Repeater
A repeater is an electronic device that operates on onlv the physical layer of the OSI model. A repeater connects two or more cable segments and retransmits any incoming signal to all other segments. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data.
For example, the maximum distance that a signal can travel on an Ethernet cable segment is 500 metres, but one repeater can double the effective length of an Ethernet to 1,000 metres.
Repeaters are not capable of connecting two dissimilar network technologies. They operate at the physical layer and are used to generate or replicate signals. They do nothing to change the data.
  Hub
A hub is a central network device that connects network nodes, such as workstations an   servers in a star topology. A hub may a so be referred to as a concentrator, an is a device t at can    have multiple inputs and outputs. all active at one time. Hubs can:  Provide a central unit from which to connect multiple nodes into one network.Permit large numbers of computers to be connected on single or multiple LANs.
Reduce network congestion by centralizing network design.
Provide multi-protocol services, such as Ethernet-to-FDDI connectivity. Consolidate the network backbone.
Enable high speed communication.
Provide connections for several different media types (e.g. coax, twisted pair, fibre).
Enable centralized network management.
Hubs are sometimes called multistation access unit (MAU).
A multistation access unit (MAU) acts as a central hub on a token ring network. Used exclusively on token ring networks, MAUs can perform the following tasks:
Connect workstations into a logical ring through a physical star topology. Move the token and frames around the ring. Amplify data signals. Connect in a daisy-chained manner to expand a token ring network. Provide for orderly movement of data. All token ring network devices connect to the network through an MAU, typically using type l, 2 (both shielded) or 3 (unshielded) twisted pair cable. The MAU passes frames from one node to the next using a physical star topology, but transports frames as though they were going around a ring. Operating as a central hub, an MAU functions at the OSI physical and data link layers.

There are different kinds of hubs. A passive hub acts as a pathway, allowing data to flow from one device on a segment to another. It simply resends a signal without regenerating it. Hubs can also serve as repeaters to boost the signal strength, thereby allowing longer cable runs out to individual workstations or servers. These - are considered active hubs, because they regenerate and process signals. An intelligent hub can detect errors and can provide assistance to a technician when attempting to locate a failing component, such as a cable with a high error rate, a cut cable or a failing workstation. Most simple hubs rebroadcast every packet to every port, through one special type of hub, called a switching hub, which reads the address of the destination for each packet and forwards it to the proper port.
  Bridge
A bridge is a network device that connects one LAN segment to another. Bridges increase efficiency an security beyond what repeater's are able to do. They operate at fayer-2-the data layer-of the OSI model and perform error detection, frame formatting; and frame routing. they have a variety of applications. such as extending a LAN to longer distances and to a greater number of ports. Bridges are used to:
Extend a LAN when the maximum connection limit, such as the 30-node limit on an Ethernet segment, has been reached. Extend a LAN beyond the length limit, for example beyond 185 metres with thin-net Ethernet.
Segment LANs to reduce data traffic bottlenecks. Prevent unauthorized access to a LAN.
Bridges operate at the MAC sub-layer of the OSI data-link layer. A bridge intercepts all network traffic and reads the destination address on each frame to determine if the frame should be forwarded to the next network.
If the bridge knows that the destination of a frame is on the segment as the source of the frame, it drops the frame because there is no need to forward it. If the bridge knows that the destination is on another segment. it transmits the frame to that segment only. If the bridge does not know the destination segment, the bridge transmits the frame to all segments except the source segments, a process that is called flooding.
Bridges can greatly enhance the performance of a network because they offer the ability to segment network traffic, limiting traffic to those networks where it belongs. This is in sharp contrast to repeaters, which retransmit all frames onto all connected segments. Another advantage of bridge is that it can be used as a firewall to keep intruders out of a network. A firewall is software or hardware that sources data from being accessed outside a network and that can also prevent data from leaving the network through an inside source.
  Router
A router performs some of the same bridge as a bridge such as learning, filtering and forwarding. Routers connect LAN's at the network layer o the OSI model, which enables them to interpret more information from packet traffic than bridges can. A router directs a packet to a specific network rather than unnecessarily- broadcasting that packet to all connected networks. In general. routers are used to:  Efficiently direct packets from one network to another, reducing excessive traffic.Join neighbouring or distant networks. Connect dissimilar networks.
Prevent network bottlenecks by isolating portions of a network. Secure portions of a network from intruders.
A router performs two basic activities: determining the optimal routing paths and transporting data through the network. The optimal path may be measured in several different ways, such as the fewest number of links to the destination, the least cost, or the optimum speed of the circuits along the way. The router uses a packet's destination address and routing table stored in its memory to determine how to forward the packet. Router can keep track of several possible routes to a destination and then can forward the packet along an alternate path if the primary route is busy or is out of service.
Routers communicate with one another and maintain their routing tables with the latest information about the status of the network. By analyzing updates froul other routers, a router can maintain an up-to-date picture of the network topology. The logic that routers use to determine how to forward data is called a routing algorithm.
  Brouter
A bridge router (brouter) performs both the functions of a bridge (OSI layer 2) and a router (OSI layer 3) in a single device. A bridge connects LANs that use the same protocol and ordinarily offers only one path to an interconnected LAN. A router connects one network to any number of others and is usually part of a WAN. It can offer different routes to a destination, and therefore it needs more information about destinations on the networks.
A brouter is a network device that acts as a bridge in one circumstance and as a router in another. Brouters are used to: Handle packets efficiently on a multiprotocol network that includes some protocols that can be routed and some that cannot be. Isolate and direct network traffic to reduce congestion. Join networks.
Secure a certain portion of a network by controlling who can access it.
A brouter is used on networks that operate with several different protocols, such as NetBEUI, IPX/SPX and TCP/IP and for this reason it is also called a multi-protocol router.
  Switches
A switch is a device that connects two or more network segments and allows different nodes to commumcate e smoothly with each other as they are the only two con
at t e time. unlike a hub, which rebroadcasts from all ports to all devices on a netwok a switch makes a direct connection between the transmitting transmitting device and the receiving divice.  Switches provide bridging capacity along with the ability to increase the bandwidth on existing networks  used on LAN are similar to bridges„ in that  operate at the data-link MAC sublayer (layer 2) to examine device addresses of all incoming traffic. Traditionally; a switch is a device that provides bridging functionality with greater efficiency.
A switch may act as a multiport bridge to connect devices or segments in a LAN. The switch normally has a buffer for each link (network) to which it is connected. When it receives a packet, it stores the packet in the buffer of the receiving link and checks the address (and sometimes CRC) to find the outgoing link. If the outgoing link is free (no chance of collision), the switch sends the frame to that particular link. Switches are made based on two different strategies (or fabrics): store-and-forward and cut-through. A store-and-forward switch stores the frame in the input buffer until the whole packet has arrived. A cut-through switch, on the other hand. forwards the packet to the output buffer as soon as the destination address is received.

