EC-413 SS7
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Transcript of EC-413 SS7
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Common Channel Signaling System Number 7 (SS7) 2
7.1 Introduction
Inchannel signaling
Common channel signaling
Network control signaling transition from inchannel to a common channel approach
Signaling System Number 7 (SS7) was first issued by CCITT in 1980 and
revised in 1984, 1988, and 1992.
Designed to be an open-ended common-channel signaling standard for a
variety of digital circuit-switched networks.
Specifically designed to be used in ISDN: provides internal control and
network intelligence essential to an ISDN.
Primary characteristics of SS7 optimized for use in digital telecommunication networks in conjunction with
digital stored program-control exchanges, utilizing 64-kbps digital channels
designed to meet present and future information transfer requirements for call
control, remote control, management, and maintenance
designed to be a reliable means for the transfer of information in the correct
sequence without loss or duplication
suitable for operation over analog channels and at speeds below 64 kbps
suitable for use on point-to-point terrestrial and satellite links
The scope of SS7 is immense, because it must cover all aspects of controlsignaling for complex digital networks, including the reliable routing and
delivery of control messages and application-oriented content of those
messages.
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Common Channel Signaling System Number 7 (SS7) 3
7.2 SS7 Architecture
Associated
Disassociated
Functional Architecture
Control messages in a common channel signaling system are
short packets routed through the network.
Although the network is a circuit-switched network, the control
signaling is implemented using packet switching technology.
SS7 functions could be implemented as additional functions in
circuit-switching nodes!associated signaling mode
Or, the network can have separate switching points for carrying
control packets only !disassociated signaling mode
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Common Channel Signaling System Number 7 (SS7) 4
Signaling Network Elements
Three functional entities in SS7
signaling points (SP): any point in signaling network capable of handlingSS7 control messages, e.g. an endpoint (such as a circuit switching node)
signal transfer points (STP): a signaling point capable of routing
control messages, e.g. a pure routing node
signaling links: a data link that connects signaling points
Two planes of operation: control planeand information plane
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Common Channel Signaling System Number 7 (SS7) 5
Signaling Network Structures
A complex network may have both SPs and STPs in a hierarchical
structure, SPs at lower level and STPs at one or more higher levels. Parameters related to network design and number of levels
STP capacities
number of signaling links that can be handled, signaling message transfer
time, and the message throughput capacity
Network performance
number of SPs and the signaling delays
Availability and reliability
ability of the network to provide service when STP failures
ITU-T suggestions for better reliability
In a signaling network with a single STP level
each SP that is not an STP at the same time connected to at least 2 STPs
the meshing of STPs is as complete as possible (full mesh)
In a signaling network with two STP levels
each SP that is not an STP at the same time is connected to at least two
STPs of the lower level
each STP in the lower level is connected to at least two STPs of theupper level
the STPs in the upper level are fully meshed
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Common Channel Signaling System Number 7 (SS7) 6
Example of Links Used in an SS7 Network
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Common Channel Signaling System Number 7 (SS7) 7
Protocol Architecture SS7 has a layered protocol architecture similar to that of OSI model.
Message transfer part (MTP):the lowest three levels providing a reliable butconnectionless (a datagram style) service for routing messages
signaling data link: a full-duplex physical link dedicated to SS7 (OSI L1)
includes control links between STPs, between an STP and an SP, between SPs
signaling link: a data link control protocol, corresponds to OSI layer 2
provides reliable sequenced delivery of data across signaling data link
signaling network: provides routing data across multiple STPs from
control source to control destination
Signaling connection control part (SCCP):added in 1984 version of SS7
SCCP + MTP = NSP (Network Service Part) contains different network-layer services to meet needs of NSP users
NSP is a message delivery system
ISDN user part (ISUP):controls signaling needed in an ISDN to deal with
ISDN subscriber calls and related functions
Transaction capabilities application part (TCAP):provides the mechanisms
for transaction-oriented (not connection-oriented) applications and functions
Operation, maintenance, and administration part (O&MAP):specifies
network management functions and message related to operation&maintenace
Application service elements (ASEs):add. modules to support new applicats
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Common Channel Signaling System Number 7 (SS7) 8
7.3 Signaling Link Level
corresponds to the data link control layer of OSI model
to turn an unreliable physical link into a reliable data link
reliability implies that
transmitted blocks of data are delivered with no loss or duplication
same order delivery of data blocks as they were transmitted
receiver is capable of exercising flow control over the sender
use of well-known data link control protocol (LAPD and LAPB)
Signal Unit Formats 1. MSU 2. LSSU 3. FISU
FSN
FSN
FSN
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Common Channel Signaling System Number 7 (SS7) 9
Message signal unit (MSU)
carries user data from level 4
Level status signal unit (LSSU)
carries control information needed at the signaling link level
Fill-in signal unit (FISU)
transmitted when no other signal units are available
Definition of different fields in signal units
flag:delimits the signal unit at both ends (01111111)
bit stuffing may be used as with LAPB and LAPD
Four fields for flow- (sliding-window) and error-control (go-back-N ARQ)
backward sequence number (BSN)
contains the number of last MSU successfully received at the other
side; for piggyback acknowledgement
backward indicator bit (BIB)
negative ack of BSN is indicated by inverting this bit
forward sequence number (FSN)
for numbering MSUs uniquely in modulo 128
forward indicator bit (FIB)
indications of MSU is new or retransmitted (e.g. after negative ack)
length indicator (LI)
specifies the length in octets of the following upper-level fields
cross-check on closing flag, also a signal unit type indicator
FISU has no user data field!LI = 0
LSSU has a single user data field of one octet!LI = 1 or 2
MSU has a data portion that is longer than two octets!LI = 3 to 63
service information octet (SIO)
indicates the nature of the MSU, consists of two subfields:
service indicator: specifies the type of message being carried
subservice field: indicates whether the message is national or internation.
