Yah Eric you mentioned almost all of them rightly, but I would like to specify the T1 and E1 part a bit
First thing T1 uses some bits of each timeslot (6th,12th,18th and 24th bit of every timeslot and these bits are also known as A,B,C and D bits) and since these bits are part of the timeslot only, a CAS T1 would be a true In-band signaling circuit. On to E1s, they can be used in both the flavors, but confusion mostly arises for CAS E1. A CAS E1 though uses A,B,C and D bits but all these bits are sent in the 16th timeslot, so as per my understanding a CAS E1 should be termed as out-band signaling circuit. Please correct me if I am wrong on this. Regards, Vikram -----Original Message----- From: ericbrouwers [mailto:[EMAIL PROTECTED] Sent: Friday, April 04, 2003 3:35 PM To: [EMAIL PROTECTED] Subject: Re: CAS and CCS - out-of-band and in-band [7:66607] Interesting discussion, In-band, out-of-band are very confusing terms when used in digital telecoms/datacoms. People and books will tell you contradicting things. For the right definitions you can best refer to ITU or ANSI specs. I haven't been involved in classical telecoms for years, but this is my interpretation: With in-band signalling the signalling is conveyed on the same channel (timeslot, connection, ...) as the voice or data is carried. So for example pressing digits during a phone call can be considered as in-band signalling (DTMF signals). Fax and modem calls may also be considered in-band. Early phone systems also used signalling systems like R2 to control the call (specific frequencies were used to control the call). Out-of-band systems either used specific wires or specific frequencies that were out of the normal frequency band. I can't remember exactly, I have some coursebooks somewhere, but I can't find them right now. Anyway, as far as I know in-band and out-of-band originate from the old analog telecoms world, and may be ambiguous when applied to the digital world. Nowadays, if you really want to classify digital signalling is in-band or out-of-band, I would say the following: out-of band signalling is signalling that is transported over a different channel (timeslot, connection, ...) than the data or voice. So for example: - ISDN BRI and PRI both have a dedicated signalling channel (the D-channel), so it's out-of-band. - In the European PSTN and GSM networks there are dedicated signalling links for voice trunks. The call setup is controlled by common signalling channels, hence the name common channel signalling. Signalling system number 7 (SS7), also called common channel signalling system number 7 (CCS7) is an out-of-band signalling system, since the signalling is not conveyed in the same channel as the voice (or data). Note that the signalling channels can be implemented in different ways: the signalling channel can be part of the same E1 trunk (for instance timeslot 16 or any other timeslot except 0 !!!) that carries the voice, this is called associated signalling; the signalling channel can also be in a timeslot of an E1 different to the E1 that carries the voice. The latter is called quasi-associated signalling: there are dedicated redundant signalling links that do not follow the same path as the voice trunks, only the begin- and end-points of the voice and signalling paths are the same...confusing, isn't it? ISDN BRI and PRI use the same concept as SS7: multiple voice or data calls are controlled by one signalling channel, which makes them common channel signalling and out-of-band. Channel-associated signaling is very confusing to me. $%^*!* where are those course books??? Hmm, let's use this definition from Cisco: "...Channel associated signaling: The transmission of signaling information within the voice channel...." So that seems to imply that all in-band signalling is also CAS signalling, which makes sense to me. So what about X.25? Initial call setup and data follow the same path: so CAS. Would you classify it is in-band??? So what about E1s and T1?? You can't just classify a standalone E1 as one or the other. In my opinion it depends on the application, the signalling system and the scope of reference. When you make a call to certain international locations, you may still hear some old signalling along the path, caused by inband CAS systems. A channelized E1 that carries voice or data calls is normally controlled by a common channel signalling system. However, maybe I shouldn't mention this, since it is too confusing: an E1 can also be classified as CAS, when you allocate a 2kbps signalling channel to each 64kbps channel....32*2 = 64 kbps, isn't it? And what about timeslot 0, it's always present and it is used to convey information...we should classify this as CAS, shouldn't we? You see very confusing. It really depends on your definitions and scope. I'm not familiar with T1s, I always ignored them, because we don't use them here in Europe. T1s seem to have 24 timeslots: 24 * 64 = 1536 kbps. T1s don't seem to have a dedicated timeslot 0, instead framing/sync/O&M is handled by the remaining 8kbps (1544-1536 kbps) of bandwidth. This is CAS, just like TS0 in an E1 is CAS. You may classify this CAS as in-band, although some may debate that, since you're using dedicated bits in the multiframe structure for CAS, just like an E1 is using certain bits in the multiframe structure as TS0 and most people will classify this TS0 as out-of-band... So both in T1s and E1s bits are robbed from the T1/E1 bandwidth. If a voice timeslot in a T1 is really 64kbps (and not 56 kbps as some say ....) , if I had to classify it, I would classify it as out-of-band a well, since you're not stealing bits from the individual voice channel, but from the T1 trunk.....Aarghh...We should ask a PCM E1/T1 guru!!! However, your ISDN PRI is 23 B + D, and this D-channel again is CCS (and out-of-band) ! I also assume that the voice trunks in your PSTN/GSM/PCS networks use dedicated SS7 signalling links, again CCS, out-of-band. Hope this helps? Eric Brouwers ----- Original Message ----- From: "Priscilla Oppenheimer" To: Sent: Tuesday, April 01, 2003 7:43 PM Subject: RE: CAS and CCS - out-of-band and in-band [7:66607] > alaerte Vidali wrote: > > > > The URL following states: > > Alas, the URL didn't get posted. If it's CVOICE materials, be wary. They > munged this topic. > > > > > "Channel-associated signaling, which uses E1 time slot 16 (the > > D channel) for signaling, fits into the out-of-band signaling > > category." > > Cisco and other documents consider the use of E1 time slot 16 to be > out-of-band signaling because it's an entire channel used for signaling > instead of using bits that are in-band with the data bits. > > It could be CAS or it could be CCS. Channel-Associated Signaling (CAS) means > that the channel is structured in such a way that each data channel can use > some bits for its signaling. > > Common Channel Signaling means that the signaling channel is unstructured > and could be used by any data channel that currently needs it. That may be a > gross over-simplification. Someone can correct me if I'm confused. > > It's weird that they threw ISDN into the statement (by saying the D channel > in parentheses.) ISDN PRI uses CCS, doesn't it? I guess I'm not sure about > that. I've seen that it does, but that might have been the CVOICE class, > which really did munge this topic. > > > > > "Robbed-bit signaling, which uses bits from specified frames in > > the user data channel for signaling, fits into the in-band > > signaling category." > > > > Robbed-bit is used in T1-CAS. > > That sounds right. The robbed-bit signaling used on T1s is considered both > in-band and CAS, according to most documents... > > > > > So I can say E1-CAS is classified in out-of-band signaling and > > T1-CAS is classified in in-band signaling. > > > > Is that right? > > Sounds right to me. > > Priscilla > > > I have seen questions stating that CAS is just > > one thing, in-band signaling. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=66845&t=66607 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
