Signaling & Network
Control NETW 704
MTP 3
Message Transfer Part 3 (MTP 3)
• Primary purpose is to route messages between SS7
network nodes in a reliable manner. It is equivalent to Layer
3 (Network) in the OSI model.
• It is divided into 2 categories
1) Signaling Messages Handling (SMH)
2) Signaling Network Management (SNM)
Point Codes
• Each node is identified by a Point Code (PC)
– National Point Code in national networks.
– International Point Code in international networks.
• Each MSU contains the Originating Point Code & Destination
Point Code.
• International PC is based on a hierarchical structure.
– Zone
– Network
– Signaling Point
•
National PC is also based on a hierarchical structure.
– Network
– Cluster
– Member
Message Format
• MTP3 portion of an SS7 msg consists of 2
fields: SIO & SIF.
SIO
– Subservice Field: Network Indicator & Priority.
– Service Indicator Field: MTP3 user.
Message Format
• MTP3 portion of an SS7 msg consists of 2
fields: SIO & SIF.
SIF
User Data Field: Data sent by MTP3 user.
Routing Label Field:
1- Signaling Link Selector (which runs a code to share the
load across different linksets or different links within a
linkset)
2- Originating PC (OPC).
3- Destination PC (DPC).
Signaling Messages Handling
• It is mainly responsible for transferring the
SUs to the correct destination.
Signaling Messages Handling
• It is mainly responsible for transferring the
SUs to the correct destination.
Signaling Messages Handling
• 1) Discrimination
– Determines whether this destination is destined to this node or
any other node.
• 2) Distribution
– Distributes the data according to the MTP 3 User indicated in the
service indicator field.
• 3) Routing
– Either msg coming from upper layer or msg coming from another
SP.
– Routing forwards according to the route found in DPC in Routing
Table.
Signaling Messages Handling
• Alias Point Code Routing
– Another PC other than the primary PC, used for Mated Pair STPs.
It identifies both & allow load sharing as the unique PC cannot be
used (it uniquely identifies 1 PC).
Signaling Network Management
• It is responsible for network failures.
• It handle the failures in three processes:
1) Link Management
2) Route Management
3) Traffic Management
Link Management
• Handles failures at the network’s links.
1. Activation: The process of making a link available, after
the link is aligned at level 2 & has passed the proving
period.
2. Deactivation: The process of removing a link from
service.
3. Restoration: The process of restoring a link to service
after failure. After the link is aligned at level 2 and has
passed the proving period, a Signaling Link Test is
performed at level 3.
Route Management
• Provides means to communicate route changes &
route availability using SNM msgs.
– Routing Status Messages
1. Transfer Prohibited (TFP)
2. Transfer Restricted (TFR)
3. Transfer Allowed (TFA)
4. Transfer Controlled (TFC)
– Route States
1. Prohibited
2. 2. Restricted
3. 3. Allowed
Route Management
1. Transfer Prohibited (TFP)
– When the prohibited state is reached, TFP msg is sent.
Prohibited state is when there is complete inability to route
msgs to the affected destination.
– Traffic Management is notified by TFP.
– TFP could be broadcasted or sent in response of a msg.
– On reception of TFP, Traffic Management performs FORCED
REROUTING (Explained later).
STP1
SSP
A
SSP
B
STP2
Unavailable Traffic Route
Route Management
2. Transfer Restricted (TFR)
– When the restricted state is reached, TFR msg is sent.
Restricted state is when there is limited ability to route msgs to
the affected destination.
– Traffic Management is notified by TFR.
– On reception of TFR, Traffic Management performs
CONTROLLED REROUTING (Explained Later).
STP1
SSP
A
SSP
B
TFR B
STP2
Alternate Route
Unavailable Traffic Route
Route Management
3. Transfer Allowed (TFA)
– When the allowed state is reached, TFA msg is sent.
Allowed state is reached when a route has been in
restricted/prohibited stat & routing capability has
been restored.
– Traffic Management is notified by TFA.
STP1
SSP
A
SSP
B
STP2
Restored Traffic Route
Route Management
4. Transfer Controlled (TFC)
– When there are many msgs destined to the same SP thus the
buffer is full and the link is congested.
– TFC is sent to all SPs trying to send to this destination.
• Note:- There are multiple congestion levels that indicate the
severity of the congestion.
Route Management
• Routeset Test
– While the status is prohibited or restricted, routeset test is run to check and
ensure that the status at one SP is the same as at the other SP.
– On reception of TFP/TFR, RSP/RSR msgs are sent at periodic intervals T10.
– The status is compared with the current status if they match, msg is
discarded, else if they do not match a msg with the new status is sent.
– On reception of TFA, Routeset test is stopped.
• Why it is important?
– Consider STP sent TFA msg to SSP for a previously prohibited routeset and
SSP failed to receive this message, then STP would have a routset marked
as TFA and SSP marked as TFP.
Route Management
• Routeset Test
– While the status is prohibited or restricted, routeset test is run to
check and ensure that the status at one SP is the same as at the
other SP.
