User guide

Network Engineering Packet Network Considerations
Telesyn Service Guide (Circuit Emulation Service (CES))
5-4
5.4 Network Engineering
5.4.1 Packet Network Considerations
Transport robustness is important since many characteristics of packet networks are not compatible to constant
bit rate services. Packet network congestion, blocking, QoS prioritization, and multiple paths with varying
latency, can lead to:
• jitter
• lost packets
• duplicate packets
• packets out of sequence.
All of these error conditions must engineered into the CES8 configuration; some of these are done automatically
by the card, while some are done though configurable parameters.
5.4.1.1 Packet Size
Packet size is directly related to the number of bytes taken from a T1 bit stream to create a packet for the pseudo
span. This is user configurable and can range from 1 byte to 1488 bytes. (Refer to the Telesyn User Guide for
details.)
Increasing the number of bytes per packet increases end to end latency, and thus adds delay in the TDM circuit.
For data applications such as Frame Relay, latency does not significantly degrade the service. However for voice
applications, sufficient amounts of delay can result in the need for echo cancellation.
At the other extreme, lower numbers of bytes per packet decrease transport efficiency by increasing the percent-
age of overhead bytes associated with the packet. Setting the number of bytes per packet too low doesn’t provide
enough bytes to fill the minimum Ethernet frame of 64 bytes, causing the packet to be filled with dummy infor-
mation and therefore wasting packet bandwidth.
show the relationship of NUMBYTES and the bandwidth/jitter values.
5.4.1.2 Packet Delay Variation
TDM-based data operates at a constant bit rate. When converting from packet to T1, the CES8 must have a
queued packet to “play out”. It cannot wait for a packet to arrive without causing errors in the T1. Unfortunately,
transit time across a packet network varies from packet to packet. These variations, or jitter, in the arrival time
of the incoming packets and are commonly referred to as Packet Delay Variation (PDV). PDV is a parameter
that is measured in milliseconds. PDV has many origins; multiple routes with different transit times, switch con-
gestion, mixed packet sizes, contention with traffic with marked for higher QoS, and router loading.
Generally, the more switching or routing nodes in the pseudo span path, the greater the PDV. In order to com-
pensate for PDV, the CES8 must buffer enough packets to ensure there that another packet has arrived before the