ommunications calls, processing requests, processes, and so on are stacked or held SO that they can be worked with in sequence. QVGA (Quarter Video Graphics Array) A computer display with 320 X 240 resolution, such as on a mobile phone, PDA, or handheld gaming console. radio channel The frequency band allocated to a service provider or transmitter. radio wave An electromagnetic wave of frequencies between approximately 20KHz and 3GHz. RADIUS (Remote Authentication Dial-in User Services) An authentication and access control server used to determine whether a user is allowed access to corporate resources. RADSL (Rate-Adaptive DSL) A symmetrical or asymmetrical transmission medium that has a maximum loop length of 18,000 feet (5.5 km) and is deployed as a single twisted-pair cable. RADSL adapts the data rate dynamically, based on any changes that may be occurring in Glossary RAM (random-access memory) the line conditions and based on the loop length. With RADSL, the rates can vary widely, from 600Kbps to 7Mbps downstream and from 128Kbps to 1Mbps upstream. RAM (random-access memory) A type of computer memory that can be accessed ran- domly (i.e., any byte of memory can be accessed without touching the preceding bytes). Raman amplifier An optical amplifier that uses a powerful laser source to boost the signal power in standard optical fiber. In this type of amplifier, the amplification effect is achieved by a nonlinear interaction between the signal and a pump laser within an optical fiber. RAN (radio access network) Part of a mobile telecommunication system that sits between the mobile phone and the core network. It is possible for a single handset to be simultaneously connected to multiple RANs (e.g., both GSM and UMTS RANs). RARP (Reverse Address Resolution Protocol) The TCP/IP protocol that a diskless machine uses at startup to find its address. The machine broadcasts a request that contains its physical hardware address, and a server responds by sending the machine its IP address. RBOC (regional Bell operating c