Cisco Certified ExpertSio Sin Sios Ss7
MTP2 provides the required Layer 2 functionality in the SS7 network. It is responsible for providing a reliable link for the upper-layer protocols. The explicit device name used at MTP2 is a Signaling Link Terminal (SLT). It is normally built into the signaling point fabric. The functions of MTP2 are as follows:
• Separation of signal units—MTP2 provides identification of individual signal units with a flag.
• Bit stuffing—The transmission of six consecutive 1s within a signal unit causes it to mimic a flag, which is not allowed. In the case that this occurs, alignment is lost on the link. To ensure that this does not happen, you use bit stuffing.
• Error detection—MTP2 uses cyclic redundancy checks (CRCs) in a frame-check sequence (FCS) field to verify the integrity of the transmitted frames. The FCS field is 16 bits in length.
• Link alignment—MTP2 is responsible for the initial phases of link alignment and allows the transmission of upper-layer messages.
• Signal link monitoring—Signal link monitoring is handled by two error counters; signal unit error rate monitor (SUERM) and alignment error rate monitor (AERM). The AERM is used during the initial link alignment, and the SUERM is used after the link is in service to verify link integrity.
• Flow control—Flow control is handled by the different frame types used by MTP2. Busy indicators can be sent, in times of congestion, to back off signaling points to return to a non-congested state.
MTP2 includes two different signal unit types that allow MTP2 to provide the services above. The Fill-In Signal Unit (FISU) and the Link Status Signal Unit (LSSU) are used at different intervals on the SS7 network. A third MTP2 message type, the Message Signal Unit (MSU) assits in routing call signaling through the SS7 network.
FISU
The FISU is the most basic MTP2 signal unit. The purpose of this signal unit is to fill in idle time on the signaling link and provide link synchronization. The FISU acts as a proactive way to detect errors on the signaling link in an attempt to find problems before a call is placed. What we mean by that is while there is no signaling traffic going through the SS7 network, FISUs are sent as a type of heartbeat between signaling points.
Figure 2-17 shows the different fields in a FISU signal unit. Figure 2-17 FISU Format
|
F |
B |
||||||
|
CK |
S |
LI |
B |
FSN |
B |
BSN |
F |
LI = Length Indicator (Not Used) FIB = Forward Indicator Bit
FSN = Forward Sequence Number BIB = Backward Indicator Bit
BSN = Backward Sequence Number F = Flag
The signal units are to be read from right to left, as they are transmitted with the flag bit first. The flag identifies one signal unit from another. There is documentation out there (including ITU Q.703) that identifies the FISU as having an opening and closing flag. The closing flag that is represented is merely the opening flag of the next signal unit. So, if signal unit #3 is the last signal unit transmitted it does not have a closing flag. The Length Indicator (LI) is not used with the FISU, and thus is set to 0.
The backward sequence number (BSN), backward indicator bit (BIB), forward sequence number (FSN), and forward indicator bit (FIB) all work together as part of signal unit identification and the identification of which signal units should be transmitted. The FSN identifies the sequence number of the current signal unit that is being transmitted. The BSN identifies the last good received signal unit and can be used as a way for the receiving end to identify which signal unit is retransmitted. The FIB and BIB indicate which signal units need to be resent.
In normal operation, the FIB and BIB equal the same value (0). If a unit is not received, or it does not pass the FCS, the BIB is set to the opposite value of the FIB. This indicator, with the help of the BSN, identifies which signal unit is retransmitted. The signaling point that originally transmitted the signal unit sees the BIB and uses BSN+1 to identify which signal unit is retransmitted. This function is shown in Figure 2-18.
After the signal unit in question is retransmitted, and it passes the FCS, the BIB is reset to 0. After this point, normal transmission of signal units can resume. A maximum of 127 messages can be marked for retransmission.
