1. GSM is not just about the air interface !

1a. Mobility introduces issues such as....

#1. Routing - your number does not change when
you roam. how is a call routed towards the
mobile ? This is NOT the handoff scenario.
( This is when the subscriber is not in his
home area but still is reachable using the
same phone number. This could arise in
the special case of a handoff between cells
belonging to two different network operators. )

1b. GSM is a set of standards that define the functional
entities of a Public Land Mobile Network, and their
interworking. Tech specs run into 5000 pages.
1c. GSM is a move towards a 'mobile' ISDN.

2. What is an ISDN ?

<INDIRECT DEFINITION>
If you have one single identity, internationally unique,
using which you can be in touch with the world, regardless
of what service you want to use ( and consequently, regardless
of how your service requirements like bandwidth vary ), you
are part of an ISDN.

2a. An ISDN works on a set of standards that define various
services and their requirements. This means a voice service
is viewed differently from a fax service, each with their own
defined service requirements, by the ISDN, as opposed to the
traditional PSTN, where all services are viewed as a 4KHz BW
signal to be carried from one place to another.

2b. In a more down to earth explanation, when you use a modem
to connect to the Internet, the PCM encoder at your digital
telephone exchange does not know that the information on the
line is originally digital, and it does an unnecessary A-to-D
conversion, which adds noise to the signal. An ISDN avoids this.

2c. Very essential to the working of an ISDN is the existence
of a proper signalling system. Why ? When the ISDN needs to
know about the properties of a call, there should be a signalling
exchange between the user and the network, and obviously, our
present decadic/DTMF signalling is insufficient.

2d. So, in an ISDN, a subset of the well-known SS7 signalling
protocol is used between the network and the user. The User-to-
Network signalling protocol is called DSS1. So, in a sense,
ISDN extends the concept of out-of-band signalling ( and its
advantages ) to the user in the form of DSS1.

2e. An ISDN provides two kinds of services - bearer services
and teleservices ( will not be dealt with ). Bearer services
are merely services which transport a bit stream between two
users in the ISDN. Bearer services include the 64Kbps voice
service, and the 64Kbps data service ( How are they different ? )

3. ISDN and IN ( Intelligent Network )

3a. Since an ISDN necessitates the existence of SS7, many
interesting possibilities arise because of the use of SS7.
SS7 makes it possible for intelligence to be distributed
in the network ( am i going too much on the jargon ? ).

3b. Let's take an example. Company ABC wants a toll-free
number to advertise to its customers. If the number was
491-9055 in Madras, all exchanges in Madras would be
programmed to route calls to this number as usual ( like
any other number beginning with 491 ) but not charge the
caller. This is tedious, since programming ALL exchanges
in a city is no easy task. Now, instead, if the company
was given the number 1-600-111-123, and all exchanges in
Madras were programmed to first contact a database, which
would translate the 1-600 number to the actual real world
phone number ( 491-9055 ), the calling subscriber's
exchange could route the call as usual, with the additional
detail that the call is not billed ( any call to a 1-600
number is not billed ).

3c. This database method provides distinct advantages.....

#1. When the company's phone number changes, the
database is ALONE updated, to point to the new
number.
#2. Depending on where the call comes from, the
answer of the database could vary. If an exchange
in the Adyar area asks for a translation of
1-600-111-123, the database returns the phone number
of the Adyar Domino's Pizza ( 442-4444 ), and if the
call comes from an Anna Nagar exchange, the database
returns 628-4444. This feature is even more tedious
to implement in the earlier mentioned method.

3d. The existence of such a database in a network distributes
the call routing intelligence, and the network is called an
Intelligent Network.
 

GSM addresses mobility the IN way !
GSM tackles the problem of user mobility by the use of
databases, and hence is the perfect example of an IN
implementation.

4a. GSM achieves the following objectives.....

#1. Mobility Management
#2. Connection Management
#3. Radio Resource Management ( not addressed )

4b. Before we address each of the above two issues, we need
to understand the entities in a GSM PLMN.

<"GSM/ISDN diagram" : Slide 5 ( Page 9 ) in COM3525-I8.pdf>

<"GSM PLMN view" : Slide 7 ( page 14 ) and 8
( page 15,16 ) in COM3525-I8.pdf>

4c. briefly mention MSC, HLR, VLR, AuC, EIR, BSC.
NO MENTION OF GMSC - mention later.

5. Architecture of GSM
 

5a. How is a subscriber identified ?

