A Review of the Architecture and the Underlying Protocols in the Telephone Network

1. A Review of the Architecture and the Underlying Protocols in the Telephone Network Dipak Ghosal Department of Computer Science University of California at Davis

2. Outline • History • Network Architecture • SS7 Protocol • Routing • Media Stimulated Focused Overload • Overview of Telephony Research • Current Efforts

3. History • Pre-1984 • AT&T • 1980’s saw rapid deployment of digital technology in the core network • 1984 • Breakup of AT&T into • 7 RBOCs (Regional Bell Operating Companies), • AT&T, and others • Local area carriers (LECs) serving LATA were regulated • Long distance carrier (IXC) service was opened

4. History (2) • Post 1984 • New Telecom Act in 1996 • Further deregulation of LECs (ILECs and CLECS) • Local area and long distance markets opened • Local Number Portability • Break-up of AT&T • AT&T • Lucent (Bell-Labs) • Mergers of RBOCs and CLECs

5. Outline • History • Network Architecture • SS7 Protocol • Routing • Media Stimulated Focused Overload • Overview of Telephony Research • Current Efforts

6. A Typical Regional POTS Network

7. Network Architecture

8. Circuit Network • Central Offices (End Offices) • Local aggregation points for phone lines • Wire-pair (local loop) to each telephone • Tandems • Hubs interconnecting Central Offices • Connecting to IXCs

9. Circuit Network (2) • Hierarchical organization • End office • Toll Center • Primary Center • Sectional Center • Regional Center

10. End Office

11. Signaling Network • Signaling network is the brain • Circuit network forms the the muscles • All nodes in the signaling network are called signaling points • SSP -> Service Switching Points • STP -> Signaling Transfer Point • SCP -> Service Control Point

12. Service Switching Point • This is the local exchange in the telephone network • Interfaces both the circuit network and signaling network • Generate SS7 messages from signals from the voice network • Generate SS7 query messages for non-circuit related messages • LNP has significantly altered the traffic mix

13. Signaling Transfer Point • Routers in the SS7 network • Route messages between SSPs • Support Global Title Translation for non-circuit related messages • These can be separate stand alone nodes or adjuncts to a voice switch • Many tandems used to act as STPs • Deployed as a mated pair

14. Signaling Transfer Point (2) • Hierarchy of STPs • Local and Regional STPs • International STPs • Gateway STPs • Interconnect different networks including cellular networks • Very important node in the SS7 network • Many other functions including measurements and data mining

15. Service Control Point • Interfaces to databases • 800/900 databases • HLR/VLR databases • LIDB (Line Information Databases) for calling cards • Local Number Portability Database • New Advanced Intelligent Network (AIN) services.

16. Types of Signaling Links

17. Types of Signaling Links (2) • A-Links are access links between SSP and STP or SCP and STP • B-Links are bridge links that connect mated STP pairs in the same hierarchy • C-Links are cross links between an STP and its mat • D-Links are diagonal links between STPs at different levels of the hierarchy • E-Links a extended links to connect to remote STP pairs • F-links are fully associated links

18. Types of Signaling Links (3) • Link sets are group of links with the same adjacent nodes • Route is a collection of link sets required to reach a destination • Route set is a collection of routes • Routing is hop-by-hop • A signaling point needs to know which linkset to use towards the destination

19. Addressing • Each signaling point has a address and it is referred to as the Point Code • It is a 24-bit address • 8 bits network identifier • 8 bits cluster identifier • 8 bits node identifier • Full point code routing • Partial point code routing • Cluster routing or network routing

20. Requirements • Availability objective: an unavailability of no more than 10 minutes downtime between two SPs • Lost message probability: 1 in 10**7 • Message Out-of-sequence probability: 1 in 10**10 • Performance objectives: • Maximum link utilization must be less than 40% • Various other requirements on various processing delay • Maximum message processing delay at an SP is 200ms

21. Outline • History • Network Architecture • SS7 Protocol • Routing • Media Stimulated Focused Overload • Overview of Telephony Research • Current Efforts

22. Protocol Stack

23. ISDN User Part (ISUP)

24. ISDN User Part (ISUP) • IAM – Initial Address Message • Message type, Called party number, calling party category, forward call indicators, nature of connection identifier, user service information • ACM – Acknowledge Message • ANM –Answer Message • REL – Release Message • RLC – Release Clear Message • All these message have a associated circuit identification code (CIC)

25. Database Query (TCAP)

26. Signaling Connection Control Part (SCCP) • Additional functions over MTP (network) layer to support connectionless and connection oriented services • Very similar to transport layer • Address Translations • Dialed digits to destination point codes • Particularly important for non-routable numbers such as 800/900. • GTT functionality is supported in the STP to determine which database will provide the translation.

