h which a person gains access to a computer, a network, or another electronic device. (3) The interface between a process and a communications or transmission facility. port number A number in the range 1 to 65,535 that identifies a port. The port number does not represent a physical port, such as the serial port to which a modem or a mouse might be attached; instead, it is like a regional memory address. port speed The maximum signaling rate on a digital access line. PoS (Packet over SDH/SONET) PoS (Packet over SDH/SONET) A communications protocol for transmitting packets over the circuit-switched protocols SDH and SONET. Much of the traffic on the Internet is carried over PoS links. POTS (plain old telephone service) The standard analog telephone service that most homes have traditionally used. powerline Broadband data transmission over electrical powerlines. PPP (Point-to-Point Protocol) A protocol that provides router-to-router and host-to- network connections over synchronous and asynchronous circuits. PPP is a successor to SLIP. PPTP (Point-to-Point Tunneling Protocol) A Layer 2 protocol that can work in a non-IP enterprise environment, which is one of its strengths for customers who use multiple protocols rather than using only IP. PPTP provides low packet overhead and good compression, but it has security weaknesses. presence-based services The ability to see in real-time where someone is, how that person prefers to be reached, and even what the person is doing. Presence-based services involve the ability, willingness, desire, and capability of a user to communicate across media end devices and even time and space. presentation layer Layer 6 in the OSI model, which provides services to the application layer, enabling it to interpret the data exchanged, as well as to structure data messages to be transmitted in a specific display and control format. preventive maintenance The routine checking of components to keep a system functioning. PRI (Primary Rate Interface) A bundle of ISDN circuits, primarily a PBX