ector Pattern generator counter Figure 61.9 Principles of a BER tester Error performance measurements 61/9 available time begins with the first second of a period of ten Table 61.5 Test pattern versus bit rate (0.151, 0.152, consecutive seconds each of which has a BER better than 10-3. 0.153) During available time, any second containing one or more errors is logged as an errored second (ES). Any period of 1 second with a Bit rate Test pattern BER exceeding 10-3 is classified as a severely errored second (SES) 50bit/s - 168kbit/s (0.153) and is subtracted from available time. The remainder is grouped into 60 second periods. Any of these 1 minute periods with a BER 64kbit/s(0.152) 211-1 exceeding 10-6 is classified as a degraded minute (DM). 1.544Mbit/s 215 -1,220-1 2.048Mbit/s 215-1 8.448Mbit/s 215 1 61.3.4.1 Apportionment 34.368Mbit/s 223 1 The hypothetical reference connection (HRX) defined in G.821 is 44.736Mbit/s 215 -1,220-1 shown in Figure 61.12. The connection consists of a local and medium grade section at each end of the link, and a long distance 139.264Mbit/s high grade section in the middle. A high capacity transmission system such as microwave or lightwave would be characterised as a high grade section. Table 61.6 CCITT Recommendation G.821 Typically a low grade section would be the metallic subscriber loop. Medium grade would be the connection from, say, a local exchange to a trunk switching centre. The three types of section in the network are allocated different portions of the total G.821 Performance Objectives specification. classification As shown in Tables 61.7 and 61.8, the allocation of degraded Degraded Fewer than 10% of one minute intervals minutes and errored seconds is handled slightly differently from Minutes(DM) to have a Bit Error Ratio (BER) worse severely errored seconds. Low and medium grade sections are than 10-6 allocated a block allowance of 15% of the total G.821 specification Severely Errored Fewer than 0.2% of one second intervals at each end (i.e. 1.5% DM and 1.2% ES) irrespec