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Backbone The part of your network that ties different departmental networks into a single whole. The backbone carries the bulk of the network traffic and must be designed accordingly. Backbone router A router that is used to connect autonomous systems in a large internetwork such as the Internet. Autonomous systems are large portions of an internetwork that fall under the administration of a single authority. In an autonomous system, routers exchange information with each other using routing protocols called Interior Gateway Protocols (IGPs), such as the Routing Information Protocol (RIP) and the Open Shortest Path First (OSPF) Protocol. Backbone routers are then used to connect the various autonomous systems into a single internetwork. Bandwidth on demandAny telecommunications technology that provides both a permanent, dedicated connection and the capability of quickly increasing bandwidth when needed by users. Many telecommunications devices incorporate bandwidth-on-demand features of various types. For example, some Integrated Services Digital Network (ISDN) devices used for Basic Rate Interface ISDN (BRI-ISDN) can be configured to use the second ISDN B channel only when the utilization of the first channel exceeds a certain threshold. If this threshold is exceeded for a specified period of time, the second B channel automatically opens up to facilitate and speed data transfer. Once the data rate has dropped below the threshold, the second B channel shuts down until it is needed again. The ISDN technology for accomplishing this combining of channels is called bonding. Many Asynchronous Transfer Mode (ATM) products also support various bandwidth-on-demand features. Bandwidth-on-demand technologies are typically used in bursty networking situations in which high transmission speeds and capacities are required for transporting video, voice, and data on common networking circuits. Bandwidth-on-demand configurations often involve a mix of leased-line services and circuit-switched telecommunications services, and they can save users money by opening additional circuits only on an as-needed basis. Networks that make use of bandwidth on demand can be designed to supply additional bandwidth under conditions such as
A standard form of communication on Integrated Services Digital Network (ISDN) communication systems. Basic Rate Interface ISDN (BRI-ISDN) connections consist of two B channels and one D channel. The B channels carry the voice or data between the customer premises and the telco’s central office (CO), while the D channel is used for establishing connections and signaling. BRI-ISDN is often referred to as 2B+D because of the channels that it uses. The bandwidth of each B channel is 64 Kbps, so the total bandwidth of BRI-ISDN is twice that, 128 Kbps. This bandwidth can be used as two separate communication links of 64 Kbps each, or it can be combined using bonding into a single 128-Kbps communication link. The bandwidth of the D channel is 16 Kbps. BRI-ISDN connections at customer premises can be connected directly to a switch at the telco’s central office, an ISDN call controller that is linked to the CO, an ISDN Private Branch Exchange (PBX), or some other signaling and communication equipment. Bits per second (bps) A unit used for measuring the speed of transmission of data on a network of computers—that is, the amount of information sent or received in a given amount of time. A bit is a single unit of digital information, represented by either a 1 or a 0. The total number of bits per second (bps) that can be transmitted over a network link represents the bandwidth of that link. Because most network communication takes place at thousands or millions of bits per second, the following related units are commonly used:
Black hole In an internetwork, a black hole is a router that goes down and whose absence from the network is not detected by other routers. Packets that are forwarded to the black hole are dropped; they never reach their destination, and they give no indication to the stations sending them that this is the case—therefore, the data is lost. Normally, a router will issue an "ICMP Destination Unreachable" message when it cannot forward a packet. However, with a black hole, these messages are not generated, so the user doesn’t know that data is being permanently lost. If static routing is being used, a black hole persists until the affected router is brought back on line or until other static routers have their routing tables reconfigured to take the downed router into account. If dynamic routing is used, other routers soon detect the presence of the black hole and adjust their routing tables accordingly to favor other paths. Brouter Any network device having the capabilities of both a bridge and a router. Usually, a brouter will act as a router for one protocol (for example, TCP/IP) and a bridge for all other protocols (for example, IPX/SPX). Brouters are not common in networks. Network services often send their announcements over every protocol on the network, which generates additional traffic and makes it generally disadvantageous to run more than one protocol on a single network. The solution adopted by most implementers today is to use a single protocol for all network communication on the main portion of the network, with gateways connecting to segments running other protocols. The protocol of choice for most internetworking today is TCP/IP.
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