2.4 Select the appropriate connectivity devices for various token-ring and Ethernet networks.

Connectivity devices include:
Repeaters
Bridges
Routers
Brouters
Gateways

Repeaters The most basic LAN connection device, repeaters strengthen the physical transmission signal. A repeater simply takes the electrical signals that reach it and then regenerates them to full strength before passing them on. Repeaters generally extend a single network (rather than link two networks).

Bridges are used to connects multiple networks, subnets, or rings into one large logical network. A bridge maintains a table of node addresses. Based on this table, it forwards packets to a specific subnet, reducing traffic on other subnets.

Bridged networks usually are of the spanning tree (Figure 2.1) architecture. This means that can only be one path to any destination (otherwise packets would circle the network and cause network storms). A bridge is more sophisticated than a repeater, but not as sophisticated as a router. Therefore, bridge works at Data Link layer. Routers work at Network layer and Repeaters work at the Physical layer.

Some people may be wondering why a bridge can only use spanning tree structure while a router can form multiple paths. A bridge only works at the Data Link layer. As such, it cannot distinguish one protocol from anther. A bridge can only distinguish between two networks and not between two protocols. The advantage of this feature is that you can support more protocols including non-routable protocols such as NetBEUI and LAT, which cannot be done by router. The disadvantage of this feature is that you cannot have multiple paths. A router has the added advantage of preventing broadcast storm. However a bridge just forwards all the packages not belong to its own segment.

Figure 2.3 Bridge network using Spanning Tree architecture

Because a bridge simply passes the packets from one network to another, you can connect a 10BaseT network with 10Base5 network. You can also connect an Ethernet network to a token ring network by a bridge. Please remember that a bridge just passes the packet. It NEVER does any other processing such as protocol translation.

Router: In the Windows NT environment, router helps LANs and WANs achieve interoperability and connectivity and can link LANs that have different network topologies (such as Ethernet and Token Ring). Routers match packet headers to a LAN segment and choose the best path for the packet, optimizing network performance.

In the Macintosh environment, routers are necessary for computers on different physical networks to communicate with each other. Routers maintain a map of the physical networks on a Macintosh Internet (network) and forward data received from one physical network to other physical networks. Computers running Windows NT Server with Services for Macintosh can act as routers and you can also use third-party routing hardware on a network with Services for Macintosh.

Hub: In a network using a star configuration, each workstation is connected to a special unit called a hub. The hub provides a common connection so that all of the computers can communicate with one another.

Gateway can work on any layer of OSI model. Remember a gateway can be used to connect your network to IBM mainframes. It converts DLC to your network protocols such as TCP/IP.

When you saw GSNW gateway service for NetWare, you should know GSNW is also used for making translation between Microsoft network protocol NetBEUI and IPX/SPX.

Another usage of the gateway is to convert E-mail format.