The repeater, bridge, router and gateway are all pieces of network equipment that work at various levels of the OSI model performing different tasks. The repeater network device exists in the physical layer of the OSI model and is the cheapest of all the mentioned devices. A repeater can be thought of as a line extender as connections on mediums such as 10BaseT and 100BaseT become weak beyond distances of 100 meters. The repeater receives a signal in an analog environment and replicates it to form a signal that matches the old one. In a digital environment the repeater receives the signal and regenerates it. Using a repeater in a digital network can create strong connections between the two connecting joins since any distortion or attenuation is removed. Unlike routers repeaters are restricted to linking identical network topology segments ie a token-ring to a token ring segment. Repeaters amplify whatever comes in and extends the network length on one port and sends out to all other ports (there is no calculation to find the best path to forward packets). This means that only one network connection can be active at a time.
A bridge is an older way of connecting two local area networks or two segments (subnets) of the same data link layer. A bridge is more powerful than a repeater as it operates on the second layer (data link) of the OSI network model. Messages are sent out to every address on the network and accepted by all nodes. The bridge learns which addresses are on which network and develops a routing or forwarding table so that subsequent messages can be forwarded to the right network. There are two types of bridge devices; a transparent hub bridge and a translating bridge. A translating bridge will connect two local area networks (LAN) that use different data link protocols. By translating the data into the appropriate protocol ie from token ring to Ethernet network. A transparent hub bridge will perform the same functions as a translating but will only connect two LANs that use the same data link protocol.
Routers are used in the majority of home networks today and are placed at the gateways of networks. They are used to connect two LAN’s together (such as two departments) or to connect a LAN to an internet service provider (ISP). Routers use headers and forwarding tables like a bridge to determine the best path for forwarding the packets. Routers are more complex than bridges and use protocols such as internet control message protocol (ICMP) to communicate with each other and to calculate the best route between two nodes. A router differs as it ignores frames that are not addressed to the router and use algorithms and protocols that allow them to send packets to the best possible path. A router operates at the third OSI layer (network layer) and can be dynamic or static. Once a static routing table is constructed paths do not change. If a link or connection is lost the router will issue an alarm but will not be able to change the path of traffic automatically unlike dynamic routing. Routers are slower than bridges but routers are more powerful as they can split and reassemble frames receiving them out of order also they can choose the best possible route for transmission, these extra features make routers more expensive than bridges.
Gateways connect networks with different architectures by performing protocol conversion at the application level. Gateway is the most complex device operating at all seven layers of the OSI model. Gateways are used to connect LAN’s to mainframes or connect a LAN to a wide area network (WAN) Gateways can provide the following things:
Connect networks with different protocols
Terminal emulation so workstation can emulate dumb terminals (have all computer logic on a server machine)
Provide error detection on transmitted data monitoring traffic flow.
File sharing and peer to peer communications between LAN and host.
Reference:
Carr, H. H. & Synder, C. A. (2007) Data Communications & network security. United States of America: McGraw-Hill/Irwin pg 124-129
Dennis, A. (2002). Networking In The Internet Age Application Architectures. United States of America: John Wiley and Sons, Inc
Dostálek, L., & Kabelová. A. (2006). Understanding TCP/IP. Retrieved August 6, 2006 from http://www.windowsnetworking.com/articles_tutorials/Understanding-TCPIP-Chapter1-Introduction-Network-Protocols.html