GSM Networks: BSS, NSS, Abis Interface
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 •
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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 5
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 • •
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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 7
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 • AuC’s 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 •
It’s 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
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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
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VLR is assigned a limited geographical area. While the HLR deals with tasks independent of subscriber’s 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. 10
MSC •
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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 GMSC
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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 • •
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An interface between BTS and BSC It’s a PCM 30 interface, – the transmission rate is 2 mbps – 32 channels, 64kbps each Abis interface is regarded as proprietary because GSM didn’t specify it in great detail, which leads to variations in layer-2 protocols – a BTS from company X can’t 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
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Layer 2: LAPD – Its an ISDN D-Channel protocol • Adopted by GSM from ITU Standards Q.920 and Q.921 • GSM hasn’t 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
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Control Fields – Information frame: bit 0 is 0 – Supervisory frame: first two bits are 10 – UI Frame have first two bits as 11
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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 8’s Control field is 8 bits wide and thus N(S) and N(R) get 0 to 7. 15
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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 1’s occur elsewhere.
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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
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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
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Command/Response Bit – determines if message contains • Command • Answer • Ack for a command
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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 – – – – –
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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 ack’ed by RR, RNR, REJ depending on an error occurred or not. – Contains both N(S) and N(R).
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RR Frame – – – –
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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 RNR’s N(R) indicates the last I frame that was correctly received
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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.
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SABME Frame – is sent when no layer 2 connection was established
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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 before 21 disconnecting
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UI Frame – Contains neither the N(S) nor N(R). – Doesn’t require an ack. As P=0.
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DISC Frame – Used to take layer 2 connection out of service – Waits for an acknowledgement from all peers in the form of UA
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UA Frame – Used to answer a DISC or SABME frames
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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
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