1. Lecture 10

2. Signaling The process of generating and exchanging information between network components to establish, monitor or release connections and control related network and system operations Signaling information is categorized to Supervisory signals/line signals/subscriber signals Routing signals Management signals

3. Supervisory Signals Usually between subscriber and central office Used to: Initiate call Notify of incoming call Control the call (busy signals/ dial tones/ error signals/ call end signal)

4. Routing and Management Signals Routing Signals Between offices Used for path setup and terminal status messages Management Signals Supervisory signals such as: Route updates Link failure notification Link restoration notification

5. Types of Signaling In band (in channel) signaling Loop disconnect Multi-frequency Voice frequency PCM Common channel signaling Associated Non-associated Quasi-associated

6. In-band Signaling Signaling and data use the same channel Usually signaling sent first to setup path, then data follows Loop disconnect-> pulse dialing Multi-frequency-> tone dialing Voice frequency-> use several frequencies that are either within the voice BW (300-3400Hz)->in-band signaling or outside the voice BW=> out of band signaling Pulse Code Modulation (PCM)-> digital signaling where signaling is transmitted in the same time frame as data. Signaling may use separate slots (out-slot signaling) or transmitted as user data header (in-slot signaling)

7. Common Channel Signaling (CCS) Signaling use dedicated signaling links separate from voice links One signaling link can supervise several data links Several types Associated-> data and signaling networks have same topology Non-associated-> independent data and signaling network topologies Quasi-associated-> both use same topology but not the same routes

9. Advantages of In-band Signaling Simple, requires less computation, devices and links Uses same network as data-> can be used in any system Signaling and data use same path hence experience same conditions CCS requires high reliability (link failure-> clear signal lost-> trunk reservation remains) Most terminals use in-band signaling-> converters required for CCS

10. Advantages of CCS Higher signaling capabilities than in-band Destination status can be checked before path setup hence reducing trunk cost One signaling device controls several trunks (less equipment per device) Separate voice and data, failures affect only one Better control of the network-> higher efficiency

12. Signaling System 7 ITU signaling standard Common Channel Signaling System No. 7 (SS7 or C7) Digital signaling for wired and wireless telephony Allows for national variants

13. SS7 Capabilities Basic call setup, management, and tear down Wireless services such as personal communications services “PCS” (e.g. voice mail), wireless roaming, and mobile subscriber authentication Local number portability (LNP) Toll-free (800/888) and toll (900) services Enhanced call features such as call forwarding, calling party name/number display, and three-way calling Efficient and secure worldwide telecommunications

14. SS7 Signaling Network SS7 messages use 56/64 kbps bidirectional, common channel signaling There are 3 types of nodes in SS7 Service Switching Point (SSP): an end point. The switches which actually service the voice call Signal Transfer Point (STP): an intermediate point. A packet switch that relays SS7 messages between end points Service Control Point (SCP): an end point. Provide additional services e.g. route alternatives, how to handle special numbers, voice messages

16. SS7 Signaling Network SS7 failure-> network down Hence SCPs and STPs are deployed as pairs in different locations (e.g. 2 identical SCPs) for more reliability Links between points are also deployed as pairs Traffic is rerouted upon link/node failure Error control and retransmission mechanisms employed for more reliability There are six types of links in SS7 Labeled “A” to “F”

17. SS7 Signaling Network A (Access) Link: connects an end point (SSP/SCP) to an intermediate point (STP) B (Bridge) Link: connects an STP to a peer STP (both can be in same or different networks) C (Cross) Link: connects identical STP pairs (SCP pairs do not have connections between them) D (Diagonal) Link: connects secondary and primary STPs. Similar to B hence links are usually called B/D E (Extended) Link: connects an SSP to an alternate STP. Used as a backup for A links F (Fully associated) Link: connects two end points. Usually not used if there are STPs

19. SS7 Protocol Stack

20. Message Transfer Protocol (MTP) MTP level 1: defines physical specification MTP level 2: ensures accurate transmission between two end points of a link. It employs error checking, sequencing and retransmission techniques for reliability MTP level 3: routes messages to destinations. Avoids failed links and congested areas ISDN User Part (ISUP): defines the protocol to setup, manage and release trunk circuits between terminating exchanges. Used for ISDN and non-ISDN calls. Calls within the same switch do not use ISUP signaling

21. SS7 Protocol Stack Telephone User Part (TUP): used to create and terminate end calls in analog circuits. Used for old systems, replaced by ISUP Signaling Connection Control Part (SCCP): provides connectionless and connection-oriented transport layer services. It translates services requested by upper layers to certain destination point code and subsystem number (similar to IP address and port number) Transaction Capabilities Applications Part (TCAP): supports the exchange of non-circuit related (authentication, roaming, device identification, PIN, etc)

22. SS7 Messages There are 3 types of MTP level 2 messages Fill-In Signal Unit (FISU): empty messages sent regularly (every 150ms) in both directions if there is no data to transmit Link Status Signal Unit (LSSU): carries 1 or 2 octets identifying the state of the link to the other end Message Signal Unit (MSU): carry all types of signaling data in the Signaling Information Field (SIF)

24. MTP Level 2 Message Fields Flag: indicates start of message Backward Sequence Number (BSN): sequence number of last received message (piggybacked Ack) Backward Indicator Bit (BIB): indicates that the message was not received (negative Ack) Forward Sequence Number (FSN): sequence number Forward Indicator Bit (FIB): set if a message is retransmitted (due to a BIB) Length Indicator: based on its value the message is identified as FISU, LSSU or MSU

25. MTP Level 2 Message Fields Service Information Octet (SIO): consists of 2 parts each 4 bits. First part (subservice field) specifies whether this message includes the network indication (e.g. national or international) and priority (0-3). Second part (Service Indicator) defines service type as follows Service IndicatorMTP Service 0Signaling Network Management Message (SNM) 1Maintenance Regular Message (MTN) 2Maintenance Special Message (MTNS) 3Signaling Connection Control Part (SCCP) 4Telephone User Part (TUP) 5Data User Part (call and circuit-related messages) 6 7Data User Part (facility registration/cancellation messages)

26. MTP Level 2 Message Fields Signaling Information Field (SIF): contains the upper layer message. FISU and LSSU do not contain SIO or SIF fields Cyclic Redundancy Check (CRC): used to detect errors in the received message

27. MTP Level 3 Message Fields These messages are routed through the network until it reaches the destination Includes the source address (originating point code “OPC”) and destination address (destination point code “DPC”) The source can specify the link to be used via a field called signaling link selection (SLS) field Different ITU and ANSI message formats Need translation device to communicate

28. MTP Level 3 Message Fields

29. Basic Call Setup In SS7 (ISUP) IAM: Initial Address Message ACM: Address Complete Message ANM: Answer Message REL: Release Message RLC: Release Complete Message

30. Basic Call Setup In SS7 (ISUP) User dials No. S1 sends IAM message with dialed No to destination switch (or next hop switch) requesting specific truck reservation Destination (or next hop) reserves connection, rings destination (or send IAM to next hop) and sends acknowledgement (ACM message) S1 sends ring tone to dialing user Destination lifts handle Destination sends answer message to S1 S1 ensures user is connected to trunk and starts billing

31. Basic Call Setup In SS7 (ISUP) User hangs phone Switch (S1 in fig) sends message to destination switch (or next hop) to release reservation (REL message) Destination switch (or next hop) releases the reserved channel and acknowledges sender of REL (RLC message) Each of these messages have its own message format Other messages exist which handle cases like busy user, network busy, user unavailable, etc