Abstract—The paper examines the performance of bit-interleaved parity (BIP) methods in error rate monitoring, and in declaration and clearing of alarms in those transport networks that employ automatic protection switching (APS). The BIP-based error rate monitoring is attractive for its simplicity and ease of implementation. The BIP-based results are compared with exact results and are found to declare the alarms too late, and to clear the alarms too early. It is concluded that the standards development and systems implementation should take into account the fact of early clearing and late declaration of alarms. The window parameters defining the detection and clearing thresholds should be set so as to build sufficient hysteresis into the system to ensure that BIP-based implementations yield acceptable performance results. Keywords—Automatic protection switching, bit interleaved parity, excessive bit error rate I. INTRODUCTION HIS paper examines the performance of bit-interleaved parity (BIP) methods in error rate monitoring, especially for high bit error rates. The term high here is relative. For example, when the BIP calculation is taken over 801 frames, a bit error rate (BER) greater than 10 -3 is high, in that for values of BER above this level there is noticeable disparity between the BIP-based and exact probabilities of bit error in the BIP word. When the number of frames is 9720 the transition level is at a BER of 2 10 -3 , and so on. The reason for this is given by (3) and (4) and depicted in Fig.4. The analysis presented here can be applied in SONET systems employing automatic protection switching (APS). Technical standards exist that specify requirements (and objectives) for declaring and clearing alarms. Details of SONET frame structure and automatic protection switching can be found in telecommunication standards[1-4] and other texts [5-8]. The protection mechanism can be of two types [6], the 1:1 and 1:n protection mechanisms. Fig.1 (a) depicts the 1:1 protection architecture where a protection interface is paired with each working interface. Fig.1 (b) depicts the other the 1: n protection architecture, consisting of a single protection facility for several working interfaces. In either case, when a working interface fails, traffic is automatically switched over to the protection interface. The failed working facility is marked with an in both halves of Fig.1. V. K. Oduol is with the Department of Electrical and Information Engineering, University of Nairobi, Nairobi, Kenya (+254-02-318262 ext.28327, vkoduol@uonbi.ac.ke) Cemal Ardil is with the National Academy of Aviation, Baku, Azerbaijan The SONET STS-1 frame structure is given in Fig.A1 of the Appendix. For the purposes of this paper, the bytes of interest are the three BIP bytes B1, B2 and B3 whose scopes are as follows. The B1 byte is used to detect parity errors per frame. This is done for the first STS-1 frame in the STS-n multiplexed frame. It monitors section level bit errors. The B2 byte is used to monitor line-level bit errors, and the B3 byte is used to monitor path-level bit errors, inclusive of the path overhead. A single interleaved parity byte is used to provide error monitoring across a particular segment along the end-to-end SONET path. This parity byte performs a parity check on the previous Synchronous Transport Signal level 1 (STS-1) frame. During the parity check, the first bit of the BIP octet is a parity check on the first bit of all octets of the previously scrambled STS-1 frame. The second bit of the BIP octet is used exactly the same way, i.e. it is a parity check on the second bits of each octet of the previous STS-1 frames, and similarly for the other bits. Head end switch Tail end switch Signal divert ed Working facility protection facility (a) 1:1 Protection Head end switch Tail end switch Signal diverted Working facility protection facility Working facility Working facility (a) 1: n Protection Fig.1 protection switching When the bit errors exceed certain thresholds, i.e. when there is an excessive bit error rate condition, the system may declare an alarm. These conditions when present must be declared within time limits specified by telecommunication standards [1,3]. The rest of the paper is organized as follows: Section II presents a discussion and determination of the bit error probability in the BIP word. Section III presents the declaration of the alarm using a sliding window, where first BIP-Based Alarm Declaration and Clearing in SONET Networks Employing Automatic Protection Switching Vitalice K. Oduol and Cemal Ardil T World Academy of Science, Engineering and Technology International Journal of Electrical and Computer Engineering Vol:5, No:12, 2011 1939 International Scholarly and Scientific Research & Innovation 5(12) 2011 scholar.waset.org/1307-6892/6347 International Science Index, Electrical and Computer Engineering Vol:5, No:12, 2011 waset.org/Publication/6347