– On reception of TFP/TFR, RSP/RSR msgs are sent at periodic intervals
T10.
– The status is compared with the current status if they match, msg is
discarded, else if they do not match a msg with the new status is
sent.
– On reception of TFA, Routeset test is stopped.
TFA A
SSP
A
STP1
SSP
B
Unavailable Traffic Route
Traffic Management
• Deals with signaling traffic, the actual msgs generated by
MTP3 users.
• Procedures
1. Changeover.
2. Emergency Changeover.
3. Time-Controlled Changeover.
4. Changeback.
5. Time-Controlled Diversion.
6. Forced Rerouting.
7. Controlled Rerouting.
8. Link Inhibiting.
Traffic Management
1. Changeover
– When a link becomes unavailable and there are other links in the linkset that
can be used.
– Traffic is changed over to one of the other links or to an alternate linkset.
– Change Over Order (COO) msg is sent with the last correctly received SU in the
FSN.
– Change Over ACK (COA) is sent back with last correctly received SU by the other
end in the FSN.
– SSP A with one link in each linkset to STP 1 and STP 2. When the link to STP 2
fails, SSP A detects the failure and performs a changeover to the STP 1 linkset.
The changeover is made to a new linkset because no other links are available in
the same linkset. If more links were available in the STP 2 linkset, the
changeover would be to a new link in the same linkset.
STP1
SSP
A
SSP
B
STP2
Traffic Management
2. Emergency Changeover
– It is possible that a node cannot determine the last acknowledged message
when a link fails. An example is the failure of the signaling terminal hardware.
Typically, the signaling terminal hardware contains the receive buffers and
keeps track of the FSN for incoming signaling units. There is no way to
determine where the request for retransmission should start if this information
is lost. In this case, an Emergency Changeover (ECO) is sent to the far end to
initiate a changeover. The ECO does not contain the last accepted FSN field
because the last good message cannot be determined.
3.
Time-Controlled Changeover
– When there is no route to send the COO & COA.
– A timer T1 is set and when it times out, the system changes over automatically.
STP1
SSP
A
2.Changeover
after T1 expiration
STP2
SSP
B
Traffic Management
4. Changeback
– When COO & COA have been sent and the link that has failed was restored.
– Change back and send SUs on the restored link.
5. Time-Controlled Changeback (Diversion)
– When there is no route to send change back msg.
– A timer T3 is set and when it times out, the system changes back
automatically.
– As shown, the SSP A – STP 2 linkset that was unavailable has been restored.
Assuming that SSP A set its routing table to load share between STP 1 and
STP 2 for traffic destined to SSP B, the MSUs previously diverted to STP 1
should now be sent to STP 2.
STP1
SSP
A
2.Changeback
after T3
expiration
STP2
SSP
B
Traffic Management
6. Forced Rerouting
– When Traffic Management is forced to move traffic
away from an unavailable route.
– It is sent in response to a TFP msg sent by Route
Management.
STP1
SSP
A
SSP
B
STP2
Unavailable Traffic Route
Traffic Management
7. Controlled Rerouting
– When Traffic Management moves traffic away from an
unavailable route in a controlled way.
– It is sent in response to a TFR msg sent by Route
Management.
– SSP A receives a TFR from STP 1 for SSP B. SSP A has a
routeset for destination SSP B with two routes in the
routeset. SSP A performs controlled rerouting of traffic
from STP 1 to STP 2.
STP1
SSP
A
SSP
B
STP2
Unavailable Traffic Route
Traffic Management
7. Controlled Rerouting (Cont)
– When the route from STP1 to SSP B is restored, STP 1
sends a TFA to indicate that full routing capability
toward SSP B has been restored. SSP A performs
controlled rerouting again, this time shifting traffic
from the STP 2 route to the STP 1 route using the
same basic procedure.
STP1
SSP
A
SSP
B
STP2
Unavailable Traffic Route
Traffic Management
8. Link Inhibiting
– Signaling link management inhibiting is used to prevent user
traffic on the links while leaving the links themselves in service.
This process is useful for isolating links for testing purposes.
The inhibit procedure uses the Link Inhibit (LIN) and Link
Inhibit Acknowledgement (LIA) messages to communicate
between the two nodes concerning the linkset being inhibited.
STP1
SSP
A
SSP
B
STP2
Unavailable Traffic Route
Traffic Management
8. Link Inhibiting (Cont)
– A maintenance engineer at STP 1 must perform testing on a link that
has had intermittent problems. The engineer issues the command at
a maintenance terminal to place the link in an inhibited state so it is
not used by normal user traffic. STP 1 sends a LIN message to SSP A.
Because SSP A has other links available for routing, it determines
that it can safely remove the link from traffic service and respond
with an LIA back to STP 1 in acknowledgement. Because SSP A has
only 1 per linkset, it performs a controlled reroute of traffic to STP 2
linkset.
STP1
SSP
A
3.Controlled
Reroute to STP2
SSP
B
STP2
Unavailable Traffic Route