 monitoring is done in quasi-real time and while operational (i.e., in traffic). In Chapter 8 we discussed PCM systems and the digital network including T1 (DS1) and E1 hierarchies. Let us quickly review their in-service performance monitoring capabilities. This capability is not unique to T1- and E1-type systems. For example, SONET and SDH have even more sophisticated capabilities, often referred to as OA&M (operations, administration, and maintenance). ATM is (will be) rich in such network management aids. 21.5 SYSTEM DEPTH - A NETWORK MANAGEMENT PROBLEM CSMA/CD LAN CSMA/CD LAN Bridge Bridge Frame Frame relay relay 384 k 384 k E1 channel E1 channel E3 multiplex E3 multiplex Microwave Microwave terminal terminal 585 km Figure 21.1 A typical multilevel network requiring a network management system. Note multiple convergences. DS1 or T1 has a frame rate of 8000 frames a second. Each frame is delineated with a framing bit or F-bit. With modern alignment and synchronization algorithms, to main- tain framing repetition of the F-bit 8000 times a second is excessive and unnecessary. Advantage was taken of F-bit redundancy by the development of the extended superframe (ESF). The ESF consists of 24 consecutive DS1 frames. With 24 frames we expect to have 24 framing bits. Of these, only six bits need to be used for framing, six are used for a cyclic redundancy check (CRC-6) on the superframe, and the remaining 12 bits form a 4-kbps data link for network control and maintenance. It is this channel that can serve as transport for network management information. It can also serve as an ad hoc test link. Our present concern is in-service monitoring. For instance, we can get real-time error performance with the CRC-6, giving us a measure of errored seconds and severely errored seconds in accordance with ITU-T Rec. G.821 (Ref 4). Such monitoring can be done with test equipment such as the HP4 37702A or HP 37741A. Permanent monitoring can be carried out using typically Newbridge network monitoring equipment, which can monitor an