SS7
SIGNALING: -
Definition
Signaling
System 7 (SS7) is architecture for performing out-of-band signaling
in support of the call-establishment, billing, routing, and
information-exchange functions of the public switched telephone
network (PSTN). It identifies functions to be performed by a
signaling-system network and a protocol to enable their performance.
1.
What is Signaling?
Signaling
refers to the exchange of information between call components
required to provide and maintain service.
As
users of the PSTN, we exchange signaling with network elements all
the time. Examples of signaling between a telephone user and the
telephone network include: dialing digits, providing dial tone,
accessing a voice mailbox, sending a call-waiting tone, dialing *66
(to retry a busy number), etc.
SS7
is a means by which elements of the telephone network exchange
information. Information is conveyed in the form of messages. SS7
messages can convey information such as:
I’m forwarding to you a call placed from
212-555-1234 to 718-555-5678. Look for it on trunk 067.
Someone
just dialed 800-555-1212. Where do I route the call?
The
called subscriber for the call on trunk 11 is busy. Release the call
and play a busy tone.
The
route to XXX is congested. Please don’t send any messages to
XXX unless they are of priority 2 or higher.
I’m
taking trunk 143 out of service for maintenance.
SS7
is characterized by high-speed packet data and out-of-band
signaling.
2.
What is Out-of-Band Signaling?
Out-of-band
signaling is signaling that does not take place over the same path as
the conversation.
We
are used to thinking of signaling as being in-band. We hear dial
tone, dial digits, and hear ringing over the same channel on the same
pair of wires. When the call completes, we talk over the same path
that was used for the signaling. Traditional telephony used to work
in this way as well. The signals to set up a call between one switch
and another always took place over the same trunk that would
eventually carry the call. Signaling took the form of a series of
multifrequency (MF) tones, much like touch tone dialing between
switches.
Out-of-band
signaling establishes a separate digital channel for the exchange of
signaling information. This channel is called a signaling link.
Signaling links are used to carry all the necessary signaling
messages between nodes. Thus, when a call is placed, the dialed
digits, trunk selected, and other pertinent information are sent
between switches using their signaling links, rather than the trunks
which will ultimately carry the conversation. Today, signaling links
carry information at a rate of 56 or 64 kbps. It is interesting to
note that while SS7 is used only for signaling between network
elements, the ISDN D channel extends the concept of out-of-band
signaling to the interface between the subscriber and the switch.
With ISDN service, signaling that must be conveyed between the user
station and the local switch is carried on a separate digital channel
called the D channel. The voice or data that comprise the call is
carried on one or more B channels.
Why Out-of-Band Signaling?
Out-of-band
signaling has several advantages that make it more desirable than
traditional in-band signaling.
It allows for the transport of more data at higher
speeds (56 kbps can carry data much faster than MF out pulsing).
It
allows for signaling at any time in the entire duration of the call,
not only at the beginning.
It
enables signaling to network elements to which there is no direct
trunk connection.
3.
Signaling Network Architecture
If
signaling is to be carried on a different path from the voice and
data traffic it supports, then what should that path look like? The
simplest design would be to allocate one of the paths between each
interconnected pair of switches as the signaling link. Subject to
capacity constraints, all signaling traffic between the two switches
could traverse this link. This type of signaling is known as
associated signaling, and is shown below in Figure 1.
Figure
1. Associated Signaling
Associated
signaling works well as long as a switch’s only signaling
requirements are between itself and other switches to which it has
trunks. If call setup and management was the only application of SS7,
associated signaling would meet that need simply and efficiently. In
fact, much of the out-of-band signaling deployed in Europe today uses
associated mode.
The
North American implementers of SS7, however, wanted to design a
signaling network that would enable any node to exchange signaling
with any other SS7–capable node. Clearly, associated signaling
becomes much more complicated when it is used to exchange signaling
between nodes which do not have a direct connection. From this need,
the North American SS7 architecture was born
4.
The North American Signaling Architecture
The
North American signaling architecture defines a completely new and
separate signaling network. The network is built out of the following
three essential components, interconnected by signaling links:
Signal switching points (SSPs) —SSPs
are telephone switches (end offices or tandems) equipped with
SS7-capable software and terminating signaling links. They generally
originate, terminate, or switch calls.
Signal
transfer points (STPs)—STPs are
the packet switches of the SS7 network. They receive and route
incoming signaling messages towards the proper destination. They
also perform specialized routing functions.
