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Failure of SS7 Links
Problem — With a single link to the adjacent signaling
point, service is disrupted if the link goes down for some
reason (i.e., layer 1 alarm, congestion).
Solution — Link resiliency is achieved by using multiple
links between a local point code and an adjacent point
code. By definition, such links are said to belong to the
same link set, which can contain up to 16 links. Ideally,
the links of a link set should not be carried over a unique
physical medium (such as an E-1 or T-1 trunk) but,
instead, should be split over independent physical trunks.
Link failure management is a standard MTP3 operation
and is not an SIU520-specific feature. In other words,
failure between links in a same link set will happen in a
completely transparent way for the user application.
Details — In an SIU520 system configuration, two
commands are used in config.txt to configure link sets
and links: MTP_LINKSET and MTP_LINK. In this
example, two SS7 links are defined between local point
code 0x100 and adjacent point code 0x200 on time
slot 16 of PCM ports 1 and 2. (Also see Figure 3.)
Application Note Building Fault-tolerant SS7 Systems Using the Intel
®
NetStructure™ SIU520 SS7 Signaling Gateway
4
Figure 3. SIU Connected to Adjacent Node with Two Links in Link Set
SIU520
SSP/SCP
A
) Load sharing between link 0 and link 1 under normal conditions
Link id 0, slc 0
Link Set id 0
Link id 1, slc 1
Point Code
0x100
SSP/SCP
B) Traffic sent over link 1 under failure of link 0
SIU520
Point Code
0x100
Point Code
0x200
Point Code
0x200
* MTP_LINKSET <linkset_id> <adjacent_spc> <num_links> <flags> <local_spc> <ssf>
MTP_LINKSET 0 0x200 2 0x0000 0x100 0x08
* MTP_LINK <link_id> <linkset_id> <link_ref> <slc> <bpos> <blink> <bpos2>
*<stream> <timeslot> <flags>
MTP_LINK 0 0 0 0 0 0 0 0 16 0x0006
MTP_LINK 1 0 1 1 0 1 0 1 16 0x0006