- Int'l Mobile Subs. Identity IMSI
( 15 digit number unique worldwide
Skycell has 404 40 *** and RPG has
404 41 *** IMSI )

- Mobile Station ISDN - MSISDN : This is
the mobile's phone number eg. 91 98410 26613
( part of the country's numbering plan )

- Phone numbers are just for convenience of
users. System level operation always refers
to users differently ( Equipment Number in
landline switches, IMSI in GSM ).

- GSM always talks in terms of IMSI wherever
possible, and the HLR maps MSISDN to IMSI.

5b. IMEI ( Int'l Mobile Equipment Identity ) ?

-19 digit number

- assigned and pre-programmed into equipment
by manufacturers, like hard-wired Ethernet
addresses on NICs.

- used to prevent fraud.

- on any GSM phone, press *#06# to view IMEI
of that mobile phone.

- Equipment Identity Register ( EIR ) has
white, grey and black-listed IMEI.

- If some equipment was stolen, those IMEI
are put in the black list in the EIR, and
any calls made from equipment with that
IMEI are blocked and logged ( in fact,
before a call is made, even the registration
procedure will be prevented )

<"IMSI/IMEI/MSISDN" : slide 4 of gsm1.pdf>

5c. HLR

- database : supports only store/retrieve

- HLR stores IMSI of customer against MSISDN

- HLR also has data on where to find the
customer.

- HLR stores customer profile

- limitations arise because of a storage capacity
issue or SS7 message processing performance issue

- HLR from Nokia can support upto 60K subscribers
( check ! )

5d. VLR

- database for temporary storage of subscriber
information.

- when a subscriber is not in the geographical
area where his HLR is ( say, he is roaming ),
then the VLR has a copy of his profile which
is supplied by the HLR during the registration
procedure.

- stores location information within the PLMN;
cells grouped together to form location area
( LA ) to reduce paging traffic; VLR stores
this LA.

- VLR saves repeated database requests to HLR.

- often NOT separated from the MSC ( why ? )

5e. MSC

- the switch/exchange in the GSM PLMN

- all switch vendors market existing landline switch
hardware ( Ericsson AXE, Alcatel E10, Siemens
EWSD ... ) - everything is in the software !

- For example, Nokia markets the DX200 landline
switch, and Nokia's GSM solution is also centred
around the DX200 hardware in a MSC/VLR configuration.

- MSC speaks to HLR using Mobile Application Part,
which is at the application layer of the SS7 stack.

- When mobile subscriber initiates a call, the MSC
plays the role of a normal switch ( CO ) routing
the call. When a call to a mobile subsciber is made,
MSC interrogates the HLR, and if the subscriber is
in the area served by THIS MSC, the call is
established. If HLR responds saying subscriber is
not in this MSC area, the MSC routes the call to
wherever the subscriber is. If the subscriber has
enabled other options like voicemail, the MSC
routes the call appropriately.

- MSC is the point of interconnection of PLMN and
other networks ( PSTN, ISDN ).

5f. BSC

- The BSC marks the beginning of the Radio Subsystem
of the PLMN.

- Functions include handoffs, paging, and transparent
trasfer of signalling information between MSC and
the MS.

- Standardised interface between BSC and MSC
( A-interface ); uses SCCP ( connection-oriented
transport layer protocol of SS7 stack ) for signalling.

- If handoffs are intra-cell, or inter-cell, within
cells under the control of the same BSC, the MSC
is not aware of the handoff at all.

- If handoff is between cells controlled by different
BSC under the same MSC ( more than one BSC under
one MSC possible ), then the MSC is informed because
incoming voice/data path of user as seen by the
MSC changes

- does not do any switching/concentration; all channels
on the air interface have a one-to-one correspondence
with bearer channels between the BSC and MSC on the
A-interface.

- Since BSC has N*64Kbps channels to MSC, BSC also does
rate adaptation from 13Kbps to 64Kbps ( and vice-versa ).

5g. BTS

- This is what is there at each of the cell sites that
are seen around the city.

- essentially only radios ( transceivers ) present

- little or no intelligence present

BTS-to-BSC interface standardised ( A-bis interface )

<"GSM System Architecture" : slide 9,10
( pages 17,18,19 ) of COM3525-I8.pdf>

<"GSM Protocol Layers for signalling" :
Slide 15 ( page 29 ) in COM3525-I8.pdf>
 

6. Procedures in GSM

Now that we have an idea of the GSM architecture in the
fixed part of the PLMN, we can see the various procedures
that are followed by entities in the PLMN.

