Signaling System 7 :
SS7 (signaling System 7) or "out-of-band"
signaling
is the
most widely used signaling system where the signals are
transmistted on a
seperate physical channel from the call channel.
Because of that, SS7 network can examine all parts of a call
route for free
lines and allocate them at once. This leads to a more efficient
network and
fast call setup times, generally about 3 seconds where as in
early CAS systems
this might take about 20 seconds.
Also it has many more advantages compared to CAS (Channel
Associated Signaling)
or "in-band" signaling where signals and the call are
transmitted on the same
channel.
SS7 allows telecommunication networks to offer wide ranges of
services such as
telephony, fax transmission, data transfer, ISDN and IN
(Intelligent Networks).
SS7 is popular because :
- Flexible and expandible message types allow transmission of
more info per
signal
- Reliability
- Capacity up to 64Kbps.
In SS7 networks, the signals are transmitted on dedicated
channels seperated
from the call channel. The signal data are encapsulated in a
packet (also
called frame or signal unit) that is encoded digitally before
transmission. For
this, the switching office must be equipped with the electronic
processors to
encode the incoming and to decode the outgoing messages.
Each SS7 message is made up of 5 fields.
- Address of the destination office : This is the address of the
destination
office also named as DPC (Destination Point Code)(目的信令点编码).
- Address of the originating office : This is the address of the
originating
office also named as OPC (Originating Point Code). This OPC does
not identify
the caller. Instead, it's the address of the office where the
caller is
attached.
- Message packet number is to synchronize the messages at the
destination
office in case they arrive out of order in which they are sent.
- Data field contains messages, all the information, circuit
identification
number (CIC)(电路识别码) to identifiy the call which the signal refers.
- Error checking field is to check if there is any error occured
during the
transmission.
SS7 Signaling Types :
SS7 supports 2 types of signaling : Circuit related signaling
and non-circuit
related signaling.
Circuit related signaling is used to setup, maintain and release
a call's
circuit. Every circuit related signal contains a CIC (Circuit
Identification
Code) in the data field of the message body. With this code, the
switch can
identify which call circuit refers which signal.
Non circuit related signaling messages don't refer to a
particular circuit so
they don't carry CIC. They are used to exchange special
information between the
switching offices.
SS7 Signaling Transfer Modes :
The signaling and the call are transfered on different channels
in 3 types of
modes.
- Associated Signaling Mode : In this mode, the messages follow
the same path
as the related call. They are transfered in seperate channels
but follows the
same route with the call channel.
- Non-Associated Signaling Mode : In this mode, the messages are
not required
to travel the same path as the call to which they refer. Also
each message can
travel in different paths too. This mode is more efficient but
cause problems
if the messages arrive out of the order in which they are sent.
It's generally
not used.
- Quasi-Associated Signaling Mode : This signaling mode is a
special case of
non-associated signaling mode. In this mode signals follow a
different path
relative to the call but the same path relative to each other to
make sure the
messages arrive in the same order in which they are sent.
Steps of a Basic SS7 Call:
1) The caller takes the phone "off-hook", dial the destination
number. The
subscriber signaling pass this information to the local calling
office.
2) The local originating office which use SS7, encapsulates the
dialed number
and the CPC (calling party category)(主叫用户类别) information in to the first
signal IAM
(Initial Address Message)(初始地址报文) to setup the call to the destination
office. In some
cases IAM can be replaced with IAI (Initial Address Message with
Additional
Information) (带有附加消息的初始地址报文)to pass more information.
3) On the route to the destination, each receiving office checks
the DPC
(Destination Point Code) with its own Point Code to see if the
message is
destined to itself. If not, it transfers the message to the next
office in the
route. When the destination office finally receives the IAM or
IAI, it checks
the subscriber number to see if it's free. If free then sends
back the ACM
(Address Complete Message)(地址完全消息).
4) At this point, the voice circuit is opened, ring back tone is
put on the
circuit back to the caller and ringing current is sent to the
dialled number's
phone.
5) When the called subscriber answers, the destination switching
office sends
back ANC (Address Charge Message) to the first office to begin
call charging.
6) When the conversation is over, to release the call circuit,
the originating
switching office sends CLF (Clear Forward) and the destination
switching office
sends back the RLG (Release Guard) (释放监护信号)signals.
SS7 and ISUP:
SS7 signaling system is used both in PSTN (Public Switched
Telephone Network)
and ISDN (Integrated Services Digital Network). ISUP is the ISDN
User Part used
for SS7 signalling. It's the most widely used signaling.
ANI service might be an example of a ISDN service where ISUP(综合业务数字网用户部分)
signaling is
involved. ISDN Network offers both voice and non-voice services
to ISDN
customer such as ANI (automatic Number Identification) also
known as CLIP
(calling Line Identification and Presentation) or caller ID. The
ISUP signal
IAM includes the calling lines number. ANI service allows
callers number to be
displayed at the dialled numbers ISDN terminal by reading the
calling number
from the IAM message.
Another example of a ISUP in ISDN network is the User-to-User
text messaging
between ISDN customers. These messages can be sent with USR
(User-to-User)
message which is read by the called numbers ISDN terminal.
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