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36 Chapter 1: Introduction to the EIU
297-8991-910 Standard 03.01 August 1999
System-wide limitations
The EIU is collocated in an LPP shelf with other ASUs such as the link
interface unit (LIU7) and frame relay interface unit (FRIU). The exact
configuration of ASU-type units depends on the applications. The LPP is
connected to the DMS-bus through eight DS30 links in a load-sharing
arrangement.
Each DS30 has a transfer capacity of approximately 256 Kbyte/s. A single EIU
can route long messages (1518 bytes) from the LAN to the DMS-bus at a rate
that can overload the DS30 and cause the link to fail (SysB state). This link
failure causes traffic to switch to an alternate DS30 link. If the system
maintains the level of traffic that caused the initial failure, each DS30 link
topples one by one until the LPP is isolated from the DMS-bus. Further, as
each DS30 link overloads, all pending messages on that link are lost, including
SS7 messages.
Note: When deployed, fiber SR128 links through the MS will provide
enhanced capacity to alleviate link capacity overload.
The IP throttling feature throttles IP messages to and from the EIU to provide
a measure of protection against DS30 link overload. Application groups can
engineer throttling to permit them to override the defaults. Complete
protection is still not guaranteed due to other message sources, such as SIPC,
MTS, logs, and OMs. Further, multiple EIUs on the same LPP can still
simultaneously send a large message on the same link which, when combined
with other messages in the LMS RX FIFO queue, can still cause an overload.
IP throttling has also been implemented to protect the links between the MS
and EIUs that are on FLIS or SNSE LIS.
A problem common to all ASU types is that the F-bus receive buffers can be
overloaded. If this happens too many times within a certain time interval, the
rate adaptor detects the error and fails the link. Engineering rules are required
to ensure any ASU type is not overloaded. Refer to Provisioning Rules for LPP,
SSLPP, and SNSE LIS, System Engineering Bulletin SEB 92-02-001.
Limitations associated with maintenance
The operating company can datafill a maximum of eight EIUs in the LIUINV
table. That is, the maximum number of EIUs on a switch is eight. As a further
limitation, each LPP can have a maximum of four EIUs. The FLIS can have up
to eight EIUs.
These limitations are not only a datafill issue. Other factors must be studied
before these values can be increased, such as traffic load through an LPP or
FLIS, EMI emissions, and routing issues. Currently, with 8 EIUs and 28 LIU7s
in a 36 processor LPP configuration, the emissions are just within allowable
limits.