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WAN capacity planning: Building a methodology

Successful WAN capacity planning depends on many factors and how they work together. This article provides network managers with a starting point for understanding how to measure and predict network capacity and provides a 10-step methodology to simplify the process.

When managing wide area network (WAN) capacity, it's not enough to buy additional leased lines and hope for the...

best. The key question when addressing network capacity planning is, "How much is enough?" Network managers must decide if it makes sense to configure the minimum amount of bandwidth or to over-engineer the network. They must regularly review network performance to know what capacity is needed, now and in the future. And they must always balance the need to meet user performance expectations against the available budget.

WANs are typically built with high-capacity node-to-node links such as DS3 (45 Mbps) channels, with lower-capacity links such as DS1 (1.544M bps) connecting from the major nodes to office locations. Internet-based networks, such as virtual private networks (VPNs), use low- to high-speed access (based on the requirements) from the originating user site to an Internet node, and the same or similar configuration at the terminating site.

Predicting application performance
Before deploying a new application on the network, networking professionals must first assess its possible impact on WAN resources. The following factors should be considered, as they can affect the delivery of data (e.g., frames, packets and segments):

  • Latency: Because it can take a long time for packets to be delivered across WANs, be sure to use protocols in which receivers acknowledge delivery of data, as this can measure round-trip time

  • Packet loss: It is possible, even in the best networks, for intermediate devices to lose packets. This may be due to errors, to overloading of the intermediate network, or to intentional discarding of traffic to enforce a particular service level

    Best practices for WAN design and capacity planning
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    WAN capacity planning: Building a methodology

    Determining WAN bandwidth requirements
  • Retransmission: When packets are lost in a reliable network, they are retransmitted. However, this incurs two delays: 1) the delay from resending the data; and 2) the delay resulting from waiting until the data is received in the correct order before forwarding it up the protocol stack

  • Throughput: This describes the amount of traffic a network can carry, usually in terms such as kilobits or megabits per second.

These factors, and others -- such as the impact of network signaling, compression and encryption -- affect WAN performance. Fortunately, capacity planning solutions are available to ensure that WAN traffic is effectively managed to benefit all users.

The many facets of capacity planning
Network managers perform many tasks that factor into capacity planning, such as performance measurement, forensic analysis, load-testing and/or load generation. They are constantly receiving and collecting data from these activities. They also collaborate with application developers and IT departments who in turn depend on them to deliver network services. Let's briefly examine some of these activities:

  • Performance measurement: Network managers measure WAN performance using different parameters, such as per-port metrics (traffic volume on port 80 between clients and servers and the elapsed time) or end-user metrics (speeds of key functions)

  • Forensic analysis: Network operators can use sniffers to break down transactions by protocol and locate problems, e.g., retransmissions or protocol negotiations

  • Load generation: Software, network appliances or managed service providers can generate scripted traffic; this helps determine when network capacity has been reached.

Additional capacity planning factors include:

  • WAN design or topology
  • Existing voice and data traffic on the network
  • Devices connected to the network, e.g., routers, switches, PBXs, domain controllers
  • Connectivity to the public switched telephone network (PSTN), if needed
  • Network redundancy and resilience
  • Network security requirements
  • Growth of data volumes over time
  • Number of concurrent users
  • User response times
  • Long-running versus intermittent applications

Capacity planning methodology
Let's assume that WAN capacity planning will be performed once the network has been installed and is running. This makes sense because network operation is ever-changing. Thus, capacity planning ought to be a fairly regular activity. The steps below outline a simple methodology for capacity planning success.

  1. Understand the network's activities, e.g., the work it's expected to support. This includes applications such as email, voice communications, and remote access
  2. Discuss short-, medium- and long-term service and usage requirements of network users
  3. Gather network performance data from endpoints, routers, switches and other devices
  4. Review network performance audit reports, if available
  5. Analyze bandwidth usage and whether the existing network infrastructure is sufficient for the demand
  6. Determine the number and type of devices the network will need to support at the present and in future time frames (six months, 12 months)
  7. Use capacity planning and network design tools to analyze network configurations
  8. Based on results of network tools, determine the aggregate amount of bandwidth required, and correlate this into the type of circuit(s) that will support the traffic;
  9. Analyze the need for network redundancy and how this will translate into additional capacity;
  10. Test and validate the new WAN configuration using traffic generators as mentioned earlier along with network design software.

While capacity planning can take volumes to describe adequately, in this article we have offered a simple yet effective approach to the subject.

About the author: Paul F. Kirvan, FBCI, CBCP, CISSP, has more than 35 years experience in telecommunications and information technology as a practitioner, consultant, author and educator. He also specializes in business continuity and disaster recovery.

This was last published in March 2009

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