User guide

Packet Network Considerations Network Engineering
5-5
Telesyn Service Guide (Circuit Emulation Service (CES))
current packet has been played out. The CES8 provides buffers that can be configured to absorb up to 60ms of
PDV. Increasing the depth of these buffers adds latency and thus delay to the T1.
As mentioned above in the discussion on packet size, latency may not degrade some data services but voice ser-
vices may require echo cancellation to handle excessive delays.
5.4.1.3 End-to-End Latency
Many services carried over T1 are delay sensitive. For example, in voice applications end to end transport latency
greater than 25ms generally requires the use of echo cancellers to remove audible reflections from far end 4 to2
wire hybrid circuits. Therefore it is often important to understand the total end to end transport delay when engi-
neering a T1 over Ethernet circuit.
Calculations of end to end latency must include the following:
1. Packet size - The number of bytes from the T1 stream that are used to form the Ethernet packet.
2. Switch transit delay - The delay incurred as a packet is switched or routed through nodes in the Ethernet net-
work.
3. PDV Buffering - The depth of the jitter buffer on the DCEI8 adds to the end to end delay.
The total latency is determined by adding the contribution of each the factors above. For example, in an EPSR
configuration the end to end delay varies depending on which direction traffic is circulating on the ring.