Section 4 – GSM Signaling
BSSMAP
BSS management messages (BSSMAP) between MSC and BSS (BSC/ BTS),
which are necessary for resource management, handover control, paging
order etc. The BSSMAP messages can either be connection less or
connection oriented.
Section 4 – GSM Signaling
Initial MS Messages
These messages are passed unchanged through BSS, but BSS
analyses part of the messages and is not transparent like DTAP
messages.
Between BSS and MSC, the initial MS message is transferred in the
layer 3 information in the BSSMAP.
The Initial MS messages are:
• CM Request
• Location update request
• Paging response
Section 4 – GSM Signaling
LAPDm
Link Access Procedures on the Dm channel (LAPDm) is the layer 2
protocol used to convey signaling information between layer 3
entities across the radio interface. Dm channel refers to the
control channels, independent of the type including broadcast,
common or dedicated control channels.
LAPDm is based on the ISDN protocol LAPD, used on the Abis
interface. Due to the radio environment, the LAPD protocol can
not be used in its original form. Therefore, LAPDm segments the
message into a number of shorter messages.
Section 4 – GSM Signaling
Data exchanged between the data link layer and the physical layer
is 23 octets for BCCH, CCCH, SDCCH and FACCH. For SACCH
only, 21octets are sent from layer 2 to layer 1.
LAPDm functions include:
• LAPDm provides one or more data link connections on a
Dm channel. Data Link Connection Identifier (DLCI) is used for
discriminating between data link connections.
• It allows layer 3 message units be delivered transparently
between layer 3 entities.
• It provides sequence control to maintain the sequential order of
frames across the data link connections.
Section 4 – GSM Signaling
LAPDm Frame Structure
info
N(R)
P/F
length
N(S)
0
command
0
0
1
address
SAPI
CR
1
Section 4 – GSM Signaling
Sequence Number: N(S) send sequence number of the
transmitted frame. N(R) is receive sequence number.
P/F : All frames contain the Poll/Final bit. In command frames, the
P/F bit is referred to as the P bit. In response frames, the P/F bit
is referred to as the F bit.
Service Access Point Identifier: Service Access Points (SAPs) of a
layer are defined as gates through which services are offered to
an adjacent higher layer.SAP is identified with the Service
Access Point Identifier (SAPI).
SAPI = 0 for normal signaling of DTAP & RR
SAPI = 3 for short message services
Section 4 – GSM Signaling
LAPDm has no error detection and correction. It is used in two modes:
•
•
Acknowledge &
Unacknowledged
and having a different structure for both.
Section 4 – GSM Signaling
LAPD
All signaling messages on the Abis interface use the Link Access
Procedures on the D-channel. (LAPD protocol). LAPD provides two
kinds of signaling:
•
•
unacknowledged information
acknowledged information
LAPD link handling is a basic function to provide data links on the 64 kbps
physical connections between BSC and BTS.
Section 4 – GSM Signaling
Links are provided for operation and maintenance (O&M) of the
links, for O&M of the BTS equipment and for transmission of
layer 3 Abis messages.
Each physical connection can support a number of data links
(logical connections). On each physical connection each data
link is identified by a unique TEI/SAPI
Section 4 – GSM Signaling
LAPD has three sub signaling channels
1.
RSL (Radio signaling Link), deals with traffic management,
TRX signaling.
2.
OML (Operation & Maintenance Link), serves for maintenance
related info and transmission of traffic statistics.
3.
L2M (Layer 2 Management), used for management of the
different signaling on the same time slot.
Section 4 – GSM Signaling
LAPD Frame Structure
Flag
FCS
N(R )
info
P/F
N(S)
length
0
command
TEI
1
address
SAPI
CR
Flag
0
Section 4 – GSM Signaling
LAPD Frame structure is made up of:
Flag: Indicates the beginning and end of each frame unit. Flag has
a pattern of 01111110.
FCS: Frame Check Sequence, provides the error checking for the
frame. If error is found frame will be retransmitted.
Command: It has two types of structure, in acknowledge mode it
has N(S) and N(R ). N(S) is a sequence number of frame sent
and N(R ) is the sequence number of the frame expected to
receive next.
Section 4 – GSM Signaling
C/R: This bit indicates whether it is command or response.
P/F: In command frames, the P/F bit is referred to as the P bit and
the other end transmits the response by setting this bit to F.
TEI: Terminal Endpoint Identifier, is a unique identification of each
physical entity on either side like each TRX within a BTS have a
unique TEI.
Section 4 – GSM Signaling
SAPI: Service Access Point Identifier, used to identify the type of link.
SAPI = 0 for RSL
SAPI = 62 for OML
SAPI = 63 for L2ML
Each LAPD link is identify by SAPI/TEI pair.
