a-network link. Virtual Tributary Layer STS Layer Line Layer Section Section Section Section Section Section FIGURE 5-9 SONET Transmission Segment Another method of linking facilities is the mid-span meet, commonly used by carriers to link or interface between networks. This is depicted in Figure 5-10 and shows links between LECs and IXCs. Local Exchange Inter-Exchange Local Exchange Access Facilities Facilities Facilities Access Lines Lines Lightwave Terminal Add Drop Multiplex Equipment Ring Medium FIGURE 5-10 Linked SONET Transmission Facility PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS A variation on the light wave terminal equipment (LTE) is the add- drop multiplexer (ADM). SONET transmission capacity is built around the 51 Mbs STS. An LTE breaks out all the STS in the optical carrier (OC) and makes them available. If the LTE is used at a location where all STS are not terminated in local routing/switching equip- ment, then any transit streams must be connected directly to other LTE facing other routes. The ADM breaks out and terminates a limited quantity of STS from the optical carrier. For example, the office in which the terminal is located might only require 1 or 2 STS from an OC-3, OC-12, or higher rate transmission facility. Using an ADM would be less costly in terms of capital investment and operations expense. NETWORK FACILITIES Basic network facilities include access and transport. These facilities are built with switching and transmission equipment. Generally speaking, the transmission equipment meets certain ANSI, ITU, NEBS, or other standards with respect to input and output signals, control interfaces and protocols, and network management. Since deregulation and divestiture, manufacturers have sought market acceptance of products across a combination of public carrier net- works and private or enterprise networks. Corporate buyers not in the network services business have continued to press for improved network operations and uptime. These basic trends have driven manufactu