ITU-T G.822
CONTROLLED SLIP RATE OBJECTIVES ON AN INTERNATIONAL DIGITAL CONNECTION
Organization: | ITU-T |
Publication Date: | 1 November 1988 |
Status: | active |
Page Count: | 5 |
scope:
Scope and considerations
The end-to-end slip rate performance should satisfy the service requirements for telephone and non-telephone services on a 64-kbit/s digital connection in an ISDN.
The slip rate objectives for an international end-to-end connection are stated with reference to the standard digital Hypothetical Reference Connection (HRX) of Figure 1/G.801 [1] of 27 500 km in length.
It is assumed that international switching centres (ISC) are interconnected by international links which are operating plesiochronously, using clocks with accuracies as specified in Recommendation G.811. It is recognized that one slip in 70 days per plesiochronous interexchange link is the resulting maximum theoretical slip rate, taking into account clock accuracies according to Recommendation G.811 only, and provided that the performance of the transmission and switching requirements remain within their design limits.
In the case where the connection includes all of the 13 nodes identified in the HRX (Recommendation G.801) and these nodes are all operating together in a plesiochronous mode, the nominal slip performance of a connection could be 1 in 70/12 days or 1 in 5.8 days. However, since in practice some nodes in such a connection would be part of the same synchronized network a better nominal slip performance can be expected (e.g. where the National Networks at each end are synchronized. The Nominal Slip Performance of the connection would be 1 in 70/4 or 1 in 17.5 days).
Note - These calculations assume a maximum of four international links.
In a practical international end-to-end connection containing both international and national portions, the slip rate may significantly exceed the value computed from n plesiochronous interexchange links due to various design, environmental and operational conditions in international and national sections. These include:
a) configuration of the international digital network,
b) national timing control arrangements,
c) wander due to extreme temperature variations,
d) operational performance characteristics of various types of switches and transmission links (including diurnal variations of satellite facilities),
e) temporary disturbances on transmission and synchronization links (network rearrangements, protection switching, human errors, etc.).
Note - The maximum number, n, of plesiochronous interexchange links is under study.
A threshold of slip performance is a suitable compromise between desired service requirements and normally achievable performance. Slip levels according to category (b) (see Table 1/G.822) exceeding this threshold will begin to affect performance and can cause some services to be considered degraded. In order to ensure that a trend of performance has been identified, the threshold rate must be measured over a sufficient period to record a significant number of slips. An objective limit is placed on the total time that the threshold is exceeded during the period of one year. The performance objectives are intended to represent a uniform set of specifications.
Slip is one of several contributing factors to impairment of a digital connection. The performance objectives for the rate of octet slips on an international connection of 27 500 km in length or a corresponding bearer channel are given in Table 1/G.822. Further study is required to confirm that these values are compatible with other objectives, e.g. the error performance as listed in Recommendation G.821.
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