6a. Subscription Information addition

- when a new subscriber is to be allowed in a network,
he is given an IMSI ( a SIM card is given to him at
the retail outlet, and the SIM contains the IMSI,
as programmed into it by the operator )
- He is also given an MSISDN, and the HLR is updated
with an MSISDN-IMSI mapping, along with his service
profile ( can he roam, can he make long distance calls,
does he subscribe to a voicemail service ... )
- The subscriber inserts the SIM card into a mobile
terminal ( phone ) and can start using the service.
- This event marks the identification of a subscriber's
home PLMN, because in any GSM network in the world,
the subscriber is now identified as belonging to
so-and-so PLMN, by means of his IMSI.
 

6b. IMSI attach and IMSI detach

- When a subscriber inserts his SIM card into the phone,
and turns on the phone, the network ( more specifically,
the MSC ) notices a new IMSI trying to register. Let
us assume the subscriber is turning on his phone for
the first time in his home network. The MSC identifies
the IMSI as the IMSI of this network and hence interrogates
the HLR. The HLR responds with the subscriber's profile,
which is stored in the VLR of the network. Simultaneously,
the HLR also notes the fact that the subscriber is now
reachable at so-and-so VLR.
- If we assumed the case of the subscriber turning on his
phone in a network other than his home network, the
visited MSC would realise that the IMSI is not the visited
network's IMSI. It would then interrogate the subscriber's
HLR ( back in his home network ) through the SS7 network,
and obtain the subscriber profile, which would be stored
in the VLR of the visited network. Again, the subscriber's
HLR ( in his home network ) would mark the subscriber as
reachable using the VLR of the visited network.
- The above is the IMSI attach procedure.
- IMSI detach procedure is performed when the subscriber
turns off his phone, or is un-reachable for a long time.
A data value in the VLR helps in deciding whether it
is useful to try to reach the subscriber. An IMSI detach
procedure makes this value convey the information that it
is pointless to attempt to reach the subscriber. An IMSI
attach does the opposite.

Question: what happens when a subscriber does IMSI detach in one network
and does an IMSI attach in another ? will the first visited VLR still
have the subscriber profile ?

6c. GMSC ( Gateway MSC ) function

- This refers to the interrogation of the HLR, and the action
taken on receipt of the HLR's response.
- The GMSC comes into the picture only in a mobile termincated
call ( MT call ).
- GMSC has translational tables that map MSISDN to the HLR
which has IMSI/subscriber profile for that MSISDN.
- The GMSC decides whether the MT call should be completed
through entities within the PLMN ( VLR, BSC ) or
through entities external to the PLMN ( other networks ).
- If the called subscriber is in that PLMN, the call is
forwarded to the MSC, which queries the VLR using the
IMSI, and the VLR responds with the Location Area ( LA )
of the called subscriber. The subscriber is then paged.
- If the HLR responds with a VLR address that is not the
address of this PLMN's VLR, the GMSC queries that VLR
where the subscriber is present.
- The VLR of the foreign PLMN responds with a Mobile Station
Routing Number ( MSRN ), which is structurally similar to
an MSISDN in the forign VLR, and which can be used as a
Global Title for routing the call. The GMSC forwards this
MSRN to the PLMN's MSC for setup of the forwarded leg of
the MT call ( from home PLMN to visited PLMN ).
- MSRN is dynamic, valid for that call only, and alloted
from a pool of available MSRNs at the visited PLMN.

<"GSM Numbers" : Slide 4 and 5 in gsm1.pdf>

<"MT call" : slide 15 ( page 30 ) of COM3525-I8.pdf>

<"MT call signalling between ISDN and PLMN" :
Figure ( isdn-gsm.gif )>

Questions
#1. What happens when Dr. Srikanth is in Canada with his
RPG-subscribed cellphone ( roaming in Canada ) within a
few metres of his friend, who calls him from a landline
by dialling +91 98 410 26613 ?

* The GSM Tromboning Effect *

* Issues if GMSC present in Canada - routing is optimal
but billing is a nightmare - use example of calling
person, called person's PLMN and called person's visited
PLMN all in different countries *

#2. My cousin has a cellphone with phone number
98 112 01402 ( from Delhi ). He is roaming in Madras,
and to save money in calling him, I dial the MSRN and
not the MSISDN, from my phone. Will I succeed ?
 

#3. Assume there's a trombone in question #1. If Dr. Srikanth
were to immediately hang up and call his friend back,
how would things be ?
 

- pressing one button to generate the corresponding tone generates
4 messages to be exchanged between MS and MSC
- messages are START-DTMF, START-DTMF-ACK, STOP-DTMF and
STOP-DTMF-ACK.

8. Short Messaging Service - Not discussed.

END

Other references
---------------
#1. "GSM Authentication and Encryption Process" : Slide 2 in gsm1.pdf