Signal
control points (SCPs)—SCPs are
databases that provide information necessary for advanced
call-processing capabilities.
Once
deployed, the availability of SS7 network is critical to call
processing. Unless SSPs can exchange signaling, they cannot complete
any inters witch calls. For this reason, the SS7 network is built
using a highly redundant architecture. Each individual element also
must meet exacting requirements for availability. Finally, protocol
has been defined between interconnected elements to facilitate the
routing of signaling traffic around any difficulties that may arise
in the signaling network.
To
enable signaling network architectures to be easily communicated and
understood, a standard set of symbols was adopted for depicting SS7
networks. Figure 2 shows the symbols that are used to depict these
three key elements of any SS7 network.
Figure
2. Signaling Network Elements
STPs
and SCPs are customarily deployed in pairs. While elements of a pair
are not generally co-located, they work redundantly to perform the
same logical function. When drawing complex network diagrams, these
pairs may be depicted as a single element for simplicity, as shown in
Figure 3.
Figure
3. STP and SCP Pairs
5.
Basic Signaling Architecture
Figure
4 shows a small example of how the basic elements of an SS7
network are deployed to form two interconnected networks.
Figure
4. Sample Network
The
following points should be noted:
STPs W and X perform identical functions. They are
redundant. Together, they are referred to as a mated pair of
STPs. Similarly, STPs Y and Z form a mated pair.
Each
SSP has two links (or sets of links), one to each STP of a mated
pair. All SS7 signaling to the rest of the world is sent out over
these links. Because the STPs of a mated pair are redundant,
messages sent over either link (to either STP) will be treated
equivalently.
The
STPs of a mated pair are joined by a link (or set of links).
Two
mated pairs of STPs are interconnected by four links (or sets of
links). These links are referred to as a quad.
SCPs
are usually (though not always) deployed in pairs. As with STPs, the
SCPs of a pair are intended to function identically. Pairs of SCPs
are also referred to as mated pairs of SCPs. Note that a pair of
links does not directly join them.
Signaling
architectures such as this, which provide indirect signaling paths
between network elements, are referred to as providing
quasi-associated signaling.
6.
SS7 Link Types
SS7
signaling links are characterized according to their use in the
signaling network. Virtually all links are identical in that they are
56–kbps or 64–kbps bi-directional data links that support
the same lower layers of the protocol; what is different is their use
within a signaling network. The defined link types are shown in
Figure 5 and defined as follows:
Figure
5. Link Types
A Links
A
links interconnect an STP and either an SSP or an SCP, which are
collectively referred to as signaling end points ("A"
stands for access). A links are used for the sole purpose of
delivering signaling to or from the signaling end points (they could
just as well be referred to as signaling beginning points). Examples
of A links are 2–8, 3–7, and 5–12 in Figure 5.
Signaling
that an SSP or SCP wishes to send to any other node is sent on either
of its A links to its home STP, which, in turn, processes or routes
the messages. Similarly, messages intended for an SSP or SCP will be
routed to one of its home STPs, which will forward them to the
addressed node over its A links.
C Links
C
links are links that interconnect mated STPs. As will be seen later,
they are used to enhance the reliability of the signaling network in
instances where one or several links are unavailable. "C"
stands for cross (7–8, 9–10, and 11–12 are C
links). B links, D links, and B/D links interconnecting two mated
pairs of STPs are referred to as either B links, D links, or B/D
links. Regardless of their name, their function is to carry signaling
messages beyond their initial point of entry to the signaling network
towards their intended destination. The "B" stands for
bridge and describes the quad of links interconnecting peer pairs of
STPs. The "D" denotes diagonal and describes the quad of
links interconnecting mated pairs of STPs at different hierarchical
levels. Because there is no clear hierarchy associated with a
connection between networks, interconnecting links are referred to as
either B, D, or B/D links (7–11 and 7–12 are examples of
B links; 8–9 and 7–10 are examples of D links; 10–13
and 9–14 are examples of interconnecting links and can be
referred to as B, D, or B/D links).
E Links
While
an SSP is connected to its home STP pair by a set of A links,
enhanced reliability can be provided by deploying an additional set
of links to a second STP pair. These links, called E (extended) links
provide backup connectivity to the SS7 network in the event that the
home STPs cannot be reached via the A links. While all SS7 networks
include A, B/D, and C links, E links may or may not be deployed at
the discretion of the network provider. The decision of whether or
not to deploy E links can be made by comparing the cost of deployment
with the improvement in reliability. (1–11 and 1–12 are E
links.)
