Configuring the System for Random SLS Generation

The Random SLS Generation feature can alleviate problems of the EAGLE not load-sharing between all links within a linkset. This feature is available for both ITU and ANSI traffic.

The ITU protocol uses a 4 bit Signaling Link Selection (SLS) field with no modification of SLS values by intermediate nodes and a one-to-one mapping of SLS values to signaling links. These rules can be overly restrictive in situations where they are not necessary.

For both ITU and ANSI, the feature allows the user to have the EAGLE ignore the incoming SLS value and randomly generate a new 8-bit SLS value to select an outgoing linkset and a link. For ITU only, the original 4-bit SLS value is not changed and is still contained in the outgoing message. The newly generated SLS is used for link selection only. For ANSI, the original SLS value in the outgoing MSU can be replaced with the SLS value generated by the feature. This is done by appropriately setting SS7OPTS:SLSREPLACE parameter.

Messages destined for a particular destination are randomly distributed across all the links to that destination using an internally generated random 8-bit SLS. This means that this feature does not follow the ITU protocol requiring that all messages with the same SLS value must use the same signaling link. Also, correct sequencing of Class 1 messages is not guaranteed. Random SLS generation applies to all Class 0 and Class 1 SCCP messages.

This feature is implemented with one of these values for the randsls parameter of the chg-stpopts command.

class0 – Applies the Random SLS feature to Class 0 ITU SCCP messages and associated service. For example, Random SLS Generation would apply to Class 0 UDT, XUDT, and UDTS, XUDTS messages. Class 1 messages would still use the standard ITU method for link selection.
all – Applies the Random SLS feature to all ITU SCCP messages
off – Turns off the Random SLS feature.
perls – Applies the Random SLS feature on a specific linkset instead of applying the Random SLS feature system-wide. To use the randsls with ANSI, the value for randsls must be specified as perls. For more information about random SLS generation on a specific linkset, refer to Per-Linkset Random SLS.
Caution:
If the randsls parameter value of the chg-stpopts command is all, thus activating the Random SLS feature for Class 1 ITUSCCP messages, and the value of the class1seq parameter of the chg-sccpopts command is on, there is no guarantee that UDT/XUDTITU Class 1 messages are delivered to the remote node in the order in which they were received. To ensure that Class 1 UDT/XUDTITU messages are delivered to the remote node in the order in which they were received, the randsls parameter value should be set to either off or class0 if the value of the class1seq parameter of the chg-sccpopts command is on.

For ITU linksets, this feature is available as a system-wide option as well as on a per-linkset basis. For ANSI linksets, this feature is available only on a per-linkset basis. The Random SLS feature is applied to incoming messages on ITU linksets as shown in Table 3-23.

Table 3-23 ITU Random SLS Rules

System-Wide RANDSLS Value (in the RTRV-STPOPTS Output)	RANDSLS Value for the Outgoing Linkset	Random SLS Action
OFF	N/A	The Random SLS feature is not applied on any ITU message.
ALL	N/A	The Random SLS feature is applied on all ITU SCCP messages.
CLASS0	N/A	The Random SLS feature is applied on all ITU SCCP CLASS0 messages.
PERLS	OFF	The Random SLS feature is not applied on any ITU message on the specified linkset.
PERLS	ALL	The Random SLS feature is applied on all ITU SCCP messages on the specified linkset.
PERLS	CLASS0	The Random SLS feature is applied on all ITU SCCP CLASS0 messages on the specified linkset.

The Random SLS feature is applied to incoming messages on ANSI linksets as shown in Table 3-24.

Table 3-24 ANSI Random SLS Rules

System-Wide RANDSLS Value (in the RTRV-STPOPTS Output)	RANDSLS Value for the Incoming Linkset	Random SLS Action
OFF	N/A	The Random SLS feature is not applied on any ANSI message.
ALL	N/A	The Random SLS feature is not applied on any ANSI message.
CLASS0	N/A	The Random SLS feature is not applied on any ANSI message.
PERLS	OFF	The Random SLS feature is not applied on any ANSI message on the specified linkset.
PERLS	ALL	The Random SLS feature is applied on ANSI SCCP and ISUP messages on the specified linkset.
PERLS	CLASS0	The Random SLS feature is applied on all ANSI SCCP CLASS0 messages on the specified linkset.

