Telecoms Networks

Mohamed Mokdad

HES – Biel/Bienne

Agenda

• OSI Model

• Networks modelling

• Connection types

• Interfaces

• SS#7– Transfer Message Part– User Parts

• Standards

OSI Layers dynamics

1

2

3

4

5

6

7

1

2

3

4

5

6

7

Transport network

Signalling network

A real SS#7 network

Management network

Network Overlaying

The networks

Core

Edge Access

Current architectures

xDSL

Fixnet

Wireline

Access

GSM-GPRS-UMTS

Wi-Fi

Mobile

PS Core (IPSS IP Based)

Telephony Gateway

Telephony Gateway

CS Core

(POTS, ISDN)

GSM Core

Cellular

Access

Wireless

Access

PS Core (ATM)

Telephony Gateway

Telephony Gateway

Networks modelling

• 1 component = 1 objet– Switch– Access Point– Router

• 1 objet = Σ fonctions

• 1 Dialog protocole with the object– MIB & SNMP

ISDN Interfaces

NT1NT2TE1

TATE2

R S T S/T

GSM Interfaces

BS/MSCMT0

BS/MSCMT1TE1

BS/MSCMT1TATE2

BS/MSCMT2TE2

UmSR

Circuit/Packet Switched?

• Circuit switching– 1 circuit dedicated to a connection– Connection must exist before data transfer– Connection oriented

• Paquet switching– 1 paquet can follow any connection– Connection doesn’t exist before data transfer– Connectionless

Interconnexion

• PTS (Provider of Telecom Services)– What is it?

• Interconnexion– Transport– Signaling– Billing

• TDM & IP voice services interconnection– Présentation and discussion

SS#7 example = CCBS ?

SS#7 Architecture - STP

examines the destination of messages it receives, consult a routing table, and send the messages via Cross Links - C links.

SS#7 Architecture - SEP

End point in the SS#7 in the same way that a telephone is an end point in the PSTN. This is located in the local switch and is addressed by the Signalling Point Code. These are linked via Access Links - A links.

SS#7 Architecture – B&D Links

Bridge Links - B links - connect 2 STPs in a redundant way and at the same hiercharchy level.

SS#7 Architecture – D Links

SS#7 Architecture – E Links

Extended links - E links – can be connected for redundancy and reliability reasons to another pairs of STPs.

SS#7 Architecture – F Links

SEP can be providing services to another SEP (data, special processing, data concentration, etc.). Connection is made via Fully Associated Links – F Links.

SS#7 - The Userparts

Possible Future extentions.

Mapping OSI-SS#7

The Layered Model

• Different networks can share common resources.e.g. “transport mechanism” for signalling

• New specific user partse.g. MAP integration afterwards

• Easier protocol conversions and encapsulation in different networks, e.g. not just SS#7, but genrally speaking

SS#7 - Userparts

OMAP: Operations, Maintenance and Administration PartASE: Application Service ElementTUP: Telephone User PartDUP: Data User PartTCAP: Transaction Capabilities Application Part ISUP: ISDN User PartSCCP: Signalling Connection Control Part MTP: Message Transfer Part

OSI-Primitives

OSI-Primitives - 2

Message Transfer Part

MTP level 1 considers the links, the control of clocking, and all of the physical considerations of sending messages over wires. Electrical Design Engineers job.No interest for software designers

MTP level 2 simply monitors and reports to the layer above. It assembles the outgoing messages into packets known as signalling units.

MTP level 2 Messages

Up to 279 bytes packet sizeLock on “flag” pattern: 0111110 - zeros at both ends & 1s in the middle.Bit stuffing could be necessary

# of bits >

Bit Stuffing

Byte Stuffing

MTP level 2 Messages

Backward Sequence Number (BSN). Backward Indicator Bit (BIB)Forward Sequence Number (FSN) Forward Indicator Bit (FIB)Length Indicator (LI)

MTP 2 - Transmission

MTP 2 – Basic Error Correction

Basic Error Correction

• When the packet is received at the transmitting side the difference in the FIB and the BIB trigger a series of actions:– Transmission is stopped.– Messages beginning with FSN #19 are

retransmitted in sequence.– Messages which were in the buffer before

FSN #19 are deleted.– Transmission begins again

MTP 2 - Frames

SS#7 – MTP level 3

Signalling Network Management (SNM) Signalling Message Handling (SMH)

Major part of the job of Level 3 is to control traffic on the traffic resources over which it has control, e.g. congestion, redirection, changeover, changeback

Signalling Network Management

• Force Rerouting

• Controlled Rerouting

• Changeover

• Changeback

• MTP Restart

Signalling Message Handling

Userparts

• SCCP: Signalling Connection Control Part– Sequence control and delivery

• ISUP: ISDN User Part– Digital networks

• TUP: Telephone Users Part– Analog telephony

• DUP: Data Users Part– Data

Architecture

Stack & message coding

ISUP Example

SS#7 - Call Setup

IAM: Initial Address MessageACM: Address Complete Message - Switch can alert- Off Hook: ANM: ANswer Message - All switches connect the path- Busy: The caller can release - REL - the call setup

SS#7 – Call Release

REL – Release - and – RLC – Release Complete

SS#7 - Simple Call Setup

RingBack Tones

SS#7 - More complete Figure

SS#7- IAM Fields

SS#7 - TCAP Queries

SS#7

SS#7 - ISDN SS CCBS

SS#7

AB

SS#7 - TCAP Message Structure

SS#7 - 0800 Example

SS#7 - The ITU Standard

SS#7 - The ITU Standard

SS#7 - The ITU Standard

2 Signalling messages2.1 Address Complete Message (ACM): A message sent in the backward direction indicating that all the address signals required for routeing the call to the called party have been received.2.2 Answer message (ANM): A message sent in the backward direction indicating that the call has been answered. In semi-automatic working, this message has a supervisory function. In automatic working, this message is used in conjunction with charging information in order to:– start metering the charge to the calling subscriber (see Recommendation Q.28 [2]); and– start measurement of call duration for international accounting purposes (see Recommendation E.260 [3]).

SS#7 - The ITU Standard

2.46Unequipped Circuit Identification Code message (UCIC): A message sent from one exchange to another when it receives an unequipped circuit identification code.2.47User Part Available message (UPA): A message sent in either direction as a response to a user part test message, to indicate that the user part is available.2.48User Part Test message (UPT): A message sent in either direction to test the status of a user part marked as unavailable for a signalling point.2.49User-to-user information message (USR): A message to be used for the transport of user‑to‑user signalling independent of call control messages.

SS#7 and IP

• SIGTRAN– SIGnalling TRANsport

• SCTP– Stream Control Transport Protocol

• Interworking SS#7 and MGCP– Media Gateway Control Protocol

SIGTRAN Stack

SCTP

IMS or some thing else

• Which one to use– SIGTRAN?– SS#7?– IMS?

• 3GPP Signalling– Also valid for VoIP (SIP)

• IMS– IP Multimedia Subsystem