Chapter 8. Link Efficiency Tools

QoS Exam Topics

This chapter covers the following exam topics specific to the QoS exams:

• Explain the various link efficiency mechanisms and their function

• Identify the Cisco IOS commands required to configure and monitor CB header compression

• Given a list of link speeds and a specific delay requirement, determine the proper fragment size to use at each link speed and identify the typical delay requirement for VoIP packets

• Identify the Cisco IOS commands required to configure and monitor Multilink PPP with Interleaving

• Identify the Cisco IOS commands required to configure and monitor FRF.12

Most WAN links are leased from a service provider, with one of the variables affecting the pricing being the bandwidth on the link. For instance, the distance and the bandwidth, or clock rate, on the link affect leased line pricing. Frame Relay service providers base their prices in part based on the access rate of the access links into the Frame Relay network, and the committed information rate (CIR) of the various virtual circuits (VCs).

If the offered load on the network consistently exceeds the bandwidth or clock rate of the link the traffic must flow across, unpredictable behavior can result. For example, queues consistently fill, causing more delay, jitter, and drops. If the offered load far exceeds the clock rate for a period of time, most data applications slow down significantly, with voice and video streams possibly even becoming unusable. Depending on how you configure quality of service (QoS) in the network, some traffic types may perform as expected, but with a likely result of allowing some other traffic types to degrade even more quickly, because most QoS tools by design end up favoring one type of traffic over another.

This chapter covers two classes of QoS tools that directly impact the usage of the bandwidth in a networkcompression tools and link fragmentation and interleaving (LFI) tools. Compression tools compress the number of bytes in a packet so that fewer bytes need to be sent over a link.

LFI tools directly impact serialization delaysand serialization delay is impacted by actual link bandwidth. The slower the link, the longer it takes to serialize a packet. If a small packet must wait on a large packet to be serialized onto a link, the small packet may experience too much delay, particularly on slow-speed links. LFI tools reduce the delay experienced by a short packet by breaking larger packets into smaller pieces, and by transmitting the original small packets in between the fragments of the original larger packets. Smaller packets get better service, and in many cases, smaller packets are part of a delay-sensitive application, such as Voice over IP (VoIP).