Line rate and bit rate

Rajendra had an interesting problem:

Recently I got confused with the term line-rate. Is it the packets being switched across the switch fabric or control packets destined to the protocol tasks or both or something else?

The line rate is a physical layer term that has nothing to do with the line cards or switching fabrics. It indicates the actual speed with which the bits are sent onto the wire (and is thus also known as physical layer gross bit rate). The data transfer rate (commonly known as bit rate) is the transfer rate offered by the physical layer to the data link layer. If you want to be precise, you should call it physical layer net bit rate.

Two well-known physical layer technologies with different line rate and data transfer rate are ISDN (actually the I.430 recommendation) with 160192 kbps line rate and 144 kbps data transfer rate and Gigabit Ethernet (the 802.3z recommendation) with 1.25 Gbps line rate (due to 8b/10b encoding).

This article is part of You've asked for it series.


  1. In ITU-T rec I.430 I found the following:

    "2.1 Services required from the physical medium

    Layer 1 of this interface requires a balanced metallic transmission medium, for each direction of transmission, capable of
    supporting 192 kbit/s.".

    So where the 160 kbit/s line rate exist?
  2. Ahmm ... it's one of those "we always knew that ..." things. Time to dig deeper into the issue :(
  3. I-430 clearly states (section 5.4.1) "The nominal transmitted bit rate at the interfaces shall be 192 kbit/s in both directions of transmission". The 160 kbps figure comes from the U interface (used in US), ANSI standard T1.601.

    Thanks for your feedback, fixed the post.

    Lesson learned: always check the standards first.
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