Any networking technology that gives users access to essential network services from remote locations. Remote access to a company network can be either dial-up access through a modem or dedicated access through a leased line. Remote access typically gives remote users access to the following services on a company network:

• File and print services
• Client/server applications such as database applications
• Applications for remote network administration

A bridge that connects two geographically separated networks by using a telecommunications service such as Plain Old Telephone Service (POTS), leased lines, or a circuit-switched service. A remote bridge has at least one local area network (LAN) port, such as an RJ-45 jack for an unshielded twisted-pair (UTP) LAN connection to a switch or a hub, and at least one serial port, such as an RS-232 port or V.35 interface. The serial port is synchronous for digital lines or asynchronous for modems. The bridge might have both synchronous and asynchronous serial ports. Remote bridges can also be enabled for Simple Network Management Protocol (SNMP) and have other diagnostic and support features such as out-of-band management (OBM) support.

Stands for Radio Guide, a U.S. Army specification for grades of transmission lines. RG specifications refer to forms of coaxial cable used in networking, as in the following examples:

• RG-8:

Also known as N series cable, which is a coaxial cable with an impedance of 52 ohms. RG-8 looks like thicknet Ethernet cabling but is actually a lower grade and does not perform as well. True thicknet cabling is labeled as IEEE 802.3 cabling, has a diameter of 3/8 inches, and is yellow or orange.

• RG-58:

Often called thinnet, which is a form of coaxial cabling with an impedance of 50 ohms and a diameter of 3/16 inches used in 10Base2 Ethernet networking. Subdesignations of this standard include RG-58 /U, which has a solid copper core, and RG-58 A/U, which has a stranded copper core.

• RG-59:

Another name for CATV or cable television cabling, which is a form of coaxial cabling with an impedance of 75 ohms.

• RG-62:

A form of coaxial cabling with an impedance of 93 ohms that is used in ARCNET networks.

A networking topology in which network stations are connected along a single path whose ends are joined to form a circle. The circle might be logical only; the physical arrangement of the cabling might be starlike, with a hub or concentrator at the center. A ring network is based on a ring topology.

The ring topology is commonly used in the following kinds of networks:

• Token ring networks:

The ring of a token ring network is concentrated inside a device called a Multistation Access Unit (MAU).

• Fiber Distributed Data Interface (FDDI) networks:

The ring in this case is both a physical and logical ring and usually runs around a campus or collection of buildings to form a high-speed backbone network.

A family of push-and-click connectors for twisted-pair wiring in telephone and network wiring. RJ stands for Registered Jack. RJ types define both a jack or receptacle (female) and a plug (male) type of connector. The most common types of RJ connectors are as follows:

• RJ-11 connector:

A 4-wire or 6-wire telephone-type connector that connects telephones to wall plates. RJ-11 supports up to six wires, but usually only four are used with the two-pair twisted-pair cabling commonly found in telephone cabling.

• RJ-45 connector:

An 8-wire telephone-type connector used with twisted-pair cabling for connecting computers, wall plates, patch panels, and other networking components. RJ-45 is the standard type of connector for both unshielded twisted-pair (UTP) and shielded twisted-pair (STP) cabling in star-topology Ethernet networks such as 10BaseT and 100BaseT4. RJ-45 is defined in International Organization for Standardization (ISO) standard 8877.

• RJ-48 connector:

An 8-wire telephone-type connector used with twisted-pair cabling for connecting T1 and 56-KB digital data service (DDS) lines. RJ-48 uses the same jack as RJ-45 but uses a different pinning, with one pair of wires to transmit signals, one pair to receive signals, one pair for drain, and one unused pair (reserved for future use). RJ-48 connectors come in three varieties: RJ-48C and RJ-48X for connecting T1 lines, and RJ-48S for connecting 56-KB DDS lines.

Generally, a specific path along which packets can be forwarded on an internetwork by a router. Specifically, a TCP/IP utility for viewing and modifying the internal routing table on a computer running Microsoft Windows 2000, Windows NT, Windows 98, or Windows 95. This internal routing table contains routing information that determines how the computer delivers packets to local and remote hosts on the network. If a multihomed server running Windows 2000 or Windows NT is used, you can use the route command to configure the routing table for static routing. (In Windows 2000, you can also use Routing and Remote Access to configure static routing.)

Example

Typing route print at the command prompt displays the routing table of the local computer. Typing route add 172.16.25.0 mask 255.255.255.0 172.16.10.1 metric 2 adds a new route to the routing table, specifies that any packets destined for the network with network ID 172.16.25.0 should be forwarded to the router interface 172.16.10.1 in the local network, and specifies that packets sent along this route will traverse two hops on the network.

