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11/21/99 1
PSTN
• A T1 (1.5 Kbps) is a time-multiplexed signal of 24 DS0 (64 kbps)• A T-S-T (Time-Space-Time) division multiplexed switch maps bits arriving in
one time slot to another time slot on an output port
T-S-TSwitch
DS0 lines T-S-TSwitch
T-S-TSwitch
T-S-TSwitch
TrunksT1
Bidirectional DS0 circuit
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Role of SS7 network• To set up a phone call, messages are sent from one T-S-T switch to the next until
the called party is reached.• These messages are called ISUP messages (see protocol slide) and sent through a
packet-switched connectionless network called the SS7 network.• The packet switches in the SS7 network are called STPs, which perform MTP L3
datagram forwarding• STPs are comparable to IP routers; the network layer protocol they operate on is
MTP L3 instead of IP; the addresses they use are called Point Codes.• The SS7 network is also used to carry TCAP messages (see protocol slide) to
provide value-added services, such as 800-number, calling card calls, etc. These messages are sent from the switches to SCPs.
• With the help of the SS7 network, the PSTN is now considered an “Intelligent Network,” allowing the network to offer value-added services beyond the basic phone call service (called Plain Old Telephone Service or POTS).
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SS7 network architecture
STP STP
STP STP
T-S-T Switch
T-S-T Switch
T-S-T Switch
T-S-T Switch
T-S-T Switch
T-S-T Switch
T-S-T Switch
T-S-T Switch
Databases(e.g. calling card)
STP: Signal Transfer Point: datagram router that forwards MTP L3 datagrams
Databases(e.g. 800)
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STPSwitch
SS7 protocol stack
MTP L1
MTP L2
MTP L3
SCCP
TCAP
MTP L3-L1 Message Transfer Part Level 3 to Level 1TCAP Transaction Capabilities Application PartSCCP Signaling Connection Control PartISUP ISDN User Part
SCCP
ISUP
MTP L1
MTP L2
MTP L3
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MTP L3
8 24 24
NetworkIdentifier
NetworkCluster
NetworkCluster Member
8 8 8
SLS Signaling link SelectionDPC Destination Point Code
OPC Originating Point Code
• Connectionless (datagram) NL protocol• Addressing scheme: point code (14 or 24 bits)
MTP L2Header SLS OPC DPC
MTP L3 Header
ISUP or TCAP/SCCPmessage
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Basic call setup (using ISUP)
SSP
SSP
DTMF Signaling IA
M
IAM
ACM ACM
RINGRESPONSE
ANMANM
IAM Initial Address MessageACM Address Complete MessageANM Answer Message
SSP
SSP
STP
IAMMTP L3packet forwarding
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Call setup with advanced services (using TCAP)
Switch
STP SCP
Switch Switch
Switch
STP SCP
STP SCP STPLocal Carrier Local Carrier
Long Distance
800-555-4400
800-555-4400
800-
555-
4400
516-555-1278
516-555-1278
ring
STP: Signal Transfer PointSCP: Service Control Point
Voice trunksSignaling links
Long distance carrier
516-555-1278
ISUPIAM