F Links
F
(fully associated) links are links, which directly connect two
signaling end points. F links allow associated signaling only.
Because they bypass the security features provided by an STP, F links
are not generally deployed between networks. Their use within an
individual
7.
Basic Call Setup Example
Before
going into much more detail, it might be helpful to look at several
basic calls and the way in which they use SS7 signaling (see Figure
6).
Figure
6. Call Setup Example
In
this example, a subscriber on switch A places a call to a subscriber
on switch B.
Switch A analyzes the dialed digits and determines that
it needs to send the call to switch B.
Switch
A selects an idle trunk between itself and switch B and formulates
an initial address message (IAM), the basic message necessary to
initiate a call. The IAM is addressed to switch B. It identifies the
initiating switch (switch A), the destination switch (switch B), the
trunk selected, the calling and called numbers, as well as other
information beyond the scope of this example.
Switch
A picks one of its A links (e.g., AW) and transmits the message over
the link for routing to switch B.
STP W
receives a message, inspects its routing label, and determines that
it is to be routed to switch B. It transmits the message on link BW.
Switch
B receives the message. On analyzing the message, it determines that
it serves the called number and that the called number is idle.
Switch
B formulates an address complete message (ACM), which indicates that
the IAM has reached its proper destination. The message identifies
the recipient switch (A), the sending switch (B), and the selected
trunk.
Switch
B picks one of its A links (e.g., BX) and transmits the ACM over the
link for routing to switch A. At the same time, it completes the
call path in the backwards direction (towards switch A), sends a
ringing tone over that trunk towards switch A, and rings the line of
the called subscriber.
STP X
receives the message, inspects its routing label, and determines
that it is to be routed to switch A. It transmits the message on
link AX.
On
receiving the ACM, switch A connects the calling subscriber line to
the selected trunk in the backwards direction (so that the caller
can hear the ringing sent by switch B).
When
the called subscriber picks up the phone, switch B formulates an
answer message (ANM), identifying the intended recipient switch (A),
the sending switch (B), and the selected trunk.
Switch
B selects the same A link it used to transmit the ACM (link BX) and
sends the ANM. By this time, the trunk also must be connected to the
called line in both directions (to allow conversation).
STP X
recognizes that the ANM is addressed to switch A and forwards it
over link AX.
Switch
A ensures that the calling subscriber is connected to the outgoing
trunk (in both directions) and that conversation can take place.
If
the calling subscriber hangs up first (following the conversation),
switch A will generate a release message (REL) addressed to switch
B, identifying the trunk associated with the call. It sends the
message on link AW.
STP W
receives the REL, determines that it is addressed to switch B, and
forwards it using link WB.
Switch
B receives the REL, disconnects the trunk from the subscriber line,
returns the trunk to idle status, generates a release complete
message (RLC) addressed back to switch A, and transmits it on link
BX. The RLC identifies the trunk used to carry the call.
STP X
receives the RLC, determines that it is addressed to switch A, and
forwards it over link AX.
On
receiving the RLC, switch A idles the identified trunk
l
network is at the discretion of the network provider. (1–2 is
an F link.)
8.
Database Query Example
People
generally are familiar with the toll-free aspect of 800 (or 888)
numbers, but these numbers have significant additional capabilities
made possible by the SS7 network. 800 numbers are virtual telephone
numbers. Although they are used to point to real telephone numbers,
they are not assigned to the subscriber line itself.
When
a subscriber dials an 800 number, it is a signal to the switch to
suspend the call and seek further instructions from a database. The
database will provide either a real phone number to which the call
should be directed, or it will identify another network (e.g., a
long-distance carrier) to which the call should be routed for further
processing. While the response from the database could be the same
for every call (as, for example, if you have a personal 800 number),
it can be made to vary based on the calling number, the time of day,
the day of the week, or a number of other factors.
The
following example shows how an 800 call is routed (see Figure 7).
Figure
7. Database Query Example
A subscriber served by
switch A wants to reserve a rental car at a company's nearest
location. She dials the company's advertised 800 number.
When
the subscriber has finished dialing, switch A recognizes that this
is an 800 call and that it requires assistance to handle it
properly.
Switch
A formulates an 800 query message including the calling and called
number and forwards it to either of its STPs (e.g., X) over its A
link to that STP (AX).
STP
X determines that the received query is an 800 query and selects a
database suitable to respond to the query (e.g., M).
