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 7 7 6 6 5 5 4 4 3 3 2 2 1 1
The networks Access Edge Core
Current architectures CS Core (POTS, ISDN) Fixnet PS Core (IPSS IP Based) Telephony Gateway Wireline Access xDSL PS Core (ATM) Mobile Wireless Access Telephony Gateway Wi-Fi Cellular Access GSM Core GSM-GPRS-UMTS
Networks modelling • 1 component = 1 objet • Switch • Access Point • Router • 1 objet = Σ fonctions • 1 Dialog protocole with the object • MIB & SNMP
ISDN Interfaces TE1 NT2 NT1 TE2 TA R S T S/T
GSM Interfaces MT0 BS/MSC TE1 MT1 BS/MSC TE2 TA MT1 BS/MSC TE2 MT2 BS/MSC R S Um
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 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 – 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 AssociatedLinks – F Links.
SS#7 - The Userparts Possible Future extentions.
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 Part ASE: Application Service Element TUP: Telephone User Part DUP: Data User Part TCAP: Transaction Capabilities Application Part ISUP: ISDN User Part SCCP: Signalling Connection Control Part MTP: Message Transfer Part
Message Transfer Part MTP level 2 simply monitors and reports to the layer above. It assembles the outgoing messages into packets known as signalling units. 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 Messages # of bits > Up to 279 bytes packet size Lock on “flag” pattern: 0111110 - zeros at both ends & 1s in the middle. Bit stuffing could be necessary
MTP level 2 Messages Backward Sequence Number (BSN). Backward Indicator Bit (BIB) Forward Sequence Number (FSN) Forward Indicator Bit (FIB) Length Indicator (LI)
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
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
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
SS#7 - Call Setup • IAM: Initial Address Message • ACM: 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