QoS: Tuning Bandwidth, Delay, Jitter, and Loss 13

minimum amount of bandwidth to a particular queue. Figure 1-4, for example, shows a twoqueue system. The first queue gets 25 percent of the bandwidth on the link, and the second queue gets 75 percent of the bandwidth.

Figure 1-4 Bandwidth Reservation Using Queuing

With regard to Cisco IOS Software queuing tools that reserve bandwidth, if both queues have packets waiting, the algorithm takes packets such that, over time, each queue gets its configured percentage of the link bandwidth. If only one queue has packets waiting, that queue gets more than its configured amount of bandwidth for that short period.

Although adding more bandwidth always helps, the tools summarized in Table 1-3 do help to improve the efficient utilization of bandwidth in a network.

Table 1-3 QoS Tools That Affect Bandwidth

Delay

All packets in a network experience some delay between when the packet is first sent and when it arrives at its destination. Most of the concepts behind QoS mechanisms relate in some way to delay. Therefore, a deeper look into delay is useful. Take a look at Figure 1-5; this sample network is used often in this book.

201

301

At what points will delay occur in this network? Well, at all points, in actuality. At some points in the network, the delay is so small that it can just be ignored for practical purposes. In other cases, the delay is significant, but there is nothing you can do about it! For a fuller understanding, consider the following types of delay:

Each of these types of delay is explained over the next several pages. Together, the types of delay make up the components of the end-to-end delay experienced by a packet.

Serialization Delay

Imagine you are standing at a train station. A train comes by but doesn’t stop; it just keeps going. Because the train cars are connected serially one to another, a time lag occurs between when the engine car at the front of the train first gets to this station and when the last car passes by. If the train is long, it takes more time until the train fully passes. If the train is moving slowly, it

QoS: Tuning Bandwidth, Delay, Jitter, and Loss 15

takes longer for all the cars to pass. In networking, serialization delay is similar to the delay between the first and last cars in a train.

Serialization delay defines the time it takes to encode the bits of a packet onto the physical interface. If the link is fast, the bits can be encoded onto the link more quickly; if the link is slow, it takes longer to encode the bits on the link. Likewise, if the packet is short, it does not take as long to put the bits on the link as compared with a long packet.

Use the following formula to calculate serialization delay for a packet:

#bits sent

-------------------------

Link speed

Suppose, for instance, that Hannah send a 125-byte packet to Server1. Hannah sends the packet over the Fast Ethernet to the switch. The 125 bytes equal 1000 bits, so at Fast Ethernet speeds, it takes 1000 bits/100,000,000 bits per second (bps), or .01 ms, to serialize the packet onto the Fast Ethernet. Another .01 ms of serialization delay is experienced when the switch sends the frame to R1. (I ignored the data-link header lengths to keep the math obvious.)

Next, when that same packet leaves R1 over a 56 kbps link to R2, serialization takes 1000 bits/ 56,000 bps, or 17.85 ms. The serialization component over Fast Ethernet is insignificant, whereas serialization becomes a more significant number on lower-speed serial links. Figure 1-6 shows the various locations where the packet from Hannah to Server1 experiences serialization delay.

201

301

As Figure 1-6 shows, serialization delay occurs any time a frame is sent. On LAN links, the delay is insignificant for most applications. At steps 3 through 6 in the figure, the serialization delay is 17.85 ms, 7.8 ms, .02 ms, and .65 ms for the 125-byte packet, respectively. Also note

QoS: Tuning Bandwidth, Delay, Jitter, and Loss 17

Propagation delay occurs as the bits traverse the physical link. Suppose, for instance, that the point-to-point link between R1 and R2 is 1000 kilometers (1,000,000 meters) long. The propagation delay would be as follows:

1,000,000

----------------------- = 004.8 ms 2.1 × 108

Figure 1-7 shows two contrasting examples of serialization and propagation delay.

Figure 1-7 Serialization and Propagation Delay for Selected Packet and Link Lengths

Server 1

As you can see in Figure 1-7, the length of the link affects propagation delay, whereas the size of the packet and link speed affect serialization delay. The serialization delay is larger for larger packets, but the propagation delay is equal for different-sized packets, on the same link. One common misconception is that the link speed, or clock rate, affects propagation delay—it does not! Table 1-5 lists the various propagation delays and serialization delays for parts of Figure 1-6.

Table 1-5 Example Serialization and Propagation Delays with Figure 1-6

continues