STP
X forwards the query to SCP M over the appropriate A link (MX). SCP
M receives the query, extracts the passed information, and (based on
its stored records) selects either a real telephone number or a
network (or both) to which the call should be routed.
SCP
M formulates a response message with the information necessary to
properly process the call, addresses it to switch A, picks an STP
and an A link to use (e.g., MW), and routes the response.
STP
W receives the response message, recognizes that it is addressed to
switch A, and routes it to A over AW.
Switch
A receives the response and uses the information to determine where
the call should be routed. It then picks a trunk to that
destination, generates an IAM, and proceeds (as it did in the
previous example) to set up the call.
9.
Layers of the SS7 Protocol
As
the call-flow examples show, the SS7 network is an interconnected set
of network elements that is used to exchange messages in support of
telecommunications functions. The SS7 protocol is designed to both
facilitate these functions and to maintain the network over which
they are provided. Like most modern protocols, the SS7 protocol is
layered.
Physical Layer
This
defines the physical and electrical characteristics of the signaling
links of the SS7 network. Signaling links utilize DS–0 channels
and carry raw signaling data at a rate of 56 kbps or 64 kbps (56 kbps
is the more common implementation).
Message Transfer Part—Level 2
The
level 2 portion of the message transfer part (MTP Level 2) provides
link-layer functionality. It ensures that the two end points of a
signaling link can reliably exchange signaling messages. It
incorporates such capabilities as error checking, flow control, and
sequence checking.
Message Transfer Part—Level 3
The
level 3 portion of the message transfer part (MTP Level 3) extends
the functionality provided by MTP level 2 to provide network layer
functionality. It ensures that messages can be delivered between
signaling points across the SS7 network regardless of whether they
are directly connected. It includes such capabilities as node
addressing, routing, alternate routing, and congestion control.
Collectively,
MTP levels 2 and 3 are referred to as the message transfer part
(MTP).
Signaling Connection Control Part
The
signaling connection control part (SCCP) provides two major functions
that are lacking in the MTP. The first of these is the capability to
address applications within a signaling point. The MTP can only
receive and deliver messages from a node as a whole; it does not deal
with software applications within a node.
While
MTP network-management messages and basic call-setup messages are
addressed to a node as a whole, other messages are used by separate
applications (referred to as subsystems) within a node. Examples of
subsystems are 800 call processing, calling-card processing, advanced
intelligent network (AIN), and custom local-area signaling services
(CLASS) services (e.g., repeat dialing and call return). The SCCP
allows these subsystems to be addressed explicitly.
Global Title Translation
The
second function provided by the SCCP is the ability to perform
incremental routing using a capability called global title
translation (GTT). GTT frees originating signaling points from the
burden of having to know every potential destination to which they
might have to route a message. A switch can originate a query, for
example, and address it to an STP along with a request for GTT. The
receiving STP can then examine a portion of the message, make a
determination as to where the message should be routed, and then
route it.
For
example, calling-card queries (used to verify that a call can be
properly billed to a calling card) must be routed to an SCP
designated by the company that issued the calling card. Rather than
maintaining a nationwide database of where such queries should be
routed (based on the calling-card number), switches generate queries
addressed to their local STPs, which, using GTT, select the correct
destination to which the message should be routed. Note that there is
no magic here; STPs must maintain a database that enables them to
determine where a query should be routed. GTT effectively centralizes
the problem and places it in a node (the STP) that has been designed
to perform this function.
In
performing GTT, an STP does not need to know the exact final
destination of a message. It can, instead, perform intermediate GTT,
in which it uses its tables to find another STP further along the
route to the destination. That STP, in turn, can perform final GTT,
routing the message to its actual destination.
Intermediate
GTT minimizes the need for STPs to maintain extensive information
about nodes that are far removed from them. GTT also is used at the
STP to share load among mated SCPs in both normal and failure
scenarios. In these instances, when messages arrive at an STP for
final GTT and routing to a database, the STP can select from among
available redundant SCPs. It can select an SCP on either a priority
basis (referred to as primary backup) or so as to equalize the load
across all available SCPs (referred to as load sharing).
ISDN User Part (ISUP)
ISUP
user part defines the messages and protocol used in the establishment
and tear down of voice and data calls over the public switched
network (PSN), and to manage the trunk network on which they rely.
Despite its name, ISUP is used for both ISDN and non–ISDN
calls. In the North American version of SS7, ISUP messages rely
exclusively on MTP to transport messages between concerned nodes.