Figure 2-18 Normal Retransmission Procedure
Figure 2-18 Normal Retransmission Procedure
SSP SSP
Now that you understand how the fields technically work, it is important to point out that they do not operate in this manner when used in FISU frames. The FSN of a FISU is actually the last known good, received MSU on the signaling link. This FSN does not change until another MSU is received. The next time MSU transmission pauses, the FISU assumes the FSN of that last MSU again. The following example shows that if the last MSU frame number was 5, your FISU frame number will be 5. If the last MSU frame number was 24, the FISU frame number will be 24.
In the case of the FISU, it is sent between signaling points during idle time, and it has no significance to the network. For this reason, there is no need to retransmit lost or damaged FISUs.
LSSU
The LSSU is responsible for communicating the current status of the signaling link to the signaling points. In normal operation, the LSSU is not sent between signaling points. It is only sent when an event occurs that affects the link's ability to transmit and receive MSUs. Similar to the FISU, the LSSU is created based on the last MSU that was transmitted on the link. For this reason, the LSSU is not retransmitted on the signaling link. The signal unit format for the LSSU is shown in Figure 2-19.
Figure 2-19 LSSU Format
|
F |
B |
|||||||
|
CK |
SF |
S |
LI |
B |
FSN |
B |
BSN |
F |
CK = Check Bits LI = Length Indicator (LI=1 or 2) FSN = Forward Sequence Number BSN = Backward Sequence Number SF = Status Field
S (Spare) = Not Used FIB = Forward Indicator Bit BIB = Backward Indicator Bit F = Flag
All fields within the LSSU have the same function that they do for the FISU with the exception of the LI, which is used in LSSUs, and the addition of the Status Field (SF). The LI field indicates in LSSUs whether the SF is 8 or 16 bits. LI=1 causes the SF to be 8 bits and LI=2 causes the SF to be 16 bits. If the receiving SLT cannot process a two-octet SF, the second octet is ignored and only the first octet is processed. The SF indicates what the current status of the signaling link is by using four different values and two other values to transport information about the type of proving that is used during the link alignment process:
• Status Indicator Out of Alignment (SIO)
• Status Indicator Out of Service (SIOS)
• Status Indicator Busy (SIB)
• Status Indicator Processor Outage (SIPO)
• Status Indicator Normal (SIN)
• Status Indicator Emergency (SIE)
The SIO indicates one of two things, either that the link is out of alignment because of a 1s density violation (different than the 1s density violation in T1) or that it is the beginning of the alignment process. In the first case, the SIO is sent if six consecutive 1s are detected. The transmission of six consecutive 1s, as stated earlier in the chapter, mimics a flag, and that is not allowed. You will also see the SIO at the beginning of the link alignment procedure a little later in the chapter.
A SIOS indicates that the signaling point is not able to send or receive MSUs (link down), and it is also seen when a signaling point is turned up. SIOS continues until the initial alignment phase has begun.
A SIB indicates there is congestion at the transmitting signaling point. When congestion is detected, the T5 timer (sending SIB timer) is started and SIBs are sent to the remote signaling point. When the receiving signaling point receives the LSSUs with SIB, it starts the T6 timer (remote congestion guard timer). Every LSSU after that point with SIB resets the T7 timer (excessive delay timer) until a LSSU is received without SIB. After there are no more LSSUs with SIB, the T6 timer is shut off and normal operation resumes. Figure 220 displays the congestion timer process by using the T5, T6, and T7 timers.
Figure 2-20 Congestion Timer Operation
Figure 2-20 Congestion Timer Operation
- SSP SSP
A SIPO indicates that the transmitting signaling point cannot send messages to the functional levels of 3 or 4. SIPO does not mean that all links in the link set are affected, and it is possible for MTP3 to still be functional (just not communicated with). When SIPO is indicated locally, all MSUs received are discarded and LSSUs are sent with SIPO.
NOTE In the case that SIPO is indicated locally, Local Processor Outage (LPO) is declared on the transmitting side.
After the far end receives the LSSUs with SIPO, it terminates the transmission of MSUs and begins the transmission of FISUs, with the FSN set to the last transmitted MSU. The receiving end is in Remote Processor Outage (RPO).
