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8/12/2019 26000484 GSM Networks BSS NSS Abis Interface a Wonderful Book Personal Favotite Sufyan
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GSM Networks: BSS, NSS, Abis Interface
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Table of Contents
BSS
NSS Abis Interface
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BSS
Provides a connection between the MSs of a limited area and
the NSS. BSS consists of:
One or more BTSs
One BSC
One TRAU
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BTS
Provides a connection to MS via the Air interface
On the NSS side, it provides a connection to BSC via Abis-interface. The size of an BTS has considerably reduced from 91
GSN recommendations allow a BTS to host up to 16 TRXs. However, a
majority host one to four TRXs.
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Architecture and functionality of a BTS
TRX Module
Consists of
a low frequency part for digital signal processing
A high frequency part for GMSK modulation
O&M (Operations and Maintenance)
Contains one central unit that administers all the parts of the BTS
Clock
Clock Generation and Distribution are part of O&M
Derive reference clock from PCM signal on Abis-interface, as BTS internal
clock, hence the clock is synced
If clock un-synced, errors in call handling, handover, etc may occur Filters
Wide-band filters for 900MHz, 1800MHz etc
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BTS Configurations
Standard
All BTSs are assigned different Cell Identities.
A no. of BTSs form a location area (sometimes even one)
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Network Switching Subsystem
Responsibility
Control and database functions required fro setup of call connection Features
Encryption
Authentication
Roaming Consists of
MSC (Mobile Switching Center)
HLR (Home location Register) and AuC (Authentication Center)
VLR (Visitor location Register)
EIR (Equipment Identity Register)
The Subsystem are connected directly/indirectly via the worldwide SS7
network
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NSS
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HLR and AuC
PLMN requires access to at least one HLR as a permanent
data store. HLR maintains subscriber-specific data such as Ki.
AuC is implemented as an integral part of HLR, however the
AuC can be implemented independently and the H-interface
is used to connect it to HLR
AuCs major function is to calculate the authentication-triplets.
HLR passes on this data to VLR which use them for
authentication or ciphering
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VLR
Its a database similar to HLR but it differs in functionality.
HLR is responsible more for static functions while,
VLR provides more dynamic subscriber data management
Roaming
When user travels from one location to another, the data passes from one VLR
(old) to another VLR (new)
Apart from the handover of data from old VLR to new VLR, the new VLR can
request additional data from HLR
VLR is assigned a limited geographical area. While the HLR deals with
tasks independent of subscribers location but the HLR has a constraint, it
strives to have low access times A VLR might have more than one MSC.
An HLR may have more than one VLR.
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MSC
Tasks
Assignment of user channels towards the BSS Perform and control inter-MSC handover
MSC with an interface to other networks is called Gateway-MSC.
Any MSC without Gateway functionality has to route external calls through G-
MSC
Since each MSC has a VLR, a collection of VLRs make a PLMN.
The geographical interdependence has caused the integration of the VLR
into the MSC
B-interface connects VLR to MSC. But due to the above its not used.
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EIR
Due to the separation of subscribers identity from the MS makes it easier
for the black market to operate.
EIR helps in identifying, track, bar stolen equipment.
Like the HLR and VLR, EIR also has a database
The white list contains all the approved types of mobile stations;
The black list contains those IMEIs known to be stolen or to be barred for
technical reasons.
The gray list allows tracing of the related MS
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Abis-Interface
An interface between BTS and BSC
Its a PCM 30 interface,
the transmission rate is 2 mbps
32 channels, 64kbps each
Abis interface is regarded as proprietary because GSM didnt specify it ingreat detail, which leads to variations in layer-2 protocols
a BTS from company X cant be used with a BSC by company Y. Fixed mapping of Air interface traffic channel to Abis interface time slot.
BTSs can be connected in a serial configuration or a star config.
Serial are of 2 types
Line Ring
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Signaling in Abis Interface
Abis utilizes layers 1 to 3 of the OSI
protocol stack
Layer 1 : D-Channel
Layer 2: LAPD
Layer 3: TRXM, RLM, CCM, DCM etc
Layer 2: LAPD
Its an ISDN D-Channel protocol Adopted by GSM from ITU Standards
Q.920 and Q.921
GSM hasnt implemented all the
features of the ITU standard.