Transaction Capabilities Application
Part (TCAP)
TCAP
defines the messages and protocol used to communicate between
applications (deployed as subsystems) in nodes. It is used for
database services such as calling card, 800, and AIN as well as
switch-to-switch services including repeat dialing and call return.
Because TCAP messages must be delivered to individual applications
within the nodes they address, they use the SCCP for transport.
Operations, Maintenance, and
Administration Part (OMAP)
OMAP
defines messages and protocol designed to assist administrators of
the SS7 network. To date, the most fully developed and deployed of
these capabilities are procedures for validating network routing
tables and for diagnosing link troubles. OMAP includes messages that
use both the MTP and SCCP for routing
10.
What Goes Over the Signaling Link?
Signaling
information is passed over the signaling link in messages, which are
called signal units (SUs).
Three
types of SUs are defined in the SS7 protocol.
message signal units
(MSUs)
link
status signal units (LSSUs)
fill-in
signal units (FISUs)
SUs
are transmitted continuously in both directions on any link that is
in service. A signaling point that does not have MSUs or LSSUs to
send will send FISUs over the link. The FISUs perform the function
suggested by their name; they fill up the signaling link until there
is a need to send purposeful signaling. They also facilitate link
transmission monitoring and the acknowledgment of other SUs.
All
transmission on the signaling link is broken up into 8-bit bytes,
referred to as octets. SUs on a link are delimited by a unique 8-bit
pattern known as a flag. The flag is defined as the 8-bit pattern
"01111110". Because of the possibility that data within an
SU would contain this pattern, bit manipulation techniques are used
to ensure that the pattern does not occur within the message as it is
transmitted over the link. (The SU is reconstructed once it has been
taken off the link, and any bit manipulation is reversed.) Thus, any
occurrence of the flag on the link indicates the end of one SU and
the beginning of another. While in theory two flags could be placed
between SUs (one to mark the end of the current message and one to
mark the start of the next message), in practice a single flag is
used for both purposes.
11.
Addressing in the SS7 Network
Every network must have an addressing scheme, and the SS7 network is
no different. Network addresses are required so that a node can
exchange signaling nodes to which it does not have a physical
signaling link. In SS7, addresses are assigned using a three-level
hierarchy. Individual signaling points are identified as belonging to
a cluster of signaling points. Within that cluster, each signaling
point is assigned a member number. Similarly, a cluster is defined as
being part of a network. A three-level number defined by its network,
cluster, and member numbers can address any node in the American SS7
network. Each of these numbers is an 8-bit number and can assume
values from 0 to 255. This three-level address is known as the point
code of the signaling point. A point code uniquely identifies a
signaling point within the American SS7 network and is used whenever
it is necessary to address that signaling point.
A
neutral party assigns network numbers on a nationwide basis. Regional
Bell operating companies (RBOCs), major independent telephone
companies, and interexchange carriers (IXCs) already have network
numbers assigned. Because network numbers are a relatively scarce
resource, companies' networks are expected to meet certain size
requirements in order to be assigned a network number. Smaller
networks can be assigned one or more cluster numbers within network
numbers 1, 2, 3, and 4. The smallest networks are assigned point
codes within network number 5. The cluster to which they are assigned
is determined by the state in which they are located. The network
number 0 is not available for assignment and network number 255 is
reserved for future use.
12.
Signal Unit Structure
SUs of each type follow a format unique to that type. A high-level
view of those formats is shown in Figure 8.
Figure
8. Signaling Unit Formats
All
three SU types have a set of common fields that are used by MTP Level
2. They are as follows:
Flag
Flags
delimit SUs. A flag marks the end of one SU and the start of the
next.
Checksum
The
checksum is an 8-bit sum intended to verify that the SU has passed
across the link error-free. The checksum is calculated from the
transmitted message by the transmitting signaling point and inserted
in the message. On receipt, the receiving signaling point
recalculates it. If the calculated result differs from the received
checksum, the received SU has been corrupted. A retransmission is
requested.
Length Indicator
The
length indicator indicates the number of octets between itself and
the checksum. It serves both as a check on the integrity of the SU
and as a means of discriminating between different types of SUs at
level 2. As can be inferred from Figure 8, FISUs have a length
indicator of 0; LSSUs have a length indicator of 1 or 2 (currently
all LSSUs have a length indicator of 1), and MSUs have a
length-indicator greater than 2. According to the protocol, only 6 of
the 8 bits in the length indicator field are actually used to store
this length; thus the largest value that can be accommodated in the
length indicator is 63. For MSUs with more than 63 octets following
the length indicator, the value of 63 is used.
