SS#7

T-110.300Jouni Karvo, Timo Kiravuo

Telephone exchanges• Subscriber lines are connected to the telephone

exchanges– Originally each customer had a direct connection

(electrical pair) to the exchange

• Current exchanges are complex and expensive– A need to reduce their number

• Physical cabling becomes an obstacle– Concentrators are used to reduce the number of cables

• As the number of telephone exchanges is decreasing, the hierarchy of telephone exchanges is flattening

The structure of a telephone exchange

• a) Control unit• b) I/O• c) Switching

fabric

The most important tasks for telephone exchanges

• Connecting to subscriber terminal equipment and other exchanges

• Telephone number analysis and translation• Collecting accounting data for billing• Logging data for the use of authorities• Switching calls

I/O

• I/O circuitry connects the telephone exchange to the outside world. The I/O is arranged in hot-swappable I/O cards.

• There are several types of I/O cards (part of which are also used in concentrators)– subscriber line cards (which contains circuits for

connecting several subscribers)– PDH/SDH/ATM cards

Subscriber line cards have several tasks:

• [B] Giving power to the terminal equipment (battery) • [O] Over voltage protection• [R] Ringing• [S] Supervising the status of the terminal equipment (e.g. if

the handle is on or off hook in an old style telephone)• [C] Coder and Decoder (A/D or D/A converters, A-law/? -law

(de)coding )• [H] Hybrid - a transformer between two half-duplex and one

full-duplex connection• [T] Testing the status of the subscriber line

• Memory rule; BORSCHT, the red soup.

Switching fabric

• The task of the switching fabric is to transmit the PCM streams between two I/O ports.

• Both Time and Space Division multiplexing are used.• One measure of the size of the exchange is the capacity of

its switching fabric.• It is easy to find books describing design of switching fabrics

– in a sense it is one of the "easy'' but interesting parts of thetelephone exchanges. We will cover the structure of the switching fabrics only briefly;

• Two main types of switching fabric are used: the crossbar switch and the interconnection network.

Crossbar switches

• Often used in small scale exchanges

• Usually has an electronic switch for each line– To connect n lines, n^2

switches are needed– A typical space division switch

– Uses physical space for connections

– Solid state technology

Interconnection Network

• Used in larger switches– Different types available

• Interconnection networks are made from a number of smaller switching elements.

• The idea in interconnection networks is to save in the number of crossbars using special topologies.

Time Division Switching

• Time Division Multiplexing is used in combination with the above mentioned structures. There are different structures: Time-Space, Space-Time, Time-Space-Time etc.

• The idea in Time division Multiplexing is to rearrange the time slots in a PCM frame (E1).

• Time slot interchanger (TSI):

Time-Space-Time switching

• In Time-Space-Time switching, the internal crossbar works at a higher rate than the input line rate– Any time slot can be switched to any time slot– The crossbar configuration changes for each time slot

Signaling

• The network made of exchanges and links must be co-ordinated in order to make calls possible

• The purpose of signaling is just to do this: it– establishes calls,– monitors the calls, and– tears down the calls.

• In-band signaling was used earlier– MF tones waste of resources– Security problems

• A new signaling system was created:– Common Channel Signaling Subsystem #7

Common Channel Signaling Subsystem #7

• SS#7 is a packet switched network on top of the telephone network

• It is used to transport signaling information– It controls the trunk connections– SS#7 does not transport user data

• SS#7 packets use either a dedicated 56 or 64kbps channel (such as E1 TS1) or dedicated links.

• If signaling does not work, the whole telephone network is "dead''. Thus, stringent reliability requirements.– There is a lot of redundancy; every part of the signaling system

has at least one spare

SS#7 elements

Operator A Operator B

• The actual telephone exchanges switching the actual telephone traffic are called Signal Switching Points (SSP) (circles).

• The packet switches (routers) of the SS#7 network are called Signal Transfer Points (STP) (squares with a diagonal).

• The databases for advanced call-processing capabilities are called Signal Control Points (SCP) (cylinders).

SS#7 overlay network:

SS#7 Protocol Stack

• SS#7 uses a packet switched protocol set to carry its mission.

• The protocol stack generates a packet switched network over a circuit switched one

• MTP-x protocols (Message Transfer Part) protocols provide transport of SS#7 messages– MTP-2 is the link level protocol– MTP-3 provides network level functionality

– Routing to specific elements

SS#7 Protocol Stack

• The upper parts (layers) provide functions for call management– Telephone User Part (TUP) for traditional telephone signaling– Data User Part (DUP) for datacom users– ISDN User Part (ISUP) and Broadband ISDN User Part (BISUP)

for ISDN and ATM users.

Twisted pair / fiber

PDH/SDH/ATM

MTP-1

MTP-2

MTP-3

SCCP

TCAP

OMAPISUPBISUPDUPTUP

Signaling Connection Control Part (SCCP)

• Provides connections for the parts above it• Incremental routing

– Global Title Translation, GTT (compare to DNS)

• Sub-addressing for applications within an element– SSN = Subsystem Number (compare to ports in UDP

and TCP).

• Four classes of service available: – [0] basic connectionless class,– [1] sequenced connectionless class (messages arrive in

order),– [2] basic connection oriented class, and – [3] flow control connection oriented class.

SS#7 Protocol Stack Above SCCP

• Operations, Maintenance and Administration Part (OMAP) provides services for the operator – Include updating routing tables, and diagnostics

• Transactions Capabilites Application Part (TCAP) provides database transaction service for the upper level protocols (such as INAP).

SS#7 Environment Adaptation

• SS#7 is used for the core network • For the access network, there are different signaling stacks:• For ISDN: LAPB/LAPD corresponding MTP-2 on top of

which Q.931 corresponding to ISUP.• For B-ISDN:

– MTP layers 1-2 can be replaced by a specific Signaling AAL protocol (SAAL

– Three layer protocol: bottom: AAL5, then SSCOP (Service Specific Connection Oriented Peer-to-Peer Protocol), then SSCF (Service Specific Coordination Function). SSCOP+SSCF = Service Specific Convergence Sublayer.

– BISUP can be replaced with ATM Forum UNI or ITU-T Q.2931.