igure 3-11 Section SONET VTs. Group Group Group Overhead Line Overhead (STS-1) 125 us Chapter 3 Figure 3-12 Section Group Group Group Group Overhead VT groups. columns columns columns columns Overhead (STS-1) 125 us VT Path Overhead VT1.5 VT1.5 VT1.5 VT1.5 Figure 3-13 Pointer mechanism. STS-1 pointer indicates where SPE begins Byte 1 STS-1 Synchronous Payload Envelope (SPE) Traditional SONET Architectures Concatenated Payloads For data/broadband applications, the STS-1 is short on capacity. Hence, SONET offers the solution of concatenating STS- 1s to provide the necessary aggregate bandwidth. Figure 3-14 illustrates the concatenation of three STS-1s to 155.52 Mbps. Beyond STS-3, concate- nation is done in multiples of STS-3c: for example, STS-12c. 3.2.3 SONET Network Elements Although key SONET-oriented network elements are compatible at the OC-n level, vendors may include supplementary distinct features. For example, one vendor may offer an ADM with access at OC-3c only, whereas another may offer simultaneous access at OC-3c and OC-12c rates. Other vendors may offer Ethernet-based services or IP-routing functions. Figure 3-15 sketches out the key NEs. Figure 3-14 Concatenation mechanism (OC-3c example). STS-1 #1 STS-1 #2 STS-1 #3 STS Path Overhead Section Overhead Overhead Transport Overhead STS-3c Synchronous Payload Envelope (SPE) (TOH) Chapter 3 Figure 3-15 SONET Add/Drop Mux Key SONET network elements. Broadband Digital Crossconnect SONET Switch Digital Loop Carrier Path is lost. Ring A Ring B Signal is repeated, allowing traffic to pass along alternate route. Regional Node Signal Channels are dropped and repeated at each node and passed onto the next node. Users Users Add/Drop Multiplexers (ADMs)(Also Viewable as Edge Devices) An ADM can multiplex various inputs into an OC-n signal. At an add/drop site, only those signals that need to be accessed at that site are dropped or inserted. The other traffic continues through the NE without requiring spe- cial pass-through units or other signal processing. An ADM 