Transcript Signaling System No. 7 (SS7) protocol stack.

PSTN
Bidirectional DS0 circuit
DS0 lines
T-S-T
Switch
T-S-T
Switch
Trunks
T1
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T-S-T
Switch
T-S-T
Switch
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
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Role of SS7 network
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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
Databases
(e.g. 800)
T-S-T
Switch
T-S-T
Switch
STP
T-S-T
Switch
STP
T-S-T
Switch
T-S-T
Switch
T-S-T
Switch
STP
STP
T-S-T
Switch
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T-S-T
Switch
STP: Signal Transfer Point: datagram router
that forwards MTP L3 datagrams
Databases
(e.g. calling card)
3
SS7 protocol stack
Switch
I
S
U
P
TCAP
SCCP
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STP
SCCP
MTP L3
MTP L3
MTP L2
MTP L2
MTP L1
MTP L1
MTP L3-L1 Message Transfer Part Level 3 to Level 1
TCAP Transaction Capabilities Application Part
SCCP Signaling Connection Control Part
ISUP ISDN User Part
4
MTP L3
• Connectionless (datagram) NL protocol
• Addressing scheme: point code (14 or 24 bits)
MTP L3 Header
MTP L2
SLS
Header
8
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OPC
24
DPC
24
Network
Identifier
Network
Cluster
Network
8
8
8
Cluster Member
ISUP or TCAP/SCCP
message
SLS Signaling link Selection
DPC Destination Point Code
OPC Originating Point Code
5
Basic call setup (using ISUP)
SSP
DTMF
Signaling
SSP
SSP
STP
MTP L3
packet forwarding
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SSP
IAM Initial Address Message
ACM Address Complete Message
ANM Answer Message
6
Call setup with advanced services
(using TCAP)
516-555-1278
Local Carrier
SCP
Long distance carrier
STP
STP
ISUP
IAM
Switch
Local Carrier
Switch
Long Distance
SCP
STP
STP
Switch
Switch
Signaling links
Voice trunks
800-555-4400
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STP: Signal Transfer Point
SCP: Service Control Point
SCP
ring
516-555-1278
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