The settings for this feature are independent of the ITU SLS Enhancement feature settings for individual linksets. These settings are defined by the slsocbit (Use of the Other CIC BIT capability) and slsrsb (SLS Bit Rotation capability) parameters of the ent-ls and chg-ls commands. The randsls parameter, however, overrides the slsrsb parameter for SCCP messages. If the randsls parameter value is perls, the randsls parameter also overrides the islsrsb (SLS Bit Rotation on Incoming Linksets) parameter of the ent-ls and chg-ls commands for Class 0 SCCP messages and ISUP messages on ANSI linksets. These parameters are described in greater detail in Commands User's Guide and in ITU SLS Enhancement. Note that the ent-ls or chg-ls commands do not prevent the user from provisioning the slsrsb or islsrsb parameters.

With the implementation of this feature, a maximum of 16 links continues to be supported in a single linkset to a destination. However, it is now possible to have up to 32 links in a combined linkset to a destination, with a maximum of 16 links per linkset. The 32 links is a change from the current EAGLE maximum of only 16 links per combined linkset, which is due to ITU protocol restrictions. If more than 16 links are used in a combined linkset, the operator needs to be aware that a maximum of 16 links can be used by non-Random SLS traffic over the linkset. The non-Random SLS traffic continues to operate under the rules of the ITU protocol.

Figure 3-32 shows an example of a combined linkset from node A to nodes B and C, with 8 links per linkset. Since 8 bits allows for values 0-255 (decimal), the figure shows how these values are internally mapped to the links of the combined linkset. For ease of reading, not all values are shown.

Figure 3-32 Random SLS Mapping to a Combined Linkset

Figure 3-33 shows the mapping for a 4-link single linkset between nodes D and E. When an MSU is to be transmitted, a random 8 bit SLS is generated internally and a link is selected according to this predetermined mapping.

Figure 3-33 Random SLS Mapping to a Single Linkset

The 4 bit SLS in the outgoing message is equal to the SLS that the EAGLE received. There is no change to the SLS value in the SS7 message.

In a non-failure condition, the process for mapping the internally generated SLS values to SLC (Signaling Link Code) values for specific links is as follows:

A “random” 8-bit SLS value is generated. In reality, a single table of 256 unique SLS values, initially generated in random order, exists in the EAGLE. A counter is maintained for each linkset in the EAGLE that causes the linkset to cycle through the random values in the table as messages are routed out on that linkset. For a combined linkset, the counter for the first linkset in the EAGLE's linkset table is used.
For a combined linkset, the first bit is used to select the linkset and then is ignored when selecting the SLC. For a single linkset, the first bit is used when selecting the SLC. In all cases, the fifth bit is ignored when selecting the SLC. This is due to internal ANSI-based processing in the EAGLE.
The changed SLS value (with fifth and possibly also first bits ignored) is then divided by the number of links in the linkset (not a combined linkset) and the remainder gives the SLC value. For example, in Figure 3-32, the SLS value 78 is mapped to SLC 7 in linkset LS1 as follows:
1. The binary equivalent for decimal number 78 is 01001110.
2. The fifth bit is ignored leaving the binary number 0101110.
3. The least significant bit is used to select linkset LS1 and is then ignored, leaving the binary number 010111.
4. The decimal equivalent of the binary number 010111 is 23. When the number 23 is divided by the number of links in the linkset, in this example, eight, a remainder of seven remains, thus SLC 7 on linkset LS1 is chosen for the outgoing message.
In the example shown in Figure 3-33, the SLS value 78 is mapped to SLC 2 in LS1 (the only linkset) as follows:
1. The binary equivalent for decimal number 78 is 01001110.
2. The fifth bit is ignored leaving the binary number 0101110.
3. The decimal equivalent of the binary number 0101110 is 46. When the number 46 is divided by the number of links in the linkset, in this example, four, a remainder of two remains, thus SLC 2 on linkset LS1 is chosen for the outgoing message.
  Table 3-25 shows the mapping for a combined linkset with 16 links in each linkset. This table is discussed in more detail in the next section.

