Protocols

GSM Network and Services

Interfaces and protocols - even more three letter acronyms

GSM Network and Services 2G1723 Johan Montelius

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Interfaces

EIR F BSC C

A-bis

HLR

D

A

B MSC

VLR

E

Um

G VLR MSC GSM Network and Services 2G1723 Johan Montelius

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Signaling protocols BSS Abis

Um

A

CC MM RR

RR

RR

BTSM

BTSM

LAPDm

LAPDm

LAPD

LAPD

radio

radio

E1

E1

GSM Network and Services 2G1723 Johan Montelius

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Signaling protocols BSS •

CC MM

•

LAPD – Link Access Procedure D channel, the link layer defined for ISDN, provides retransmission, error detection ... E1 – the physical layer most often used for ISDN, 30 traffic channels and one signaling channel all of 64kbps RR

RR

RR

BTSM

BTSM

LAPDm

LAPDm

LAPD

LAPD

radio

radio

E1

E1

GSM Network and Services 2G1723 Johan Montelius

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Signaling protocols BSS •

CC MM

•

RR – radio resource management – idle mode: monitoring broadcast channels – dedicated mode: paging, request, measure quality, power control, encryption, handover.. BTSM – BTS management, the BSC controls the radio resources of the BTS RR

RR

RR

BTSM

BTSM

LAPDm

LAPDm

LAPD

LAPD

radio

radio

E1

E1

MS GSM Network and Services 2G1723 Johan Montelius

BTS

BSC 5

Signaling protocols – MS/BSC/MSC CM MM RR

DTAP

DTAP

SCCP

SCCP

MTP3

MTP3

MTP2 MTP1

BSC GSM Network and Services 2G1723 Johan Montelius

SS7

MTP2 MTP1

MSC 6

SS7 – signaling system number 7 • MTP – message transfer part, defines layer one to three. – MTP1: could be E1 – MTP2: link layer, error detection, retransmission... – MTP3: addressing in the form of signaling points, one interface has a unique address, limited address range

• SCCP – signaling connection control part – segmentation, global addressing, sub addressing (similar to UDP ports), each mobile has a MSC unique SCCP address GSM Network and Services 2G1723 Johan Montelius

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DTAP/MM/CC • Direct transfer application part – Will enclose a MM message so that it is transparently shipped over the BSC and BT to the MM layer at the mobile phone

• Mobility management – location area updating, paging, authentication ...

• Connection Management – Call control, SMS, supplementary services GSM Network and Services 2G1723 Johan Montelius

CM MM DTAP SCCP MTP3 MTP2 MTP1

MSC 8

Signaling protocols – BSC/MSC • BSS Application Part – the MSC will direct the BSC, for example when doing handover between two BSCs BSSAP

BSSAP

SCCP

SCCP

MTP3

MTP3

MTP2

MTP2

MTP1

MTP1

BSC GSM Network and Services 2G1723 Johan Montelius

MSC 9

Signaling protocols – MSC/HLR/VLR/... • TCAP - transaction capabilities application part, defined in the SS7 stack • MAP – mobile application part, this is the application layer protocol used by the noes for example register updating and query

MAP TCAP SCCP SCCP MTP3 MTP2 MTP1

GSM Network and Services 2G1723 Johan Montelius

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Which are important • • • • • • •

MAP BSSAP / BTSM / RR MM CM LAPDm Radio ... and of course the traffic channels

GSM Network and Services 2G1723 Johan Montelius

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Services – what is offered • Bearer services – basic bit pipes in various sizes • Tele services – Speech, SMS, fax, teletext ... • Supplementary services – Controls the tele services

GSM Network and Services 2G1723 Johan Montelius

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Bearer services • Circuit switched data services – up to 9.6 kbps – up to 57.6 kbps using 4x14.4 HSCSD (how) • Circuit swicthed data comes as: – tranparent or non-transparent, the network does error detection and retransmission • GRPS has introduced new bearer services – all data services are now over GPRS

GSM Network and Services 2G1723 Johan Montelius

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Teleservices Voice, TS11 Emergency call, TS12 Fax, TS61 SMS, – mobile terminating TS21 – mobile originating TS22 • Message Handling System, TS31 • Teletext TS51 – does anyone use this? • • • •