Section 4 – GSM Signaling
Exercise
Q1. Name the protocol which is transparent to BSS and what information is
used to transfer on this protocol?
Q2. Name the protocols used between
Mobile and BTS
BTS and BSC
BSC to MSC
MSC to PSTN
Section 5 – Call Handling
Call Handling
Section 5 – Call Handling
Objective
The Trainee will be able to understand:
•
•
•
•
•
Basic call concepts
Location Area concepts
Call setup in different scenarios
SMS routing
Intra and Inter MSC handovers
Section 5 – Call Handling
Introduction
Call setup is required to establish communication between a Mobile
Station and Network Subsystem (NSS). The NSS is responsible for
establishing a connection with the corresponded. Different types of calls
require different teleservices.
For the optimum utilization of the network, different location areas will be
defined to reduce the paging load on the system.
Section 5 – Call Handling
Basic Types of Calls
There are three basic types of call:
1.
Mobility Management calls: Such as Location update. These
are used to collect information about the MS and only
signaling channels are used.
2.
Service calls: Such as SMS. These calls passes very small
information, therefore signaling channels are used.
3.
User traffic calls: Such as speech or data. Large amount of
data is exchanged hence traffic channels are used.
Section 5 – Call Handling
Basic Call Setup
Section 5 – Call Handling
Subscriber on switch A places a call to a Subscriber on switch B:
1.
Switch A analyzes the dialed digits and determines that it needs
to send the call to switch B.
2.
Switch A selects an idle trunk between itself and switch B
and formulate IAM
3.
STP W receives a message, inspects its routing label, and
determines that it is to be routed to switch B.
4.
Switch B receives the message. On analyzing the message, it
determines that it serves the called number and that the called
number is idle.
5.
Switch B formulates an address complete message (ACM),
which indicates that the IAM has reached its proper destination.
Section 5 – Call Handling
6.
Switch B picks one of its links and transmits the ACM over the
link for routing to switch A.
7.
STP X receives the message, inspects its routing label, and
determines that it is to be routed to switch A.
8.
On receiving the ACM, switch A connects the calling
subscriber
9.
When and/or if the called subscriber picks up the phone,
switch B formulates an answer message (ANM),
10. Switch B selects the same link it used to transmit the ACM
11. STP X recognizes that the ANM is addressed to switch A and
forwards it over link
Section 5 – Call Handling
12. Switch A ensures that the calling subscriber is connected and
conversation can take place.
13. If the calling subscriber hangs up first switch A will generate a
release message (REL) addressed to switch B.
14. STP W receives the REL
15. Switch B receives the REL, disconnects the trunk from the
subscriber line, returns the trunk to idle status.
16. STP X receives the RLC, determines that it is addressed to
switch A.
17. On receiving the RLC, switch A idles the identified trunk.
Section 5 – Call Handling
Location Registration
When the mobile is turned on first time in the network, it has no indications
in its data about an old Location Area Identity. MS immediately inform
the network and request for the Location Update to the MSC/ VLR.
After registration MSC/ VLR will consider the MS as active and marked
the MS as “attached”.
Section 5 – Call Handling
Location Update
When the MS moves from one LA to another, it has to register. This
registration is performed when the MS detects another LAI than the
one stored. This is called location updating. This function provides
mobile subscribers with uninterrupted service throughout the GSM
coverage area so that they can:
• Be called on a permanent directory number irrespective of their
location at the time of call.
• Access the network whatever their position
Section 5 – Call Handling
There are four different types of location updating:


Normal
IMSI detach
IMSI attach
Periodic registration
Section 5 – Call Handling
Normal Update
• The Base Transceiver Station (BTS) of every cell continually
transmits the Location Area Identity (LAI) on BCCH.
• If MS detects LAI is different from the one stored in the SIM-card,
it is forced to do a location update.
• If the mobile subscriber is unknown in the MSC/VLR (new
subscriber) then the new MSC/VLR must be updated, from the
HLR, with subscriber information.
• It also consider the case of the location update timer runs out.
Section 5 – Call Handling
Section 5 – Call Handling
1.
The MS requests a location update to be carried out in the new
MSC/VLR. The IMSI is used to identify the MS.
2.
In the new MSC/VLR, an analysis of the IMSI number is carried out.
The result of this analysis is a modification of the IMSI to a Mobile
Global Title (MGT)
3.
The new MSC/VLR requests the subscriber information for the MS
from the HLR.
4.
The HLR stores the address of the new MSC/VLR and sends the
subscriber data to the new MSC/VLR.
Section 5 – Call Handling
5.
The HLR also orders the old serving MSC/VLR to cancel all
information about the subscriber since the MS is now served by
another MSC/VLR.
6.
When the new MSC/VLR receives the information from the HLR, it
will send a location updating confirmation message to the MS.