Link failure scenarios

In any situation where a link is failed, SLS values that were mapped to that link are remapped to other links of the linkset or combined linkset. This is done in the reverse order that the SLS values were originally mapped to links, of course skipping the failed link. Subsequent link failures will have their SLS values, along with SLS values from the prior failures, remapped in the same way. The odd/even mapping rule for combined linksets does not apply to the remapped SLS values under failure conditions. This is to continue to achieve the best possible load balance across all links. No MSUs should be discarded in any case.

For example, Table 3-25 shows how the internal 8-bit SLS values are distributed for a combined linkset with 16 links per linkset. It also shows what happens when one or two of the links fail. As this example shows, the SLS values that are identical after the fifth bit is dropped (for example, 0 and 16, 192 and 208, etc.) are remapped to the same link. This is why in this example the 8 different SLS values from the first failed link are remapped to only 4 links and not 8.

Table 3-25 Failure Scenarios for a 32-Link Combined Linkset

Linkset/SLC	Normal SLS Mapping	SLS Mapping for Single Link Failure	SLS Mapping for Dual Link Failure
LS1/0	0 16 64 80 128 144 192 208	Failed	Failed
LS1/1	2 18 66 82 130 146 194 210	Same as Normal SLS Mapping	Same as Normal SLS Mapping
LS1/7	14 30 78 94 142 158 206 222	Same as Normal SLS Mapping	Same as Normal SLS Mapping
LS1/8	32 48 96 112 160 176 224 240	Same as Normal SLS Mapping	Same as Normal SLS Mapping
LS1/9	34 50 98 114 162 178 226 242	Same as Normal SLS Mapping	Same as Normal SLS Mapping
LS1/12	40 56 104 120 168 184 232 248	Same as Normal SLS Mapping	40 56 … 248 225 241
LS1/13	42 58 106 122 170 186 234 250	Same as Normal SLS Mapping	42 58 … 250 161 177
LS1/14	44 60 108 124 172 188 236 252	44 60 … 252 192 208	44 60 … 252 97 113
LS1/15	46 62 110 126 174 190 238 254	46 62 … 254 64 80	46 62 … 254 33 49

LS2/0	1 17 65 81 129 145 193 208	Same as Normal SLS Mapping	Same as Normal SLS Mapping
LS2/7	15 31 79 95 143 159 207 223	Same as Normal SLS Mapping	Same as Normal SLS Mapping
LS2/8	33 49 97 113 161 177 225 241	Same as Normal SLS Mapping	Failed
LS2/12	41 57 105 121 169 185 233 249	Same as Normal SLS Mapping	41 57 … 249 192 208
LS2/13	43 59 107 123 171 187 235 251	Same as Normal SLS Mapping	43 59 … 251 128 144
LS2/14	45 61 109 125 173 189 237 253	45 61 … 253 128 144	45 61 … 253 64 80
LS2/15	47 63 111 127 175 191 239 255	47 63 … 255 0 16	47 63 … 255 0 16

Because of the large number of internal SLS values being remapped across the relatively small number of links, traffic is essentially evenly distributed across the remaining links. This is true in all cases, regardless of the original number of links or the number of failed links.

Display the existing values for the randsls parameter by entering the rtrv-stpopts command. The value for the randsls parameter is shown in the RANDSLS field. This is an example of the possible output.
```
rlghncxa03w 06-10-17 16:02:05 GMT  EAGLE5 36.0.0
STP OPTIONS
-----------------------
RANDSLS          class0
```
Note:
The rtrv-stpopts command output contains other fields that are not used by this procedure. If you wish to see all the fields displayed by the rtrv-stpopts command, see the rtrv-stpopts command description in Commands User's Guide.

If the randsls=all parameter will not be specified with the chg-stpopts command, continue the procedure with 5.

If the randsls=all parameter will be specified with the chg-stpopts command, continue the procedure with 2.
Verify the value of the class1seq parameter of the chg-sccpopts command by entering the rtrv-sccpopts command. This is an example of the possible output.
```
rlghncxa03w 06-10-17 16:02:05 GMT  EAGLE5 36.0.0

SCCP OPTIONS
---------------------------
CLASS1SEQ                on
```
If the value of the class1seq parameter is on, the randsls=all parameter of the chg-stpopts command should not be used. The class1seq=on parameter allows UDT/XUDT Class 1 ITU messages to be delivered to the remote node in the order that they were received. Using the randsls=all parameter with the class1seq=on parameter does not guarantee that UDT/XUDT Class1 ITU messages are delivered to the remote node in the order that they were received.