GSM Network and Services 2G1723 Johan Montelius

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Supplementary services • Call forwading – always/busy/no reply/not reacable • Barring – Outgoing: all or international – Incoming: all or international when roaming • Number presentation • Call waiting • Multi-party GSM Network and Services 2G1723 Johan Montelius

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Transport of voice • GSM Speech Code is converted in the TRAU (often in the BSC) to A-law (regular phone voice codec) GSC Link

Link

Cipher

Cipher

Radio

Radio

GSM Network and Services 2G1723 Johan Montelius

E1

GSC

A-law

E1

E1

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The air interface • Radio modulation – how do we turn an analog radio carrier into a digital channel • How are logical channels mapped to the physical channels. • How is bearer services coded and sent using the logical channels. • How are the logical channels used for signaling.

GSM Network and Services 2G1723 Johan Montelius

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The logical channels • Traffic channels - TCH – Full rate used for 13kbps voice or up to 14.4kbps data – Half rate used for 6.5kbps voice or low rate data • Signaling channels – Broadcast channels - BCH – Common Control Channels - CCH – Dedicated Control Channels - DCH GSM Network and Services 2G1723 Johan Montelius

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Broadcast Channels • Frequency Correction Channel (FCCH) – only visible at physical layer, helps the mobile tuen in to the right frequency • Synchronisation Channel (SCH) – Helps the mobile stay synchronized with the base station, also carries the base station identity code (colour codes) • Broadcast Control Channel (BCCH) – Information about the cell, neighbouring cells, location area identifier, structure of the Common Control Channels GSM Network and Services 2G1723 Johan Montelius

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Common Control Channels • Random Access Channel (RACH) – for the mobiles to request access • Access Grant Channel (AGCH) – for BSC to grant access • Paging Channel (PCH) - for BSC to page a mobile • Notification Channel (NCH) – for the BSC to notify a group of mobiles of Voice Broadcast Services

GSM Network and Services 2G1723 Johan Montelius

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Dedicated Control Channels • Stand alone Dedicated Control Channel (SDCCH) – duplex point to point signaling channel • Associated Controll Channels – Allways allocated together with a traffic channel or a stand alone dedicated control channel. Will controll the performance of the channel. – Slow (SACCH) – Fast (FACCH)

GSM Network and Services 2G1723 Johan Montelius

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Cell Broadcast Channel • Use to send out SMS broadcast messages, for example the “KTH/Monaco” that shows up in the screen when you connect to the Monaco network.

GSM Network and Services 2G1723 Johan Montelius

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Incomming call MS

paging (PCH)

BSS

channel req (RACH) assigned (AGCH) paging resp (SDCCH) auth req and resp (SDCCH) cipher setup (SDCCH) call setup (SDCCH) call connect (FACCH) talk (TCH) GSM Network and Services 2G1723 Johan Montelius

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Logical channels • Defined at layer 1 and provided for layer 2 (except the frequency correction channel and the synchronization channel). • Layer three of the signaling layer can add acknowledgment and retransmission. • Mapped on the physical channels: – more than one logical channel can be mapped on one physical channel – mapped in time and coding

GSM Network and Services 2G1723 Johan Montelius

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Mapping of logical channels • TCH/F + FACCH + SACCH – needs a full physical channel • TCH/H + FACCH + SACCH – half a physical channel • SDCCH + FACCH + SACCH – eight channels in one physical • BCH (BCCH/FCCH/SCH) + CCCH (RACH/PCH/AGCH/NCH) – One physical channel (or half) • A cell always needs a Broadcast channel. GSM Network and Services 2G1723 Johan Montelius

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Mapping of traffic channel 1

2

TCH TCH

13 SA

26 idle

120 ms

A physical channel is a specified time slot. Following this time slot in consecutive frames we have a sequence. Number this sequence mod 26, the traffic channel will occupy 1-12 and 14-25. The slow associated control channel occupy number 13, frame 26 is idle. The fast associated control channel is implemented using “stealing flags” in the traffic channel. GSM Network and Services 2G1723 Johan Montelius

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