1. TEL 355: Communication and Information Systems in Organizations Architecture: Signaling System 7 (SS7) Professor John F. Clark

2. Signaling Exchange of info between call components required to provide and maintain service Users signal network elements and network elements signal each other SS7 is the way network elements talk SS7 is characterized by high-speed packet data and out-of-band signaling

3. Out-of-Band Signaling Does not take place over the same path as the conversation Establishes a separate digital signal – the signaling link Takes place at speeds of 56 or 64 kbps Somewhat like ISDN except ISDN extends out-of-band signaling all the way to the user via a D channel and two B channels

4. Why Out-of-Band Signaling? Several advantages over in-band signaling Transports more data at higher speeds – 56 and 64 kbps are faster than MF outpulsing Allows signaling at any time during the duration of the call, not just the beginning Enables signaling to network elements to which there is no direct trunk connection

5. Signaling Network Architecture Associated Switching –One of the many paths between an inter- connected pair of switches is the signaling path –The rest of the paths are voice trunks –The signaling path handles all the signaling for all the voice trunks –Works well only between switches directly connected by trunks –Much more complicated when signaling between switches without a direct connection

6. N. Amer. Signaling Architecture Three essential components, connected by signaling links: –Signal switching points (SSPs) – telephone switches with SS7 software that originate, terminate, or switch calls –Signal transfer points (STPs) – packet switches that receive and route incoming signaling messages –Signal control points (SCPs) – databases that provide information for advanced call processing

7. N. Amer. Signaling Architecture II Each element is held to exacting standards Precisely defined protocols manage the routing of signaling through the network And an SS7 network is highly redundant to assure reliability –STPs and SCPs are deployed in redundant pairs –They may not be co-located, but they perform identical functions

8. N. Amer. Signaling Architecture III Quasi-Associated Switching –Mated pairs of STPs perform identical functions –Each SSP has a link to each STP of a mated pair –The STPs of a mated pair are joined by a link –The two mated pairs of associated networks are joined by four links – a quad –SCPs are usually (not always) deployed in pairs –SCPs of a mated pair are not joined by links

9. SS7 Link Types All SS7 signaling links are identical in that they: –are bidirectional data links –are 56 kbps or 64 kbps –support the lower three layers of the protocol However, they are used for different purposes

10. SS7 Link Types II –“A” links connect an STP to either an SSP or an SCP – “A” for Access –“C” links connect mated STPs – “C” for Cross –“B,” “D,” and “B/D” links connect two mated pairs of STPs – “B” for Bridge and “D” for Diagonal –Optional “E” links connect an SSP to an additional non-home pair of STPs for added reliability – “E” for Extended –“F” links bypass STPs (and SCPs) and directly connect two SSPs – “F” for Fully Associated

11. Layers of the SS7 Protocol Physical layer – physical and electrical properties of the medium – always DS-0 – thus 56 or 64 kbps Message Transfer Part (MTP level 2) – link layer functionality – error checking, flow control, and sequence checking Message Transfer Part (MTP level 3) – network layer functionality – node addressing, routing, and congestion control

12. Layers of the SS7 Protocol II Signaling Connection Control Part (SCCP) – Addresses applications within a node such as 800 call processing, calling-card processing, and advanced intelligent network (AIN) –Uses global title translation (GTT) to perform incremental routing – minimizes the need for STPs to maintain information about nodes that are far removed from them – also allows STP load sharing among mated SCPs in both normal and failure modes

13. Layers of the SS7 Protocol III ISDN User Part (ISUP) – defines the messages and protocol used over the PSN – always MTP in North America Transaction Capabilities Application Part (TCAP) – defines the messages and protocol used to communicate between applications in a node – delivered by SCCP Operations, Maintenance, and Administration Part (OMAP) – assists system administrators

14. Signaling Units SUs are 8-bit bytes (octets) and are transmitted continuously in both directions on any link that is in service There are three types of SU: –Message signal units (MSUs) – contains all the actual signaling information –Link status signal units (LSSUs) – transmits information about the signaling link –Fill-in signal units (FISUs) – occupy the link when no MSUs or FISUs are being transmitted

15. Signal Unit Structure All three types of SUs have 4 common fields: –Flag – marks the beginning of an SU and thus delimits the SUs in a transmission –Checksum – a calculation for error checking –Length indicator – indicates the number of octets between itself and the checksum – differentiates between types of SUs –BSN/BIB/FSN/FIB – confirm receipt of SUs, ensure they are in the right sequence, and provide flow control

16. Signal Unit Structure II An LSSU has an additional “Status Field” –Comprised of one or two octets –Signals the initiation of link alignment –Signals the quality of received signaling traffic –Signals the status of the processors at either end –Requires no addressing information since they are always sent between the signaling points at the end of the link

17. Signal Unit Structure III An MSU has a “Service Information Octet” –Four bits indicate the type of information in the signal information field –Two bits indicate national or international network –Two bits identify a message priority An MSU has a “Signaling Information Field” –8 to 272 octets in length –Contains all the actual signaling information, beginning with the routing label