Ones are slower than zeroes

Thinking about the implications of bit stuffing I wrote about in the SDLC post, I realized that long sequences of ones would be transmitted slower than long sequences of zeroes due to an extra bit being inserted after every fifth consecutive one. The theory would predict a 20% decrease in transmission speed.

Of course I wanted to test this phenomenon immediately. I needed real equipment with low-speed serial links (that would make the difference more pronounced and less dependent on other intra-router delays), so I started one of the BGP lab exercises; Basic BGP Setup looked like a perfect choice. We're using 64 kbps Frame Relay links in the lab with a Frame Relay switch in the middle (makes the task of designing an arbitrary WAN topology quite simple), so it's a perfect environment to test this thing. And, not surprisingly, the results confirmed the theory:

Internal-Core#ping 197.1.1.49 data 0000 size 1200 repeat 50
Sending 50, 1200-byte ICMP Echos to 197.1.1.49, timeout is 2 seconds:
Packet has data pattern 0x0000
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (50/50), round-trip min/avg/max = 608/608/632 ms
 
Internal-Core#ping 197.1.1.49 data FFFF size 1200 repeat 50
Sending 50, 1200-byte ICMP Echos to 197.1.1.49, timeout is 2 seconds:
Packet has data pattern 0xFFFF
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (50/50), round-trip min/avg/max = 724/724/728 ms

The results are almost too close to the predicted ones, but they are real :)

3 comments:

  1. It's an old trick.

    You can also correctly guess encapulation type of the first hop, after this - of second hop etc. Furthermore there were some utility for this.

    --
    mtve

    ReplyDelete
  2. Well, if you want to guess hop-by-hop encapsulation, you have to ensure that the noise in the system (i.e. other packets) is minimized. Either you do the measurements when the network is idle or take very large samples.

    I should know how to do it properly; after all, I've attended an advanced digital signal processing class a long time ago, but it's all blurry now.

    ReplyDelete
  3. I'm interested in how to check encapsulation hop-by-hop. Please write a blog if you can...

    Thanks in advance for this..

    ReplyDelete

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.