As the signaling point comes out of a congested state, it notifies MTP3 and returns to an inservice state. After the link has returned to an in-service state, the MTP2 buffers are flushed of old messages. Particularly if the PO was long in duration, old messages can refer to calls that have already been rerouted or released. Furthermore, it is required that both sides of the link synchronize their messages for proper link operation.
Initial Alignment Procedure
For proper link operation to take place, link integrity must be ensured. This process is known as link alignment, and if you can't pass alignment you can't place calls through the SS7 network.
Link alignment provides a procedure for turning up a new link between signaling points and it realigns a link that has lost alignment. In the alignment process, there are two types of alignment: SIN and SIE. The difference between the two types of alignment is the amount of time required at the proving period. Refer to Table 2-2 for a complete listing of the SS7 timers and their values as they pertain to the alignment process.
Table 2-2 SS7 Alignment Timers and Their Values
|
Timer |
Value |
Link Type |
|
T1 |
45-50s |
64 kbps |
|
Alignment Ready Timer |
500-600s |
4.8 kbps |
|
T2 |
5-50s |
Low |
|
Not Aligned Timer |
70-150s |
High |
|
T3 |
1-2s |
N/A |
|
Aligned Timer |
||
|
T4 |
T4(n) 7.5-9.5s |
64 kbps |
|
Proving Period Timer |
T4(n) 100-120s |
4.8 kbps |
|
T4 400-600ms |
64 kbps |
|
|
T4 6-8s |
4.8 kbps |
|
|
T5 |
80-120ms |
N/A |
|
Sending SIB Timer |
||
|
T6 |
3-6s |
64 kbps |
|
Remote Congestion Guard Timer |
8-12s |
4.8 kbps |
|
T7 |
0.5-2s |
64 kbps |
|
Excessive Delay Timer |
4-6s |
4.8 kbps |
NOTE The timers listed in Table 2-2 are based on ITU Q.703 and can vary based on national specifications. For example, the T4 timer can vary between 2 and 9.5 seconds.
After the type of alignment has been selected, the proving period ensures that Layer 2 is able to transmit and receive MTP3 and above traffic reliably. Integrity is verified by checking the error rate involved with the signaling link. If the proving period expires without excessive errors, the link is said to have completed link alignment. SIOS is sent in the case of the SLT being unable to send or receive MSUs for reasons other than SIPO. The following list of bulleted items describes the timers involved with the alignment process and how they are used.
• T2 Timer—Link state Is Not Aligned. During this timer, the link is not aligned and SIO is being sent from the transmitting SLT to the remote end. If a link failure occurs during this timer, T2 is restarted. If the timer expires without excessive errors, T2 is stopped and T3 is started.
• T3 Timer—Link state Is Aligned. During this timer, the SLT is aligned with the remote end and is sending and receiving SIN or SIE. The signaling link can detect and discriminate between signaling unit frames but it is not ready to send and receive upper-layer messages yet. SINs or SIEs are exchanged by the signaling points to decide which method of proving period is employed. After the exchange of SIE or SIN has occurred, T3 stops. If a link failure occurs during this timer, T3 stops and T2 is restarted.
• T4 Timer—Link state Is Proving. The proving period that was selected in T3 is used for the proving process. As far as the proving periods are concerned, SIE can take no more than one error and SIN can take no more than four. If excessive errors occur during T4, SIOS is sent and alignment is not possible. During this timer, if the point codes or SLCs do not match on both ends, the timer also fails and restarts T2.
• T1 Timer—Link state Is Aligned Ready. After the alignment procedure completes, the T1 timer (Aligned Ready Timeout) is stopped. The Aligned Ready Timeout is used to provide a window of time in which the total link alignment procedure should take place.