A networking device that is used to extend or segment networks by forwarding packets from one logical network to another. Routers are most often used in large internetworks that use the TCP/IP protocol suite and for connecting TCP/IP hosts and local area networks (LANs) to the Internet using dedicated leased lines.

The process of selecting a path through an internetwork over which to transmit packets to a destination host or hosts and then having devices called routers forward the packets to those hosts. For routing to occur, a routable protocol such as TCP/IP or IPX/SPX must be used.

A routing protocol that is used to exchange routing information between dynamic routers on Internet Protocol (IP) or Internetwork Packet Exchange (IPX) internetworks. Routing Information Protocol (RIP) was designed in 1980 to be used with the Xerox Network Systems (XNS) protocol suite but is most commonly used today in small to mid-sized TCP/IP internetworks. RIP first became popular as a result of its inclusion in release 4.2 of the Berkeley BSD UNIX platform. It is supported by Microsoft Windows NT Server and Microsoft Windows 2000 Server and has been adapted to the AppleTalk networking system as the Routing Table Maintenance Protocol (RTMP).

A variable or factor that a dynamic router can use to calculate its routing table to determine which path or route the router should use to forward a packet.

How It Works

Routing metrics enable routers to make intelligent decisions about how to forward packets to ensure that

• Packets are delivered efficiently and quickly
• Congestion does not occur over links between networks
• Packets are not lost by being dropped by overloaded or dead routers

The simplest metric used by routers to calculate routing table entries is the number of hops to a given destination network. For example, this metric is used by the Routing Information Protocol (RIP), which allows dynamic routers to communicate with each other to share routing information and synchronize the entries of their routing tables. If you need more control over the paths that packets take, you can use protocols such as Open Shortest Path First (OSPF) Protocol and Interior Gateway Protocol (IGP), which can use a number of other metrics, including real-time metrics that routers determine dynamically, such as the following:

• Load:

Generally, the number of packets being processed per second by the router or its CPU utilization. If the load on a router becomes high, the router can advise other routers to recalculate routing tables in order to divert traffic around it.

• Latency:

The time interval needed to route a packet through the router or over a specific path through the internetwork. Latency can be increased by delays due to such factors as port congestion on the router, heavy router load, bandwidth utilization of links between networks, and physical distance between networks.

Other routing metrics are manually entered into the router configuration by network administrators who have a knowledge of the physical layout and performance of the network. Such metrics can include the following:

• Bandwidth:

The total capacity of each network link to carry traffic between different networks in the internetwork.

• Reliability:

The relative amount of anticipated downtime for a given link between two networks.

• Cost:

A parameter roughly proportional to the actual cost in dollars of using each network link. Some wide area network (WAN) links might have more latency but cost much less.

• Maximum Transmission Unit (MTU):

The largest size of packet that the router can forward without segmenting the packet into subpackets. Segmentation of network traffic by routers adds additional latency to network communication.

Routing protocol

A protocol that enables the exchange of routing tables between routers in an internetwork. Routing protocols are the software implementation of specific routing algorithms, which are mathematical procedures for determining the cost of various paths or routes through an internetwork so that traffic can be routed most efficiently.

Routing protocols are generally implemented in medium to large-sized TCP/IP internetworks to simplify the administration of routing tables. Common routing protocols include the following:

• Routing Information Protocol (RIP):

Based on the distance vector routing algorithm and used in small to medium-sized internetworks. RIP is an intradomain routing protocol that can function only within a given routing domain. Microsoft Windows NT Server and Microsoft Windows 2000 Server support RIP; a multihomed machine running Windows NT or Windows 2000 can be used as a RIP router.

• Interior Gateway Protocol (IGP):

Based on the distance vector routing algorithm and used in medium-sized to large-sized internetworks. IGP is an intradomain routing protocol that can function only within a given routing domain. IGP uses a number of metrics to determine routing cost, including load, bandwidth, latency, reliability, and Maximum Transmission Unit (MTU). The router determines some of these factors dynamically as it inspects incoming traffic, while others are specified by the network administrator. IGP supports multipath routing for load balancing and fault tolerance.

• Open Shortest Path First (OSPF) Protocol:

Based on the link state routing algorithm and used in medium-sized to large-sized internetworks. OSPF is an intradomain routing protocol that can function only within a given routing domain. OSPF is also a hierarchical routing protocol that can be used within a single autonomous system. OSPF evolved from the earlier Open Systems Interconnection (OSI) reference model routing protocol called intermediate-system-to-intermediate-system (IS–IS). OSPF supports multipath routing and uses one or more routing metrics, including bandwidth, reliability, load, latency, and MTU. If OSPF uses more than one metric, it also supports type-of-service (TOS) requests for differentiating traffic.

