cular level of service. To meet these guarantees, service providers or IT staff members allocate bandwidth for certain types of traffic. Table 2-2 LAN Protocols, Devices, and Terms Protocol, Service, or Device Description Backbone The wiring running from floor to floor in single buildings and from building to building within a campus. A backbone connects switches in different wiring closets to one another. Backbones support high concentrations of traffic in carrier and enterprise networks. Blade server Blade servers are computers packaged on individual boards-blades-that plug into slots in a chassis. A chas- sis is the metal frame on which components fit, similar to a custom cabinet with slots for specific items. In high density, chassis blades are arranged vertically. In low density, three- or four-blade servers, they are arranged horizontally. Chassis are placed on racks in data centers. Vertical arrangements conserve space and can share power supplies, fans, and cabling. In addition, blades can be easily added. Bridge Bridges originally connected LANs using the same type of protocol. They had limited intelligence and generally only connected a few LANs together. Bridges are now used in LAN switches that support VLANs. The bridge connects devices (PCs, VoIP phones) together into VLANs for special treatment needed for services such as VoIP and conferencing services. Appendix Protocol, Service, or Device Description File server A specialized computer with a large hard drive. File servers provide users with access to documents and applications in central locations in LANs. The wiring center to which all devices, printers, scan- ners, PCs, and SO forth were formerly connected to within a segment of a LAN. Hubs enable LANs to be connected to twisted-pair cabling instead of coaxial cable. Only one device at a time can transmit via a hub. Higher-speed switches have replaced hubs in wiring closets. Load balancing The capability of equipment to balance traffic between networks and devices SO that one network or device i