26000484 gsm networks bss nss abis interface a wonderful book personal favotite sufyan

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

26000484 gsm networks bss nss abis interface a wonderful book personal favotite sufyan

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