• Exterior Gateway Protocol (EGP):

An interdomain routing protocol for routing between different routing domains that are connected by a routing backbone such as the Internet. EGP was specifically designed in 1984 as the protocol for communication between the core or backbone routers of the Internet. EGP does not use routing metrics—it simply keeps track of which networks are currently reachable through a given router.

• Border Gateway Protocol (BGP):

Another interdomain routing protocol created specifically to enable the core or backbone routers of the Internet to communicate with each other. BGP is superior to EGP because it can detect routing loops and use routing metrics.

• NetWare Link Services Protocol (NLSP):

Used in Novell NetWare 4.x as part of its Multi-Protocol Router (MPR). NLSP is based on a combination of OSPF routing and Novell’s Service Advertising Protocol (SAP) functions and is also based on the link state routing algorithm.

• Routing Table Maintenance Protocol (RTMP):

Used in AppleTalk networks and based on the distance vector routing algorithm. RTMP is derived from RIP.

An internal table that a computer or router uses to determine which router interface to send packets to, based on their destination network addresses. Microsoft Windows platforms automatically build their own routing tables, which are used to determine whether to forward specific packets to

• The local network segment
• A near-side router interface
• The default gateway for the segment

To view the internal TCP/IP routing table on a computer running Windows 2000, Windows NT, Windows 98, or Windows 95, type route print at the command prompt.

A typical routing table looks like the following:

Active Routes:

Network Address Netmask Gateway Address Interface Metric

127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1

172.16.8.0 255.255.255.0 172.16.8.50 172.16.8.50 1

172.16.8.50 255.255.255.255 127.0.0.1 127.0.0.1 1

172.16.255.255 255.255.255.255 172.16.8.50 172.16.8.50 1

224.0.0.0 224.0.0.0 172.16.8.50 172.16.8.50 1

255.255.255.255 255.255.255.255 172.16.8.50 172.16.8.50 1

This computer has a single network interface card (NIC) with the address 172.16.8.50. The columns of this table are as follows:

• Network Address:

A destination network address on the network

• Netmask:

The portion of the network address that must match in order for that route to be used

• Gateway Address:

Where the packet needs to be forwarded (a local NIC or a local router interface)

• Interface:

The address of the NIC through which the packet should be sent

• Metric:

The number of hops to the destination network

More properly known as RS-232C, a widely implemented serial transmission interface developed by the Electronic Industries Alliance (EIA) that is used for connecting data terminal equipment (DTE) such as computers or terminals to data communications equipment (DCE) such as modems, packet assembler/disassemblers (PADs), or serial printers. The RS-232 specification uses unbalanced lines to provide full-duplex serial communication using baseband transmission. RS-232 provides a typical data rate of 19.2 Kbps over a maximum distance of 15 meters, but the maximum data transfer rate is 115.2 Kbps. Up to two devices can be connected using RS-232. Devices cannot be daisy-chained together using RS-232. (Use RS-422/423 instead.)

RS-232 specifies the types of wires and connectors, the pinning of the connectors and the function of each wire, the voltage levels and their meanings, and control procedures such as handshaking. RS-232 cables (cables designed to use the RS-232 serial interface specification) are typically 25-wire unshielded twisted-pair (UTP) cables with DB25 type connectors or 9-wire cables with DB9 connectors. The pin assignments are shown in the following table. Note that only pins 1 through 8 and pin 20 are required for most basic RS-232 functions, which means that 9-pin DB9 connectors can be used on RS-232 serial cables for most applications.

Pin Assignments of RS-232

A high-speed full-duplex serial interface that uses balanced lines and has more immunity from noise than the RS-232 interface. (An unbalanced version called RS-423 is less frequently implemented.) RS-422 is used in industrial environments with a lot of electromagnetic interference (EMI) or where more than two serial devices need to be chained together. It is typically used for high-speed synchronous communication between data terminal equipment (DTE) and multiple daisy-chained data communications equipment (DCE).

RS-422 was originally designed to supersede RS-232, but they now coexist. RS-422 typically transmits data at rates of 230 Kbps, but the speed can be increased to around 1 Mbps. The maximum distance for an RS-422 connection is typically 300 meters. RS-422 cables typically have 25 wires and use DB37 or DB9 connectors.

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