If you wish to continue delivering UDT/XUDT Class 1 ITU messages to the remote node in the order that they were received, or if the value of the class1seq parameter of the chg-sccpopts command is off, continue the procedure with 5.

If you do not wish to continue delivering UDT/XUDT Class 1 ITU messages to the remote node in the order that they were received, continue the procedure with 3.
Change the value of the class1seq parameter of the chg-sccpopts command by entering this command.

chg-sccpopts:class1seq=off
When this command has successfully completed, this message should appear.
```
rlghncxa03w 06-10-07 00:22:57 GMT  EAGLE5 36.0.0
CHG-SCCPOPTS: MASP A - COMPLTD
```

Verify the changes by entering the rtrv-sccpopts command. This is an example of the possible output.

rlghncxa03w 08-12-17 16:02:05 GMT  EAGLE5 40.0.0

SCCP OPTIONS
---------------------------
CLASS1SEQ               off

Change the randsls parameter value. For this example, enter this command.

chg-stpopts:randsls=all
When this command has successfully completed, this message should appear.
```
rlghncxa03w 06-10-07 00:22:57 GMT  EAGLE5 36.0.0
CHG-STPOPTS: MASP A - COMPLTD
```
Verify the changes using the rtrv-stpopts command. This is an example of the possible output.
```
rlghncxa03w 06-10-17 16:02:05 GMT  EAGLE5 36.0.0
STP OPTIONS
-----------------------
RANDSLS             all
```
Note:
The rtrv-stpopts command output contains other fields that are not used by this procedure. If you wish to see all the fields displayed by the rtrv-stpopts command, see the rtrv-stpopts command description in Commands User's Guide.

If the randsls parameter value is off, all, or class0, continue the procedure with 10.
If the randsls parameter value is perls, continue the procedure by performing one of these steps.
- If Random SLS will not be applied to ANSI linksets, continue the procedure with 10.
- If Random SLS will be applied to ANSI linksets, continue the procedure with 7.
Verify the value of the slsreplace parameter by entering the rtrv-ss7opts command. This is an example of the possible output.
```
rlghncxa03w 08-12-17 16:02:05 GMT  EAGLE5 40.0.0

SS7 OPTIONS
-----------------------
SLSREPLACE   no
```
Note:
The rtrv-ss7opts command output contains other fields that are not used by this procedure. If you wish to see all the fields displayed by the rtrv-ss7opts command, see the rtrv-ss7opts command description in Commands User's Guide.

If the slsreplace parameter value is yes, continue the procedure with 10.

If the slsreplace parameter value is no, continue the procedure with 8.
Change the slsreplace parameter value. For this example, enter this command.
chg-ss7opts:slsreplace=yes

When this command has successfully completed, this message should appear.
```
rlghncxa03w 08-12-07 00:22:57 GMT  EAGLE5 40.0.0
CHG-SS7OPTS: MASP A - COMPLTD
```
Verify the changes using the rtrv-ss7opts command. This is an example of the possible output.
```
rlghncxa03w 08-12-17 16:02:05 GMT  EAGLE5 40.0.0

SS7 OPTIONS
-----------------------
SLSREPLACE   yes
```
Note:
The rtrv-ss7opts command output contains other fields that are not used by this procedure. If you wish to see all the fields displayed by the rtrv-ss7opts command, see the rtrv-ss7opts command description in Commands User's Guide.

Back up the new changes using the chg-db:action=backup:dest=fixed command. These messages should appear, the active Maintenance and Administration Subsystem Processor (MASP) appears first.

BACKUP (FIXED) : MASP A - Backup starts on active MASP.
BACKUP (FIXED) : MASP A - Backup on active MASP to fixed disk complete.
BACKUP (FIXED) : MASP A - Backup starts on standby MASP.
BACKUP (FIXED) : MASP A - Backup on standby MASP to fixed disk complete.

Figure 3-34 Configuring the System for Random SLS Generation

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