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Common Channel Signaling System Number 7 (SS7) 10
Signaling information field (SIF)
contains information for signaling network level and SS7 level 4
consists of two subfields:
routing label: a 32-bit (14 bits source and destination address each + 4
bits signaling link selection filed, used in traffic distribution)
data: user data from some SS7 application or network management data
check bits (CK)
contains an error-detecting code (CRC-16 from all except flags)
status field (SF)(only in LSSU)
used to indicate the senders view of the actual status of the link
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Common Channel Signaling System Number 7 (SS7) 11
Operation
Basic functions of the signaling link protocol
1. flow control 2. error control 3. error monitoring
flow control
Employs a sliding-window technique
Each MSU is given a new FSN, in modulo 128
LSSUs and FISUs not numbered, but carry the last MSUs FSN
All three types of signal unit can have negative acknowledgements and
piggybacked acknowledgements.
In the case of LSSU flow control, if one side is unable to keep up with theflow of data from the other side, busy indication is performed by the
status field.
For long congestion, timer control is used. Rules are:
If a receiver becomes overloaded, it must send a busy signal to stop
transmission from the other side. The receiver withholds ack of the MSUs. If
the overload condition persists, the node must repeatedly send a busy indication
at intervals of T5 time units (80-120 ms). Other side suspends tx of MSUs.
When congestion abates at the receiver, the receiver signals the end of busy
condition by resuming positive ack of incoming MSUs. Even if repeated busy indications are received every T5 time units, a node will
report the network level that the link is out of service every T6 (3-6 sec).
Error-free signal unit exchange
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Common Channel Signaling System Number 7 (SS7) 12
error control
two forms of error control:
Basic method:applies for signaling links where one-way PD is < 15 ms.
Preventive cyclic retransmission method:applies for signaling links where
one-way PD is "15 ms (include signaling links established via satellite).
Basic method is simply a go-back-N ARQ (figure below)
When the PD is long, the message unit is relatively short and the link is
idle for the most of the time!low efficiency
In this case, it is better not to wait and retransmit all unacknowledged
MSUs whenever a node has no MSUs to send. Only positive acks are sent
by the other side. Forced retransmission procedure:because of only positive acks, there
may be undetected error for a considerable period of time. When a
predetermined number of outstanding unacknowledged signal units exists,
the transmission of new units is interrupted and the retained signal units
are retransmitted cyclically until the number of unacknowledged signal
units is reduced.
Transmission of MSUs
with error correction
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Common Channel Signaling System Number 7 (SS7) 13
error monitoring
two types of signaling link error-rate monitoring
signaling unit error-rate monitor
alignment error-rate monitor
signaling unit error-rate monitor
is employed while the signaling link is in service
a means to decide a link be taken out of service due to errors
leaky bucket algorithm
using a counter initially set to zero and manipulated based on:
T: threshold above which an error is signaled to level 3
1/D: the lowest error rate(ratio of signal unit errors to signal units) that
will eventually cause an error to be signaled to level 3
counter increments one for each signal unit received in error
counter decrements one for every sequence ofDreceived signal
units, whether in error or not
an unreliable link is that when the counter reaches threshold T
for 64-kbps links: T= 64 andD= 256 (1/D= 0.004)
detects a consistent error rate not occasional surge of errors
alignment error-rate monitor
is employed while the signaling link is being initialized and aligned
alignment:transmitter and receiver are aligned with respect to the
opening flag field of each transmitted frame
provides the criteria for rejecting a signaling link for service due an
excessive error rate
using a counter initially set to zero and
increments by one for each signal unit received in error
If the counter exceeds a threshold before the end of an initial
proving period, the proving period is aborted.
Five successive failures result in the link being declared unreliable.