MTP3 is the OSI model's network layer equivalent. As stated before, the addressing is based on a point code structure, which uniquely identifies each node within the network. The MSU, found in MTP2, contains the MTP3 routing label and routes call state messages from signaling point to signaling point. Because the MSU used is signal message routing, even though it is an MTP2 message, it is discussed in this section. The MSU carries the upper-layer call control and network management messages that are used by upper-layer protocols, such as the ISDN User Part (ISUP). Figure 2-21 details the header composition of the MSU.
Figure 2-21 MSU Format
|
F |
B |
||||||||
|
CK |
SIF |
SIO |
S |
LI |
B |
FSN |
B |
BSN |
F |
CK = Check Bits
LI = Length Indicator
FSN = Forward Sequence Number
BSN = Backward Sequence Number
SIF = Service Information Field
S (Spare) = Not Used FIB = Forward Indicator Bit BIB = Backward Indicator Bit F = Flag
SIO = Service Indicator Octet
There are a couple different fields associated with the MSU: the Service Indicator Octet (SIO) and the Service Information Field (SIF). The SIO indicates which upper-layer protocol is being used on the network. Figure 2-22 details the layout of the SIO sub-field in the MSU, and Table 2-3 details the specific field values. The SIO sub-field has several bits denoted as A, B, C, and D. These bits identify which bits are read first. The A-bit is read first, followed in order until the D-bit. ANSI T1.111 identifies the bit patterns of 1110 and 1101 as reserved for national application use.
|
F |
B |
|||||||||
|
CK |
SIF |
SIO |
S |
LI |
I |
FSN |
I |
BSN |
F |
|
|
B |
B |
|||||||||
|
Number of Bits: |
16 |
<272 |
8 |
2 |
6 |
1 |
7 |
1 |
7 |
8 |
|
DCBA DCBA |
|
|
Sub-service Field |
Service Indicator |
Number of Bits:
|
D |
C |
B |
A |
Value |
|
G |
G |
G |
G |
Signaling Network Management Messages |
|
G |
G |
G |
l |
Signaling Network Testing and Maintenance Messages |
|
G |
G |
l |
G |
Spare |
|
G |
G |
l |
l |
SCCP |
|
G |
l |
G |
G |
TUP |
|
G |
l |
G |
l |
ISUP |
|
G |
l |
l |
G |
DUP—call and circuit related messages |
|
G |
l |
l |
l |
DUP—facility registration and cancellation messages |
|
l |
G |
G |
G |
Reserved for MTP Testing User Part |
|
l |
G |
G |
l |
Broadband ISDN User Part |
|
l |
G |
l |
G |
Satellite ISDN User Part |
|
l |
G |
l |
l |
Spare |
|
l |
l |
G |
G |
Spare |
|
l |
l |
G |
l |
Spare |
|
l |
l |
l |
G |
Spare |
|
l |
l |
l |
l |
Spare |
The Sub-service field contains the network indicator bits that identify which type of switch is being used. Although four bits are allocated to this field, only the D and C bits have values, which are shown in Table 2-4.
|
D |
C |
Value |
|
G |
G |
International Network |
|
G |
l |
Spare (for international use) |
|
l |
G |
National Network |
|
l |
l |
Reserved (for national use) |
You can use the value of 3 (1 and 1) in national applications that can require message priority for flow control. ANSI T1.111 specifies the use of this field for priority values of 0 through 3 only if this field does not discriminate between national and international messages. The priority codes allow for protocol-specific use of the Sub-service fields. You only use this in a closed national signaling network.
The SIF identifies an upper-layer protocol message that is used by the protocol in use, which is specified by the SIO. The format of the SIF depends on the upper-layer protocol. Some of the message types include Emergency Changeover messages, Management Inhibit messages, Traffic Restart Allowed messages, and Signaling Route Set Test messages.
The SIF length is governed by the LI in the MSU frame. Unlike the LSSU messages, the LI can have many different values. The SIF field can be up to 272 octets (272 8-bit fields), as shown in Figure 2-20. There can be up to 268 information octets, or 272, with the last four reserved for the routing label. The LI field can have a value ranging from 1-63. Values of 1-62 identify the exact number of octets that are in the SIF. A value of 63 identifies that the SIF is 63 octets or larger.
