[ronnie sahlberg <ronniesahlberg@xxxxxxxxx>]

On Fri, 16 Jul 2004 15:22:16 -0700 (PDT), Guy Harris <gharris@xxxxxxxxx> wrote:
>    the link between the BTS and BSC (are those co-located at the base
> station?) runs LAPD over some physical-layer link;

The BSC (Base station Controller) is essentially a POTS switch but for
mobile networks and is situated in a switch centre.  The BSC has
permanent links to all the RBS/BTS (Radio Base stations) it
controlles,  up to 256 RBS being controlled by each BSC depending on
make and model of the BSC.

In GSM: each cell on each RBS (up to 8 cells per RBS) has a 2 x 64000
bit/s connenction subdivided into 8  16000bit/s circuits for voice and
control.
These 16000bit/s channels are divided into one 13600bit/s channel for
voice and one 2400bit/s link for control (lapd and lapb).
(
Older generations of GSM RBS also needed a third extra 64000bit/s
channel for each cell for control but in modern versions of RBS this
controll channel (CLC) is piggybacked inside one of the 2400bit/s
circuits.
(this channel was only really used for configuration and control of
the RBS and if you started to run short on capacity in your network
and the launch were looming ahead, it was not uncommon that one just
decommissioned this timeslot after the RBS was configured and "in the
air" so that timeslot on the links could be recycled for use at other
base stations.  One shouldnt do this but hey,   if some links are
delayed, capacity is running low  and the launch is only weeks away,
which is when the government/incumbent/competition is getting ready to
roll out the trucks to measure field strength and coverage (so they
can fine you if you dont meet the regulation for signal strength and
coverage) you have to do "non-standard" things.)
)

Each such 64000bit/s circuit is in the transmission network is called
a timeslot, and very modern versions of GSM even supports
concatenating multiple timeslots in one larger circuit to get even
higher transfer rates.


Each mobile network has 5-50 BSCs depending on the networks size and
each is located in a switch centre, usually in a large city.
Each BSC is then in turn connected to the MSC (Master Switch Centre)
on an uplink which is usually n concatenated E1 or T1 connections.

You usually only have one MSC in a network and the MSC is really
(AFAIK) just a BSC that is slightly modified to talk to BSCs instead
of RBS.


I might be a bit wrong here, my memory is a bit rusty,    it was like
7+ years since I used to travel around building transmission networks
for these things.



It would however be very cool if Ethereal could start capturing from
E1 and also SDH interfaces (or T1/SoNET interfaces for your weirdo
non-european standards :-) and dissect the framing words and the clock
signals.
(I dont work with these things any more and for what I know ethereal
might already be able to do this? but it would have been useful to me
x number of years ago)



> 
>    the link between the BSC and the MSC runs "MTP" (is that one or more
> of SS7's MTP layers, e.g. MTP2?) over some physical-layer link.
> 
> So what Ethereal would do would depend on which of those protocol stacks
> is in the log file, and whether, if it's the mobile <-> base station link
> (Um interface), it contains:
> 
>    packets tagged with the particular TDMA physical channel they're on;
> 
>    packets tagged with the particular logical channel they're on;
> 
>    only LAPDm-based signalling traffic;
> 
>    or something else.
> 
> If it's the BTS <-> BSC link (Abis interface), then, if it contains only
> one channel running LAPD, a WTAP_ENCAP_LAPD encapsulation type would be
> added (or perhaps just brought back from the dead, before we changed to
> doing ISDN as WTAP_ENCAP_ISDN, with packets identified by the ISDN
> channel), the LAPD dissector set up to dissect it, and some mechanism for
> identifying the protocol running atop LAPD and added dissectors, if
> necessary, for it.  Otherwise, we'd need to know how it's structured in
> the log file.
> 
> If it's the BSC <=> MSC link (A interface), then, if it contains only one
> channel with MTP2 traffic, you could use WTAP_ENCAP_MTP2, and if it
> contains only one channel with MTP3 traffic, you could use
> WTAP_ENCAP_MTP3.  Otherwise, we'd need to know how it's structured in the
> log file.
> 
> 
> 
> 
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