Electromagnetic wave that is radiated by an antenna and produced by alternating currents that are fed to the antenna.
Also see electromagnetic and RF.

Remote Authentication Dial In User Service. A standard client-server protocol to control users access to the Internet.
A RADIUS server at an ISP handles authentication, authorization, and accounting of its subscribers accounts. It has a database of user profiles that is shared with remote access servers. Therefore, only registered and paying subscribers are granted access to the Internet.
Also see AAA.

Rate Adaptive Digital Subscriber Line. DSL technology that adjusts bit rate over time to signal quality.
Also see DSL.

Remote Access Server. A server usually located near subscribers sites that handles subscribers access to the Internet through an ISP.
Also see ISP and PoP.

A network device which receives, amplifies and re-transmits  signals. A.k.a. regenerator. Repeater works at Layer 1 of the OSI model.
Also see OSI model.

A device which connects a home network to the Internet. It lets all computers or devices in the home network share one Internet subscription/connection.
Residential gateway main function is a router but in practice it usually includes other functions such as switch, bridge, access point, and broadband modem.

Part of a computer or a network (e.g. memory, storage, port) that is allocated for a running program.

Radio Frequency. Refers to frequencies of radio waves, i.e. from 3 Hz to 300 GHz in the electromagnetic spectrum.
Bluetooth, Wi-Fi, and ZigBee are examples of standard RF technologies. Many proprietary RF technologies are also used by organizations, corporations, governments, and military forces worldwide.

Request For Comments. A standard document that is used to exchange information in the IETF about a specific protocol.

Reduced Function Device. A ZigBee device with limited functionality and can not function as a master (controller) in a network.
Also see ZigBee. Contrast with FFD.

Radio Frequency Identification. The identification of an object using radio waves (RF). RFID system consists of a reader and a tag that is attached to every object. The reader interrogates the tag using radio waves, therefore no need for line-of-sight (LOS) between the reader and the tag. The code that is read from the tag is then decoded and sent to a computer for further processing.
RFID is viewed as the next generation barcode. With RFID, products in shop shelves or in warehouse can be controlled and monitored through a network. The manufacturer and the shop owner can track the product stock real time and they can even collect customer preferences.
Also see RF.

Ring Topology is a network topology in which each node is connected to two other nodes by point-to-point links in a closed loop (ring). In ring topology, signal that is transmitted by a sending node travels in one direction around the ring, from node to node. Each node should recognize its own address and other nodes' addresses to process the signal or retransmit it to the next node.
Token Ring network is implemented using ring topology.
See picture. Also see bus, star, and mesh topologies.

Routing Information Protocol. A routing protocol that determines the best route based on the smallest number of hops between routers. As an interior gateway protocol, RIP is used within an autonomous system.
Also see IGP, routing protocol. Compare with OSPF.

Registered Jack-11. The connector that is used to terminate standard telephone cable. RJ-11 is a modular connector with four pins.

Registered Jack-45. The connector that is used to terminate Ethernet cable (i.e. Cat 5, Cat 5e, or Cat 6). RJ-45 is a modular connector that has eight pins. However, only 4 pins (pin 1, 2, 3, 6) are used for transmit and receive. They are as follows:
Pin 1 : Tx + (Transmit +)
Pin 2 : Tx -  (Transmit -)
Pin 3 : Rx + (Receive +)
Pin 6 : Rx -  (Receive -)
To identify pin number, look at an RJ-45 connector at the end of a Cat 5 cable with the tab on the bottom and the pins on the top, pin 8 is the rightmost and pin 1 is the leftmost.
See picture of RJ-45 and RJ-11 jacks. Also see Cat 5.

A wireless device ability to work continuously without losing connection when moving from one access point (base station) coverage to another.

A network component that routes a packet to its destination address across different networks. Router operates at Layer 3 of the OSI model and can connect networks with different Layer 1 and Layer 2 protocols.
Router forwards a packet to its destination by looking at a routing table. The routing table contains IP addresses of other networks routers. It doesn't contain nodes (computers) IP addresses of other networks. Router determines the best path to deliver the packet based on the routing algorithm it uses.
In a static router the routing table is configured manually, while a dynamic router can communicate with other routers and configure the routing table according to the information it receives from other routers.

An algorithm that is used by a router to communicate with other routers and determine the best path to forward a packet.
Also see IGP, EGP, BGP, OSPF, RIP.

Resource Reservation Protocol. A protocol that administers reservation request.

Real-time Transport Control Protocol. A companion protocol to RTP which provides feedback on the quality of the data transported using RTP.

Real-time Transport Protocol. A standard protocol for end-to-end delivery of real-time data, such as interactive speech, audio and video over packet-switched network. Using RTP, arriving real-time data packets can be assembled in correct order.
RTP is used in VoIP and video streaming over Internet.

Request To Send. The message that is sent by the DTE to indicate that it is ready to transmit and that the DCE must begin to prepare to receive data.
Also see DCE and DTE. Contrast with CTS.

The EIA specification for serial data communication interface.
Also see serial port and null modem cable.

Notation for Receiver.