It is expensive to transmit data over a WAN. Therefore, one of many different techniques—such as data compression, bandwidth combination, tuning window size, congestion management (queuing and scheduling), congestion avoidance, and traffic shaping and policing—can be used to optimize bandwidth usage and improve overall performance. The following sections describe these techniques.

Data Compression

Compression is the reduction of data size to save transmission time. Compression enables more efficient use of the available WAN bandwidth, which is often limited and is generally a bottleneck. Compression allows higher throughput because it squeezes packet size and therefore increases the amount of data that can be sent through a transmission resource in a given time period.

Cisco IOS software compression support includes the following data software compression types:

■ FRF.9 Frame Relay Payload Compression

■ Link Access Procedure Balanced payload compression using the Lempel-Ziv Stack (LZS) algorithm, which is commonly referred to as the Stacker (STAC) or Predictor algorithm

■ HDLC using LZS

■ X.25 payload compression of encapsulated traffic

■ PPP using Predictor

■ Van Jacobson header compression for TCP/IP (conforms to RFC 1144)

■ Microsoft Point-to-Point Compression

Bandwidth Combination

PPP is commonly used to establish a direct connection between two devices; PPP is a Layer 2 protocol for connection over synchronous and asynchronous circuits. For example, PPP is used when connecting computers using serial cables, phone lines, trunk lines, cellular telephones, specialized radio links, or fiber-optic links. As mentioned earlier, ISPs use PPP for customer dialup access to the Internet. An encapsulated form of PPP (PPPoE or PPPoA) is commonly used in a similar role with DSL Internet service.

Window Size

Window size is the maximum number of frames (or amount of data) the sender can transmit before it must wait for an acknowledgment. The current window is defined as the number of frames (or amount of data) that can be sent at the current time; this is always less than or equal to the window size.

Queuing to Improve Link Utilization

To improve link utilization, Cisco has developed QoS techniques to avoid temporary congestion and to provide preferential treatment for critical applications. QoS mechanisms such as queuing and scheduling, policing (limiting) the access rate, and traffic shaping enable network operators to deploy and operate large-scale networks that efficiently handle both bandwidth-hungry (such as multimedia and web traffic) and mission-critical applications Queuing allows network administrators to manage the varying demands of applications on networks and routers. When positioning the role of queuing in networks, the primary issue is the duration of congestion.

Note: Queuing is configured on outbound interfaces and is appropriate for cases in which WAN links are congested from time to time.

Following are the two types of queues:

■ Hardware queue: Uses a FIFO strategy, which is necessary for the interface drivers to transmit packets one by one. The hardware queue is sometimes referred to as the transmit queue or TxQ.

■ Software queue: Schedules packets into the hardware queue based on the QoS requirements. The following sections discuss the following types of queuing: weighted fair queuing (WFQ), priority queuing (PQ), custom queuing (CQ), class-based WFQ (CBWFQ), and low-latency queuing (LLQ) .

Congestion Avoidance

Congestion-avoidance techniques monitor network traffic loads so that congestion can be anticipated and avoided before it becomes problematic. If congestion-avoidance techniques are not used and interface queues become full, packets trying to enter the queue are discarded, regardless of what traffic they hold. This is known as tail drop—packets arriving after the tail of the queue are dropped.