27. Message Transfer Part (MTP) Layer 3 • Network Management • Link management • Traffic management • Route Management • Message discrimination • Message distribution • Message routing

28. MTP Layer 3 (2) • Message discrimination • Determine if the message is destined to the receiving node • If yes apply message distribution to distributed it to the appropriate application • Else, route it to the destination using the most direct route (I.e., fewest number of hops)

29. MTP Layer 3 (3) • Traffic management • Link failures • Route failures • Congestion

30. Transient A-Link Failure

31. Link Failure • Level-2 processor sends a link failure message to the Level-3 processor • Level-3 processor updates its own routing table • Level-3 processor sends out routing table update message to other Level-3 processors within the STP

32. Link Failure (2) • Send out Traffic Restricted (TFR) messages to all the SPs • Send out Traffic Prohibited (TFP) message to the mate-STP via the C-link • Send change-over message to the corresponding SP • Sends changeover signal to the Level-2 processor to re-routes messages via the C-link

33. Congestion

34. STP Architecture

35. Key Design Issue • What is the best cluster size? • Centralized architecture have few Level-3 processors • Fewer number of routing tables hence quicker update of failue information within the STP • Potential Level-3 processor overload • Distributed architectures have large number of Level-3 processors • Multiple failures can be processed in parallel • Large number of routing tables and hence delays in updating all copies • What is the priority structure for different message types in the Level-3 processor?

36. Model of Level-3 Processor

37. Network Model • 1, 8, 16, 24 A-link failures • All failures to a single STP • Simultaneous recovery after 11 seconds

38. Call Throughput

39. Key Results • A clustered architecture with 8/16 Level-2 processors per Level-3 processor performed the best • Priority of tasks was a very important factor • Dynamic priority inversion

40. Outline • History • Network Architecture • SS7 Protocol • Routing • Media Stimulated Focused Overload • Overview of Telephony Research • Current Efforts

41. Routing in Circuit Network • Dynamic Routing • Some part of the routing changes over time • Adaptive Routing • Some part of the routing is a function of the network state at the time the decision is made

42. Alternate Routing • An ordered set of routes from which the choice is made • Fixed alternate routing • A small subset of fixed route is used • The set of alternate route is scanned in some predetermined order and the call is connected on the first free path that is found • There are different methods on how the routing control is propagated

43. Alternate Routing (2) • There are different methods on how the routing control is propagated • Originating-office control • Spill-forward control • Crankback

44. Fixed Hierarchical Routing • Hierarchical organization of switches • End office • Toll Center • Primary Center • Sectional Center • Regional Center • There are specific hierarchical fan rules of how switches are connected

45. Dynamic Nonhierarchical Routing • Deployed in mid 1980s • A day is divided in to 10 traffic periods • All switches are same – no hierarchy • Routing is alternate type with the provision that alternate paths are limited to atmost two links • Long paths can result in “knock-on” effect and make the system highly sensitive to overloads • Uses crankback

46. Adpative Routing • Residual capacity adaptive routing (RCAR) • Uses occupancy information of all trunk groups periodically updated by measurements • DCR – sends calls to paths with the largest expected number of free trunks • Trunk Status Map Routing • Adaptive DNHR

47. Outline • History • Network Architecture • SS7 Protocol • Routing • Media Stimulated Focused Overload • Overview of Telephony Research • Current Efforts

48. The Problem • Media events may stimulate a large number of calls to a single number in a very short time interval • Mass Call-Ins cause focused overloads, denying service to customers trying to reach other numbers • Outages may persist for long period • Existing automated network controls protect the network, but deny service unnecessarily

49. Example of Mass Callin

50. Choke Network • Special exchange which serves many clients (e.g., radio stations) that regularly generate call-ins • Small number of trunk to this exchange • Not suitable for clients that would like to have large number of calls completed (ticket sales)