In an SS7 network, MSUs are not necessarily exchanged between adjacent neighbors (SP/STP). In a GSM system, the MSC and BSC are neighbors; how- ever, the exchange of information between the MSC and the HLR may involve several STPs. SS7 uses so-called point codes for routing and addressing MSUs.
Point codes are unique identifiers within an SS7 network. Exactly one point code, a signaling point code (SPC), is assigned to every SP and STP. An MSU has a routing label that contains the point codes of the sender (the originating point code, or OPCs) and the addressee (the destination point code, or DPC).
The routing label is, for its part, a component of the SIF. Note that neither FISU nor LSSU possesses a routing label, since those messages are exchanged only between two adjacent nodes.
Figure 8.6 shows the format of a routing label. The OPC defines the sender of the MSU, and the DPC defines its addressee. Note that addressing via SPCs works only on a national basis. The services of higher layers are needed for international addressing, in particular SCCP or ISUP, to provide the neces- sary features.
The remaining 4 bits of the routing label form the signaling link selection (SLS) field. This parameter is used to balance the load between several SS7 con- nections of a link group. If, for example, two SS7 connections are available between two network elements, all the even values of the SLS (0, 2, 4,…, 14dez) are assigned to the first link, and the odd values (1, 3, 5,…, 15dez) are assigned to the second link. This fact is important to know in the analysis of SS7 trace files.
1. Note that the service information octet and its abbreviation SIO do not have a relation to the former use of the abbreviation, which stood for status indication/out of alignment. Un- fortunately, the standards use the same abbreviation for both.
8.4.1 Example: Determination of DPC, OPC, and SLS in a Hexadecimal Trace
In the analysis of hexadecimal trace files, it generally is important to be able to convert DPC and OPC into clear text, to be able to relate the various messages to, for example, a particular MSC or BSC. As shown in Figure 8.6, DPC and OPC are each 14 bits long. The routing label, together with the 4 bits of the SLS, totals 32 bits, or 4 bytes.
Because the OPC and the DPC are 14 bits in length, it is not trivial, par- ticularly with byte (8 bits) or 16-bit-word-oriented presentations, to derive the decimal value of DPC or OPC, as illustrated in Figure 8.7. The sequence of numbers represents the hexadecimal values. The underlined part represents the routing label, that is, the SLS, OPC, and DPC. This information is decoded in clear text. At first sight, the values seem to differ.
It is important when decoding to consider the bitwise sequence of trans- mission with which the data are received by the system. The binary presenta- tion (left to right) is given in Figure 8.8.
Routing Label
LSD MSD
9E EC 0F 83 00 2E 88 CB 06 22 81 31 00 01 03 00 01 21
2E00hex= DPC=11776dez 2E20hex=OPC=11808dez Chex=SLS=12dez
Figure 8.7 Partial trace file and point codes.
1 1 0 0C } 1 0 1 1B } 1 0 0 08 } 1 0 0 08 } 0 0 1 02 } 1 1 1 0E } 0 0 0 00 } 0 0 0 00 }
DPC=10 1110 0000 0000=2E 00=11776dez
SLS=12dezOPC=10 1110 0010 0000=2E 20=11808dez
Figure 8.8 Transmission of routing label.
14 bit
SLS OPC DPC
14 bit 4
31 27 13 0 bit
Figure 8.6 Routing label (DPC, OPC, and SLS).
Possible confusion is based on the unusual length (14 bits) of OPC and DPC on the one hand, and, on the other hand, the results from the reversed way of reading/writing (right to left), a problem familiar to most programmers.
Misinterpretation can be prevented when these facts are considered.
Other representations of SPCs can be used in various national applica- tions, like the “4-3-4-3” presentation, which refers to the bits that are used per sign. The example in Figure 8.7 reads in the “4-3-4-3” presentation as follows:
DPC=2E00hex=11776dez=1011−100−0000−000=B−4−0−0 OPC=2E20hex=11808dez=1011−100−0100−000=B−4−4−0 8.4.2 Example: Commissioning of an SS7 Connection
Every SS7 connection is brought into service as presented in Figure 8.9. In the figure, an A-interface link between BSC and MSC is brought into service.
8.4.2.1 Bringing Layer 2 Into Service
After Layer 1 is established, both sides send an SIOS-LSSU, which indicates that the link is out of service and no MSU can currently be processed.
The process to bring Layer 2 into service starts with sending an SIO-LSSU. Please note the duplex characteristics of SS7. Both terminals are equal, and a link has to be established inbothdirections.
The test period, during which both sides examine the link quality, starts with sending an LSSU-SIN or an LSSU-SIE. Transmitted FISUs must not contain any errors during this test period. The link cannot go into service if an error occurs. The difference between LSSU-SIE and LSSU-SIN is the related surveillance time.
An emergency alignment is used when no alternative SS7 route currently exists and the link needs to be in service as quickly as possible.
8.4.2.2 Bringing Layer 3 Into Service
When the test time is over and no errors were detected, Layer 2 is considered to be in service and Layer 3 initiates further tests. A signaling link test message (SLTM) is used for that purpose, to transmit a number of test bytes to Layer 3 of the peer entity.
If the test bytes are correctly returned to the sender in a signaling link test acknowledgment (SLTA) message, Layer 3 is also considered to be “in traffic.”
Figure 8.10 shows examples of a SLTM and a SLTA message.
Synchronization of Layer 4 (in this case of the SCCP) follows the link establishment on the A-interface, by applying the reset procedure (described in Chapter 10).