BSN/BIB FSN/FIB
These
octets hold the backwards sequence number (BSN), the backwards
indicator bit (BIB), the forward sequence number (FSN), and the
forward indicator bit (FIB). These fields are used to confirm receipt
of SUs and to ensure that they are received in the order in which
they were transmitted. They also are used to provide flow control.
MSUs and LSSUs, when transmitted, are assigned a sequence number that
is placed in the forward sequence number field of the outgoing SU.
The transmitting signaling point stores this SU until the receiving
signaling point acknowledges it.
Because
the seven bits allocated to the forward sequence number can store 128
distinct values, it follows that a signaling point is restricted to
sending 128 unacknowledged SUs before it must await an
acknowledgment. By acknowledging an SU, the receiving node frees that
SU's sequence number at the transmitting node, making it available
for a new outgoing SU. Signaling points acknowledge receipt of SUs by
placing the sequence number of the last correctly received and
in-sequence SU in the backwards sequence number of every SU they
transmit. In that way, they acknowledge all previously received SUs
as well. The forward and backwards indicator bits are used to
indicate sequencing or data-corruption errors and to request
retransmission.
13.
What are the Functions of the Different Signaling Units?
FISUs
themselves have no information payload. Their purpose is to occupy
the link at those times when there are no LSSUs or MSUs to send.
Because they undergo error checking, FISUs facilitate the constant
monitoring of link quality in the absence of signaling traffic. FISUs
also can be used to acknowledge the receipt of messages using the
backwards sequence number and backwards indicator bit.
LSSUs
are used to communicate information about the signaling link between
the nodes on either end of the link. This information is contained in
the status field of the SU (see Figure 8). Because independent
processors control the two ends of a link, there is a need to provide
a means for them to communicate. LSSUs provide the means for
performing this function. LSSUs are used primarily to signal the
initiation of link alignment, the quality of received signaling
traffic, and the status of the processors at either end of the link.
Because they are sent only between the signaling points at either end
of the link, LSSUs do not require any addressing information.
MSUs
are the workhorses of the SS7 network. All signaling associated with
call setup and tear down, database query and response, and SS7
network management takes place using MSUs. MSUs are the basic
envelope within which all addressed signaling information is placed.
As will be shown below, there are several different types of MSUs.
All MSUs have certain fields in common. Other fields differ according
to the type of message. The type of MSU is indicated in the
service-information octet shown in Figure 8; the addressing and
informational content of the MSU is contained in the signaling
information field.
14.
Message Signal Unit Structure
The
functionality of the message signal unit lies in the actual content
of the service information octet and the signaling information field
(see Figure 8).
The
service information octet is an 8-bit field (as might be inferred
from its name) that contains three types of information as follows:
four bits are used
to indicate the type of information contained in the signaling
information field; they are referred to as the service indicator;
the values most commonly used in American networks are outlined in
Table 1
Table 1. Common
Signaling Indicator Values
Value | Function |
0 | signaling |
1 | signaling |
3 | signaling |
5 | ISDN |
two bits are used
to indicate whether the message is intended (and coded) for use in a
national or international network; they are generally coded with a
value of 2, national network
the
remaining 2 bits are used (in American networks) to identify a
message priority, from 0 to 3, with 3 being the highest priority;
message priorities do not control the order in which messages are
transmitted; they are only used in cases of signaling network
congestion; in that case, they indicate whether a message has
sufficient priority to merit transmission during an instance of
congestion or whether it can be discarded en route to a destination
The
service indicator determines the format of the contents of the
signaling information field. (Within user parts, there are further
distinctions in message formats, but the service indicator provides
the first piece of information necessary for routing or decoding the
message.)
The
first portion of the signaling information field is identical for all
MSUs currently in use. It is referred to as the routing label. Simply
stated, the routing label identifies the message originator, the
intended destination of the message, and a field referred to as the
signaling-link selection field, which is used to distribute message
traffic over the set of possible links and routes. The routing label
consists of 7 octets that are outlined below in Table 2 (in
order of transmission).
Table
2. Routing Label
Octet | Function | Number |
Destination | Contains | 3 |
Originating | Contains | 3 |
Signaling | Distributes | 1 |
Point
codes consist of the three-part identifier (network number, cluster
number, and member number), which uniquely identifies a signaling
point.
