TEL 355: Communication and

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TEL 355: Communication and
Information Systems in
Organizations

Architecture:

Signaling System 7 (SS7)


Professor John F. Clark

Signaling


Exchange of info between call components
required to provide and maintain service


Users signal network elements and network
elements signal each other


SS7 is the way network elements talk


SS7 is characterized by high
-
speed packet
data and out
-
of
-
band signaling

Out
-
of
-
Band Signaling


Does not take place over the same path as
the conversation


Establishes a separate
digital

signal


the
signaling link


Takes place at speeds of 56 or 64 kbps


Somewhat like ISDN except ISDN extends
out
-
of
-
band signaling all the way to the user
via a D channel and two B channels

Why Out
-
of
-
Band Signaling?


Several advantages over in
-
band signaling


Transports more data at higher speeds


56
and 64 kbps are faster than MF outpulsing


Allows signaling at any time during the
duration of the call, not just the beginning


Enables signaling to network elements to
which there is no direct trunk connection

Signaling Network Architecture


Associated Switching


One of the many paths between an inter
-
connected pair of switches is the signaling path


The rest of the paths are voice trunks


The signaling path handles all the signaling for
all the voice trunks


Works well only between switches directly
connected by trunks


Much more complicated when signaling
between switches without a direct connection

N. Amer. Signaling Architecture


Three essential components, connected by
signaling links:


Signal switching points (SSPs)


telephone
switches with SS7 software that originate,
terminate, or switch calls


Signal transfer points (STPs)


packet switches
that receive and route incoming signaling
messages


Signal control points (SCPs)


databases that
provide information for advanced call processing

N. Amer. Signaling Architecture II


Each element is held to exacting standards


Precisely defined protocols manage the
routing of signaling through the network


And an SS7 network is highly redundant to
assure reliability


STPs and SCPs are deployed in redundant pairs


They may not be co
-
located, but they perform
identical functions

N. Amer. Signaling Architecture III


Quasi
-
Associated Switching


Mated pairs of STPs perform identical functions


Each SSP has a link to each STP of a mated pair


The STPs of a mated pair are joined by a link


The two mated pairs of associated networks are
joined by four links


a quad


SCPs are usually (not always) deployed in pairs


SCPs of a mated pair are not joined by links

SS7 Link Types


All SS7 signaling links are identical in that
they:


are bidirectional data links


are 56 kbps or 64 kbps


support the lower three layers of the protocol


However, they are used for different
purposes

SS7 Link Types II


“A” links connect an STP to either an SSP or
an SCP


“A” for Access


“C” links connect mated STPs


“C” for Cross


“B,” “D,” and “B/D” links connect two mated
pairs of STPs


“B” for Bridge and “D” for
Diagonal


Optional “E” links connect an SSP to an
additional non
-
home pair of STPs for added
reliability


“E” for Extended


“F” links bypass STPs (and SCPs) and directly
connect two SSPs


“F” for Fully Associated

Layers of the SS7 Protocol


Physical layer


physical and electrical
properties of the medium


always DS
-
0


thus 56 or 64 kbps


Message Transfer Part (MTP level 2)


link
layer functionality


error checking, flow
control, and sequence checking


Message Transfer Part (MTP level 3)


network layer functionality


node
addressing, routing, and congestion control

Layers of the SS7 Protocol II


Signaling Connection Control Part (SCCP)



Addresses applications within a node such as
800 call processing, calling
-
card processing, and
advanced intelligent network (AIN)


Uses global title translation (GTT) to perform
incremental routing


minimizes the need for
STPs to maintain information about nodes that
are far removed from them


also allows STP
load sharing among mated SCPs in both normal
and failure modes

Layers of the SS7 Protocol III


ISDN User Part (ISUP)


defines the
messages and protocol used over the PSN


always MTP in North America


Transaction Capabilities Application Part
(TCAP)


defines the messages and protocol
used to communicate between applications in
a node


delivered by SCCP


Operations, Maintenance, and Administration
Part (OMAP)


assists system administrators

Signaling Units


SUs are 8
-
bit bytes (octets) and are
transmitted continuously in both directions
on any link that is in service


There are three types of SU:


Message signal units (MSUs)


contains all the
actual signaling information


Link status signal units (LSSUs)


transmits
information about the signaling link


Fill
-
in signal units (FISUs)


occupy the link
when no MSUs or FISUs are being transmitted

Signal Unit Structure


All three types of SUs have 4 common fields:


Flag


marks the beginning of an SU and thus
delimits the SUs in a transmission


Checksum


a calculation for error checking


Length indicator


indicates the number of octets
between itself and the checksum


differentiates
between types of SUs


BSN/BIB/FSN/FIB


confirm receipt of SUs,
ensure they are in the right sequence, and provide
flow control

Signal Unit Structure II


An LSSU has an additional “Status Field”


Comprised of one or two octets


Signals the initiation of link alignment


Signals the quality of received signaling traffic


Signals the status of the processors at either end


Requires no addressing information since they
are always sent between the signaling points at
the end of the link


Signal Unit Structure III


An MSU has a “Service Information Octet”


Four bits indicate the type of information in the
signal information field


Two bits indicate national or international
network


Two bits identify a message priority


An MSU has a “Signaling Information Field”


8 to 272 octets in length


Contains all the actual signaling information,
beginning with the routing label