Depending on the upper-layer protocol in use, the SIF can include the routing label. The routing label identifies the SLS for the MSU, the OPC, and the DPC. For ITU networks, each point code field is 14 bits and the SLS field is 4 bits for a total of 32 bits. ANSI specifies 24 bits for the OPC and the DPC and another 8 bits for the SLS for a total of 56 bits. The SLS in the ANSI network used to be set to 5 bits but was increased to 8 bits because of scalability concerns. The format of the routing label is shown in Figure 2-23.
Figure 2-23 MSU Routing Label
|
International |
SLS |
OPC |
DPC |
|
Number of Bits: |
4 |
14 |
14 |
|
North American |
SLS |
OPC |
DPC |
Number of Bits: 8 24 24
Number of Bits: 8 24 24
MTP3
MTP3 handles the routing of call-control signaling and network management messages through the SS7 network. MTP3 has two distinct areas of operation: signaling message handling and signaling network management. MTP3 acts as an interface for upper-layer user parts to communicate with each other. That being said, it is the responsibility of MTP3 to ensure that the messages originating from one signaling point are delivered to the same upper-layer user part at the destination signaling point.
The actual routing of the messages can take several different paths through the network. The messages can either be relayed through a mated STP pair or directly from one SSP to another. This ability is enabled by the use of the routing label. The routing label explicitly states the origination and the intended destination of the signal unit. Signaling message handling is broken down into three areas: message routing, message distribution, and message discrimination.
Message routing is the function of the SSP, STP, or SSP with STP functionality to analyze the signal unit to decide where to send the packet and which signaling link to send it out of. The receiving signaling point uses message distribution to deliver locally destined signal units to the proper user part. Message discrimination is the ability of the SSP to identify whether or not the received signal unit is destined for a circuit that is locally installed or not. If the signal unit is identified as not belonging locally, it is handed over to the message routing function.
Within the routing process there is a procedure for sharing the load, also known as load sharing traffic, among several links. Balanced load sharing helps to ensure that one link between signaling points does not get congested with too much signaling traffic. It is recommended that the number of links in a linkset be a power of two. This will allow for balanced load sharing.
Load sharing can be performed one of two ways:
• Load sharing across several links within the same link set
• Load sharing across several links that do not necessarily belong to the same link set
In the case of load sharing across link sets, it is referred to as a combined link set. In international network operation, it is required that both types of load sharing are supported.
The second main function of MTP3 is signaling network management. This accomplishes several things, including network failure recovery (through signaling traffic rerouting) and congestion control. Signaling network management also includes a methodology for rerouting traffic after the network failure has been corrected.
Message routing and network management, similar to every other function of SS7, are accomplished through the use of messages. Several different types of messaging within the network management fabric are identified:
• Signaling traffic management
• Link management
• Route management
Signaling traffic management is responsible for the management of the signaling link in the case of a failure. This means that it handles functions of signaling traffic rerouting in the case of a failure, signaling point restart after the failure is cleared, and signaling traffic back off in the case of congestion. Signaling link management is responsible for signaling link alignment of new links, signaling link restoration on corrected failures, and signaling link deactivation, if necessary. The signaling route management comprises several different functions to facilitate the transfer of network status information. This allows the SS7 network to test and block or unblock specific signaling routes and to define a set of signaling route test procedures.
Now that you have learned about the major components of the SS7 network, there are a couple of things that you need to make sure you obtain from your service provider if you ever need an SS7 link. These items absolutely must match between you and the service provider or you cannot operate properly in the SS7 network:
• Point codes—OPCs, DPCs, and any APCs that are required.
• CIC ranges—If these don't match on both ends, you cannot complete calls properly.
• SLC—Make sure that you are configuring to use the proper signaling link out of the link set.
• Glare—The procedure that will be used in the event that both sides select the same trunk at the same time for separate calls. This is also known as a dual seizure.
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