We started by looking at hubs and repeaters, which operate at the Physical Layer of the OSI model. A repeater is used to boost the electrical signal to improve the reachability of computers and network segments. The majority of hubs function as multiport repeaters. A hub provides a central junction box for multiple cables connected to computers or nodes. We refer to this class of hub as a simple or stand-alone hub, typically connecting only a few nodes. All devices connected to a hub belong to a single collision domain.

We also examined stackable hubs. Stackable or departmental hubs can be stacked on top of each other for larger network population requirements, typically from 24 to 72 node ports. With most stackable hubs, we also find network management software. Chassis hubs can provide the largest number of ports. They also provide a variety of modules with varying cable connection types, as well as other network components. A large common backplane, shared power supply, and cooling system are all characteristic of chassis/enterprise hubs.

A bridge is a Layer 2 device primarily used to isolate traffic to local LAN segments, and forward remote traffic across a wide area link. Bridges operate on frames and base their decisions on the destination address of a frame. Each segment attached to a bridge is a separate collision domain.

Switches are similar to bridges because they are also Data Link Layer devices that filter and forward traffic based on frame addresses. Each segment attached to a switch is a separate collision domain. However, switches dedicate the entire LAN bandwidth to each port-to-port connection. Because a switch can create multiple connections simultaneously, switches effectively multiply the total bandwidth of a LAN. Switches also operate faster than bridges, because more of their functions are performed in fast hardware, unlike the software-intensive bridge.

A router is a Layer 3 device that operates on packets and the contents of packet headers. Routers are software-intensive devices that are more complex than repeaters, bridges, and switches; however, they provide greater functionality. A router isolates traffic within entire networks or subnetworks. Unlike switches and bridges, a router does not forward broadcast frames. Thus, each segment attached to a router functions as a separate broadcast domain. Routers exchange network path information with each other, and can independently determine the best route to each packet's destination. This automatic forwarding is the basic feature of the Internet.

A gateway is typically the most complex device used in computer networks. Gateways convert entire protocol stacks to provide communication between different networking architectures.

As the functionality of an internetworking device increases, its complexity also increases. The cost of the device, and the time it takes to move information, increase as well. In other words, a repeater is less complex than a router, thus data travels faster through a repeater than through a router. However, a router provides much greater functionality than a repeater, at a correspondingly higher price. The Device and OSI Model Diagram illustrates how devices discussed in this unit relate to the OSI model. Note that a switch also understands bits, and a router also understands bits and frames.

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