Multiplex communications – Fault recovery – Bypass an inoperative channel
Reexamination Certificate
1998-01-30
2001-01-30
Vu, Huy D. (Department: 2735)
Multiplex communications
Fault recovery
Bypass an inoperative channel
C370S217000, C370S228000, C370S243000
Reexamination Certificate
active
06181676
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to wayside protection switching for a SONET/SDH radio communication network comprising a plurality of radio communication terminals and a plurality of repeaters intermediate the communication terminals.
SONET and SDH are standards for a multiplexing format for a Synchronous Optical Network. SDH is a Synchronous Digital Hierarchy.
In a SONET/SDH radio system, besides the main traffic, additional traffic can also be transmitted and carried in the main traffic overhead bytes, such as wayside (DS-1 capacity), Service channel (DS-0 capacity) etc. See ITU-RF 751-1, section 4.1.2., 1992-94. In the present invention, protection switching of wayside traffic is used as described hereinafter.
The additional traffic could consist of more than one tributary depending on the main traffic overhead capacity. To simplify the description, protection switching of two wayside tributaries is discussed and illustrated herein as examples.
The main traffic's protection switching mode is 1:n, where n is an integer greater than 1, e.g. 1 for 11. The wayside protection switching mode of this invention is 1+1.
In the present invention, when a wayside signal enters an ADD site, it is bridged to two radio channels, resulting in two adjacent channels carrying the same wayside data. This redundant configuration makes protection switching possible by selecting the better of the two radio channels when dropping the wayside traffic to a customer. The wayside radio path is protected by using unused bandwidth from an adjacent radio channel to transport a copy of the wayside data. This constitutes a redundant radio path for the wayside data. The SOA (Service Overhead Access) circuit pack is protected by having a redundant SOA circuit pack from which the wayside can be selected at a site where the wayside is dropped.
A radio communication network is subject to radio channel impairment from a variety of causes such as electrical interference, faulty equipment or radio fading. In such cases, it is necessary to have affected communication channels switched to a radio protection channel. In the case of a pair of main radio traffic terminals, switching to the protection channel (1 for n channels) is performed by a radio signal processor at the receive terminal site if a threshold value is exceeded for either line FEC (Forward Error Correction) or line PEC (Parity Error Count). FEC is a count of the number of corrected errors. The line FEC and line PEC are calculated based on the entire traffic, namely payload and overhead, and are an accumulation of section FEC and PEC from the start terminal to the end terminal.
SUMMARY OF THE INVENTION
The present invention is concerned with non-hitless protection switching (to a different path) in the case of wayside radio traffic which involves communications between repeaters forming part of a channel between two main traffic terminals. Because wayside paths could be shorter than the main traffic path, it is inappropriate to make a switching decision for wayside traffic based on line FEC and line PEC or accumulation of section FEC and section PEC from the start terminal. Rather, according to the present invention, there are three ways to separate the wayside switching decision-making from that of the main traffic:
a. use different radio overhead (ROH) bits to calculate each wayside tributary's path FEC and PEC by adding up the main traffic's section FEC and PEC on the wayside path. For each tributary, eight bits are used for FEC and four bits for PEC. If there are n wayside tributaries, then the total necessary bits for this calculation will be:
N
(total bits)=
n*
(8+4)=12
n
In accordance with method “a”:
1) At any ADD site, the FEC and PEC count bits corresponding to the tributary to be added are set to zero.
2) At any THRU site, the section FEC and PEC are accumulated to the existing counts.
3) At any DROP site, the accumulated FEC and PEC corresponding to the tributary to be dropped are compared with thresholds to determine a protection switching.
b. use radio overhead bits to calculate a common wayside path FEC and PEC. Eight bits are used for FEC and four bits for PEC plus other necessary bits. If there are n wayside tributaries, there needs to be one bit to indicate a start, two bits to represent FEC and PEC quality and n bits for tributary IDs. The total necessary radio overhead bits for this calculation will be:
N
(total bits)=(8+4)=12, when
n=
1;=(8+4)+1+2+
n=
15+
n,
when
n>
1.
A site where one wayside is dropped and the other passed through is referred to as a DROP & THRU herein, while a site where one wayside is dropped and the other not in use is called a DROP.
In accordance with method “b”, the currently preferred embodiment of the invention, a switching decision is made by using one common wayside path to calculate FEC (Forward Error Correction) and PEC (Parity Error Count), switching being effected if FEC or PEC exceed predetermined thresholds, the common wayside path, taking two paths as an example, being defined as follows:
(a) &Sgr; wayside A path+&Sgr; wayside B path
−&Sgr; (wayside A ||wayside B), if waysides A and B are overlapped (||indicates overlapped portions of the two parameters.)
(b) wayside A path OR wayside B path, if waysides A and B are not overlapped.
Fundamentally, wayside path operation is merely setting values to the above introduced ROH bytes to initiate correct wayside protection switching. The important point here is that with one common wayside path operation, it is necessary to perform switching of two waysides as independently as possible, i.e. if two waysides are not dropped at the same site, the one affected by failures should be switched individually. A simple solution to this concern is to use Poor FEC, Poor PEC and Stream Indicator bits to record whether a wayside stream and only this stream has a failure, while utilizing FEC and PEC count bits to record the path quality after the failure (if there is one) till the drop site. By doing so, the two waysides can be easily switched separately when needed. Details of a wayside path operation are given below:
ADD Site
If both waysides are added together, set the Start Indicator bit to start a wayside path and all other bits in both bytes to 0.
If waysides are added separately, set the Start Indicator bit and all other bits to 0 at the first add site. (If one wayside is added with the other one passed thru, the site will be considered as the second add site; otherwise, it will be the first add site on a wayside path.) At the second add site, check if the accumulated FEC and accumulated PEC are above the threshold. If not, no action is needed. If yes, there will be three actions:
1. set Poor FEC and/or Poor PEC bit(s) depending on which one exceeds the threshold;
2. set Stream Indicator bit corresponding to the wayside which has poor FEC and/or PEC;
3. set all FEC and/or PEC count bits to 0.
The reason for these three actions is to cope with a scenario in which a radio channel failure occurs before the second wayside stream is added. The above three actions ensure that a protection switching is performed at the drop site for the first added wayside (the Stream Indicator bit identifies the “bad” stream). It also ensures no switching for the second added wayside if the accumulated FEC and accumulated PEC (recorded by the FEC and PEC bits) are below the threshold (since all FEC and PEC bits are reset to 0 at the start of its individual path).
DROP Site
At a drop site, a protection switching is based on the following conditions:
Poor FEC and/or poor PEC bits(s) set to 1 AND the dropped wayside Stream Indicator(s) set to 1.
If both wayside streams are dropped together, the Start Indicator bit should be cleared.
If they are dropped separately, the Start Indicator bit should be cleared at a DROP site. At a DROP & THRU site, the protection switching is considered in accordance with Poor FEC and/or Poor
d'Anjou Martin
Yao Liqian
Nortel Networks Limited
Phan M.
Vu Huy D.
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