How many channels in an E-1 circuit?

In our recent series on ISDN, we mentioned in our discussion of the E-1 version of an ISDN Primary Rate Interface that one of the 32 DS-0s was already used for control purpose, so there were only 31 available DS-0s for ISDN. Hence, the format was 30B+D. This prompted an alert reader to write: “I believe E-1 is 30 channels – not 32.”

This bit of confusion prompts us to offer an explanation, especially since there is a sense in which both statements are correct. So back to a bit of basic telecomm and arithmetic. First, let’s establish that a T-1 has a bit rate of 1.544Mbps and an E-1 has a bit rate of 2.048Mbps. That’s where the confusion starts, especially since both T-1 and E-1 are built on the fundamental DS-0 building blocks of 64Kbps. Go get out your calculator, or open up an Excel spreadsheet if you no longer own a calculator.

If you multiply 64Kbps by 24 – the number of individual channels in a T-1 – you’ll find that the product is 1.536Mbps, not 1.544Mbps. Why? The extra 8Kbps is used for framing – the process that delineates one channel from another.  After all, since in both cases we’re talking about time division multiplexing as opposed to packet switching, there is no explicit “label” on each channel.

On the other hand, if you multiply 64Kbps by 32, you’ll find that the product is 2.048Mbps – an exact (integral) multiple of 64Kbps. The only problem is that all 32 channels cannot be used for data transport because there is no way to distinguish one channel from the other. Thus, in a typical E-1 format, one of the DS-0s is used for framing, and, by the way, another is used for telephony signaling. So you end up with 30 channels that are usable for transport, but the actual bit rate consists of 32 channels.

By the way, since the signaling resides in a separate DS-0 for E-1, all 64kbps in each DS-0 in the E-1 format can be used for data transport. By contrast, the traditional format for a T-1 used “robbed bit” signaling within each DS-0. This worked great for voice, but traditionally limited data transmission to 56Kbps rather than 64Kbps.

So which is better – T-1 or E-1? The answer is an undisputed “yes.” Both have their strengths. And why are there two different basic transmission speeds?  That’s an even better question, and we’ll address it in a later series.