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IP to TDM Conversion Performance

Hi all,

I am interested in any answer or document that dicusses how a Cisco 3660 can take in a 1500 byte packet on its Ethernet interface and put it on to

(i) 2xE1 lines.

(ii) FR network (1Mbps)

1. What is the time taken to break up the packet for placement onto the E1 line/FR network?

2. What is the E1 line & FR overheard?

3. How does it treat this packet with other traffic? I presume it is FIFO. Does it take the whole 1500 byte packet and encode the whole thing into lots of packets for transmission on the TSLs on the E1 or Frames on FR before handling the next 1500bytes packet?

I am a little new to this, so not sure if my questions are entirely valid. Any advice/knowledge appreciated.




Re: IP to TDM Conversion Performance

Hello Matthew,

First a 1500 Byte IP packet will be forwarded out over the interface IP routing determines to be the best path to destination. With a 3660 the whole process should not take longer than a few Micro-seconds - neglegible compared to usual transmission delays.

Relating to your post title: there will be no IP to TDM conversion. An IP packet might be transported across several different types of Media and OSI layer 2 protocols - it still remains IP. So your terminology is somewhat misleading.

Second, an E1 interface (with HDLC as default encapsulation) on a Cisco router by default will offer a MTU (maximum transmission unit) of 1500 Bytes as well, so there is no need for IP fragmentation. The same applies to Frame Relay.

About overhead: comparing E1 to Frame Relay is not really reasonable in my mind, because you are talking about two different OSI layers (E1 layer1, FR layer2).

As to Frame Relay overhead, there are at least 6 Bytes (2B flags, 2B header, 2B CRC). With different encapsulation types you get additional overhead (usually less than 8 bytes).

A.3 This is a question related to Queueing. By default Cisco uses on interfaces up to E1 speed Weighted Fair Queueing. So it will not be FIFO. WFQ is somewhat more comlex, but in short and much simplified sorts smaller packets in front and larger packets to the end of the Queue. This ONLY happens when there is more traffic than the interface can handle. In all other cases everything is sent with line speed.

Hope this helps! Please rate all posts.

Regards, Martin