ng resumes) and an access timeout (when a terminal on a LAN using a CSMA/CD access method is prevented from transmitting for a specified time after a collision occurs). time-slot interchange The interchange of time slots within a TDM frame. Time to Live A field in an IP datagram that is decremented by each router in order to guaran- tee that datagrams do not circulate on the Internet forever. time transparency The absence of delay and delay jitter through an ATM network, thus allowing high-speed transportation of real-time services. tip The contacting part at the end of a telephone plug or the top spring of a jack; the conduc- tors associated with these contacts. The other contact is called a ring. TMN (Telecommunications Management Network) A protocol model defined by the ITU-T for managing open systems in a communications network. TMN can be used in the man- agement of ISDN, B-ISDN ATM, and GSM networks. It is not as commonly used for purely packet-switched data networks. token An 8-bit word that is circulated in a unidirectional fashion among the devices on a network. token passing A network access method that uses a special bit pattern called a token that gives permission to nodes to transmit data, one node at a time. Each node is allowed to transmit a message only when it has the token. If a node has no messages to transmit when it receives the token, it passes the token to the next node. token ring A networked ring of devices that passes a special bit pattern called a token from node to node to determine which device can transmit data on the network. Token ring operates at either 4Mbps or 16Mbps. toll center The exchange that is responsible for establishing and completing national, long- distance communications. Also called the Class 4 office, transit switch, toll office, or trunk exchange. Glossary tone signaling tone signaling Transmission of supervisory, address, and alerting signals over a telephone circuit by means of tones. tone-to-dial-pulse conversion A system of converting DTMF signals to dial puls