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LAPD Frame
LAPD frames are separated into 3 frames
Information Frame gp consists of I-frame only
Supervisory Frame gp consists of RR frame, RNR Frame, REJ Frame
Un-numbered Information Frame gp consists of UI, DISC, UA, SABME
Fames
Control Fields
Information frame: bit 0 is 0
Supervisory frame: first two bits are 10
UI Frame have first two bits as 11
Formats of the LAPD control can be modulo 128 or modulo 8.
Modulo 128 CF is 16bit and thus N(S) and N(R), provides 7 bits each, thus 0
to 127
Modulo 8s Control field is 8 bits wide and thus N(S) and N(R) get 0 to 7.
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LAPD Frame
Flag
The Flag is 01111110 and is used at the start and end
Bit Stuffing stuff 0 between 5th and 6th bit if 5 consecutive 1s occur elsewhere.
Frame Check Sequence (FCS)
For Error Checking
Checksum calculated using the data b/w the start flag and FCS. And sent in the FCS (16
bit)
The Receiver does the same operation and verifies the FCS received
Address Field
Service Access Point Identifier (SAPI) defines the type of user the message is addressed
to. Its 6 bits wide.
SAPI also defines the priority for the message. E.g. SAPI 62, 63 are higher than SAPI 0.
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Address Field
Terminal End point Identifier helps in distinguishing among several
functionally identical entities
7 bit field
E.g. One TEI is assigned to each TRX, which helps while tracing
Command/Response Bit
determines if message contains
Command
Answer
Ack for a command
Extension Address Field Bits
One bit per octet in the Address Field The first one is set to 0 which indicates that this is also a part of the address
field
The second EA bit (in the second octet) indicates the last octet of the address
field
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Control Field
Its either 16 bits or 8 bits wide
Polling Bit (P) / Final Bit (F) and P/F Bit Commands Only, P-bit is used, which informs the receiver that the sender expects a response
Responses Only, F-bit is used.
If P=1 from sender, then receiver sends F=1.
LAPD allows ack of I-frame where P=0, with either an I or Supervisory Frame. However, where P=1,have to be acknowledge them immediately with a Supervisory frame
Note: In all UI frames, P=0.
Send and Receive Sequence No.
Serve the purpose of acknowledging the transfer and receipt of frames. For I frames, the sender side counter (N(S)) is incremented while the receiver side is left as is.
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Tasks of various Frames I Frame
Used to transfer Layer 3 info
The frame is acked by RR, RNR, REJ depending on an error occurred or not.
Contains both N(S) and N(R).
RR Frame Used to ack the I frame
Used for polling b/w BTS and BSC
During idle phase, RR are periodically exchanged depending on T203 (10s)
If the RRs are seen on the Abis then layer 2 is working fine
RNR Frame Used to signal no more I frames can be accepted
To continue receiving I frames an RR has to be sent to the Tx
The RNRs N(R) indicates the last I frame that was correctly received
REJ Frame
Used to signal a transmission error has been detected by analysis of FCS REJ s N(R) value indicates the first I frame that has to be repeated.
SABME Frame is sent when no layer 2 connection was established
DM Frame Indicated that the sender can no longer maintain the Layer 2 connection and will tear it down without
receiving the acknowledgment, similar to the DISC frame except that waits for the ack beforedisconnecting
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UI Frame
Contains neither the N(S) nor N(R).
Doesnt require an ack. As P=0.
DISC Frame
Used to take layer 2 connection out of service
Waits for an acknowledgement from all peers in the form of UA
UA Frame
Used to answer a DISC or SABME frames
FRMR Frame
Indicates the received message was garbled, wrong or unexpected.
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Layer 3 Information
Layer 3 information within I and UI frames follows the layer 2 header.
Because of different file formats its necessary to distinguish b/w admin
(SAPI 62,63) and connection tasks (SAPI 0).
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Bringing Abis interface into service