Fragmentation

Thus, the complete configuration for Frame Relay traffic shaping looks like:

map-class frame-relay NoBurst

no frame-relay adaptive shaping frame-relay cir 384000! (for a 384K CIR) frame-relay mincir 384000 frame-relay be 0

frame-relay bc 3840

interface serial 0 frame-relay class NoBurst

Fragmentation

One large cause of delay and jitter across WAN links is serialization delay, or the time that it takes to put a packet on a wire. For example, a 1500-byte FTP packet takes approximately 214 ms to be fed onto a 56-Kbps circuit. For optimal voice performance, the maximum serialization delay should be close to 10 ms. Thus, it can be problematic for a voice packet to wait for a large data packet over a slow circuit. The solution to this problem is to fragment the large data packet into smaller pieces for propagation. If a smaller voice packet comes in, it can be squeezed between the data packet fragments and be transmitted within a short period of time.

The sections that follow discuss some of the more common fragmentation techniques.

MTU

The maximum transmission unit (MTU) is the longest packet (in bytes) that can be transmitted by an interface without fragmentation. Reducing the MTU on an interface forces a router to fragment the large packet at the IP level. This allows smaller voice packets to squeeze through in a timelier manner.

The drawback to this method is that it increases overhead and processor occupancy. For every fragment, a new IP header must be generated, which adds 20 bytes of data. If the MTU is 1,500 bytes, the overhead is approximately 1.3%. If the MTU is shortened to 200 bytes, however, the overhead increases to 10%. In addition, shortening the MTU to force fragmentation increases processor utilization on both the router and the end host that needs to reassemble the packet.

For these reasons, shortening the MTU is only recommended as a last resort. The techniques described later in this section are more efficient, and should be used before changing the values of the MTU. When changing the MTU, size it such that the serialization delay is less than or equal to 10 ms. Thus, for a 384-kbps circuit, the MTU should be sized as follows: 384000 bps

Issue 6 January 2008 327

Page 327
Image 327
Avaya 555-245-600 manual Fragmentation, Mtu