE Voice and video applications use the terms delay and jitter, respectively, to express the responsiveness of the line and the variation in the delays.

Throughput

KEY POINT

In data transmission, throughput is the amount of data moved successfully from one place to another in a given time period.

Applications that put high-volume traffic onto the network have more effect on throughput than interactive end-to-end connections. Throughput-intensive applications typically involve filetransfer activities that usually have low response-time requirements and can often be scheduled at times when response-time-sensitive traffic is low (such as after normal work hours). This could be accomplished via time-based access lists, for example.

Packet Loss

KEY POINT

In telecommunication transmission, packet loss is expressed as a bit error rate (BER), which is the percentage of bits that have errors, relative to the total number of bits received in a transmission.

BER is usually expressed as 10 to a negative power. For example, a transmission might have a BER of 10 to the minus 6 (10-6), meaning that 1 bit out of 1,000,000 bits transmitted was in error. The BER indicates how frequently a packet or other data unit must be retransmitted because of an error. A BER that is too high might indicate that a slower data rate could improve the overall transmission time for a given amount of transmitted data; in other words, a slower data rate can reduce the BER, thereby lowering the number of packets that must be re-sent.

Reliability

Although reliability is always important, some applications have requirements that exceed typical needs. Financial services, securities exchanges, and emergency, police, and military operations are examples of organizations that require nearly 100 percent uptime for critical applications. These situations imply a requirement for a high level of hardware and topological redundancy. Determining the cost of any downtime is essential for determining the relative importance of the network's reliability.

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