Signalling System 7
Signalling System 7 is a digital communications standard rapidly coming into use on many telecommunications networks. There are many components to the SS7 system which is designed to interface a digital communications system into an ever expanding network of applications and protocols.
Signal Transfer Point (STP)
- Makes Destination based Routing Decisions
- Needs dedicated DS0's
(Timeslots/Channels on T1)
- Requires Redundancy so always
Paired into Cross Links (C links)
- Cross connect to other STP's
- Hierarchy of Switches
Signalling Point Code (SEP)
- Addresses for Network Terminating Equipment
- Linked up using paired connections
- First link called "local" connection
Access links (A Link)
- Link SEP's to STP's (Lines)
- First link called LOCAL
Bridge Links (B Links)
- Interconnect STP's (trunks)
Diagonal (D links)
- Connect levels of the hierarchy
Extended (E Links)
- Connect SEP's to other STP's for
additional redundancy
Fully Associated (F Links)
- SEP to SEP links
- Can transmit anything so long as
both sides understand format
SSP (Service Switching Points)
- Signalling Point where the
PSTN connects to SS7
- TWO TYPES
- Common Channel Signalling
Switching Office (CCSSO)
- End or Tandem offices w/SS7 Capacity
- Can use SS7 to set up a Trunk
Signalling mode call
- Service Switching Point
- Handle call set up also,
or stop calls (teardown)
- Can also perform other
functions based on software
Service Control Point (SCP)
- Database of Service Point Codes
(addresses) is at SCP
- Come in various varieties
- Provide services to the network
based on Software
- SCP's identified by SPC's and a
Sub-System Number (SSN)
- SSN's are databases, or other software
Intelligent Peripheral (IP)
- Located off network
- Provide services to SCP's via the SSN #
- Husally more hardware than soft
Services Node (SN)
- Similar to an IP, but usually more
software than hardware
Mobile Switching Center (MSC)
(Mobile/Cellular services)
- Home Location Register (HLR)
- Authentication Centers (AUC)
- Short Message Centers (SMC)
FUNCTIONAL LAYERS OF SS7
| OSI MODEL | SS7 MODEL | ||
|---|---|---|---|
| Application | OMA&P | ASE | ISDNUP |
| Presentation | TCAP | ||
| Session | |||
| Transport | SCP | ||
| Network | Other User's Parts |
||
| Data Link | MTP L2 | ||
| Physical | MTP L1 | ||
** Other User Parts
Telephone Users Part
Data Users Part
SS7 LAYERS
OAM&P
Operations Administration Management & Provisioning
ASE
Application Service Element
TCAP
- <UNKNOWN>
MTP
Message Transport Part
- L1 - Links, Clocking, physical considerations
- L2 - ITU or ANSI signalling. Message assembly of signalling units,
adds header flags (byte with zeroes at
both ends and 1's in the middle).
Bit stuffing to prevent accidentally
creating fake flags. Link monitoring,
Congestion monitoring/reporting - L3 - Congestion Control
THREE TYPES OF SIGNALLING UNITS
- Message Signal Units - Houses all SS7 msgs
- Link Status Signal Unit - Information about the Link's status.
- Fill In Signal Unit - No gaps in transmission, recognizeable data on the link
COMMON FIELDS OF SIGNALLING UNITS
NAME BITS Description
--------------------------------
- SPARE 2
- LI 6
- FIB 1 Forward Indicator Bit
- FSN 7 Forward Sequence Number
- BIB 1 Backward Indicator Bit
- BSN 7 Backward Sequence Number
- FLAG 8 Fixed pattern: (01111110)
QUEUES
-----------------------------------
Transmit Buffer
Retransmit buffer
FSN - Copies destroyed as ack'd
as received at remote MTP
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