*Posted January 14th 2015*

*Updated 6. February 2018 because of capacity improvements in
hardware shipped from this date onwards: we have improved
MTP-2 decoding capacity from 96 channels to 240 channels.*

A common question when starting a monitoring project is "how much
hardware do I need to monitor these E1 (or T1) lines?". The final
word is always the specifications:

E1/T1 Monitor 3.0

SDH Monitor 3.0

One way to get started is to look at some real-world examples:

This is a classic E1 setup. Each E1 carries 30 timeslots of voice and one timeslot of signalling. In this example, we'll assume:

- The 30 subscriber timeslots (normally voice) on each E1 are not of interest. We can ignore them.
- We want
*both*directions of the signalling.

To monitor that, we need enough ports to plug all the E1s into, and we need enough capacity to decode the MTP-2 signalling.

**Ports**: An E1/T1 Monitor 3.0 has 64 E1 receivers
(spec. 2.1.1). The site in this example has 47 E1 lines, but we
want *both* directions of them, so we need 2 x 47 = 94 E1
receivers. So we'll need two E1/T1 Monitor 3.0. We can plug the
first 32 E1s into one and the remaining 15 into the other.

**MTP-2 decoding capacity**: An E1/T1 Monitor 3.0 can monitor
240 *simplex* ordinary 64 kbit/s MTP-2 channels (spec. 2.2.1).
The site has 94 channels. So dimensioning is not affected by the
MTP-2 decoding capacity.

**Conclusion**: The site requires two E1/T1 Monitor 3.0.

Running many signalling links on the same E1 is common at the core of some networks. It's possible to put as many as 31 SS7 links on the same E1, but 16 is more common. As in the previous example, we'll ignore the remaining timeslots and we'll assume both directions are needed.

**Ports:** The site has 12 E1s. We want both directions. So we
need 24 ports. One E1/T1 Monitor 3.0 has 64 (spec 2.1.1), so we have
plenty of ports.

**MTP-2 decoding:** The site has 12 E1s x 16 signalling links x
2 directions = 384 simplex channels of MTP-2. One E1/T1 Monitor 3.0
can monitor 240. So the MTP-2 decoding capacity is the limiting factor.
384/240 = 2.

**Conclusion**: The site requires two E1/T1 Monitor 3.0.

One MTP-2 link on an E1 line can run faster than 64 kbit/s by using more than one timeslot. The formal name for this is "ITU-T Q.703 Annex A", but it's often called "high speed link", "HSL", "HSSL" or "Nx64 MTP-2". We've seen this type of signalling in networks built by NSN.

In theory, all multiples of 64 kbit/s from 128 kbit/s up to 1984 kbit/s are possible, and Corelatus hardware can handle all of them. In practice, 31 x 64 = 1984 kbit/s is the most common. As for the earlier examples, we'll assume both directions are wanted.

**Ports:** We have plenty.

**MTP-2 decoding:** The site has 8 E1s x 1 signalling link x 2
directions = 16 channels. The site has 8 E1s x 1 signalling link x 2
directions x 31 timeslots = 496 timeslots of signalling.

One E1/T1 Monitor 3.0 can monitor up to 240 channels and up to 248 timeslots. In this case, the number of timeslots is the limiting factor. 496/248 = 2.

**Conclusion:** The site requires two E1/T1 Monitor 3.0.

There is more than one way to run SS7 at high speed. Some networks use ATM-on-E1 to transport SS7. This is also sometimes called "high speed link", "HSL" or "HSSL", leading to confusion because the same descriptions are used for "ITU-T Q.703 Annex A". We've seen this type of signalling in networks built by Ericsson.

The most common way to run ATM on E1 lines is to use all timeslots except for 0 and 16. That gives 30 x 64 = 1920 kbit/s. We'll assume both directions are wanted.

**Ports:** We have plenty.

**ATM decoding:** The site has 8 E1s x 1 signalling link x 2
directions = 16 channels. The site has 8 E1s x 1 signalling link x 2
directions x 30 timeslots = 480 timeslots of signalling.

One E1/T1 Monitor 3.0 can monitor up to 16 channels of ATM and up to 496 timeslots (spec. 2.2.4). So one E1/T1 Monitor 3.0 has just enough.

**Conclusion:** The site requires one E1/T1 Monitor 3.0.

Permalink | Tags: GTH, telecom-signalling