Gateways
If two networks operate according to different network protocols. a gateway is used to connect them. Gateways usually operate at OSI layer  or higher. and basically translate the protocols to allow terminals s on two dissimilar  networks to communicate. some   gateway s a so translate data codes for example. from ASCII to EBCDIC Code.
This capability would  be useful on a LAN when a communication server routes traffic from a PC-based network using ASCII to an IBM mainframe that uses the EBCDIC Code. Gateways can be either/or combinations of hardware and software. They may be implemented an a specially designed circuit card or by using  specialized software in a standard PC. An Internet service provider (ISP). which connects users in _a home to the _ Internet, is a gateway. The computer routing traffic in an organization from individual workstations to an outside network's web server is a gateway.
Gateways can suffer from slow performance because of protocol translation, so their performance must be considered and tested when a gateway installation is contemplated. A dedicated computer acting as a gateway. if it is of reasonable speed, usually eliminates any performance problems.
Gateways perform an important role in allowing an organization to interconnect different types of LA s, so t at, the network appears as a single entity to the user. Figure 6.2 illustrates the difference between gateways and bridges.
The term gateway is used in many contexts. but in general it refers to a software or hardware interface that enables two different types of networked systems or software to communicate. For example, you might use a gateway to:
            Convert commonly used protocols (e.g. TCPIIP) to a specialized protocol (for example, an SNA: Systems Network Architecture).
       Convert message formats from one format to another. Translate different addressing schemes.
Link a host computer to a LAN.
      Provide terminal emulation for connections to a host computer. Direct electronic mail to the right network destination. Connect networks with different architectures.

There is a wide range of applications for gateways. They can function at any of the OSI layers. The most traditional type of gateway is a network device that translates one type of protocol to another, that has a very different structural composition. This type of gateway operates at the network layer of the OSI model. One of the best examples of this is a gateway that translates IBM's Systems Network Architecture (SNA) protocol for mainframe communications to another protocol, such as TCP/IP. The problem with this type of gateway is that it is relatively slow compared to other solutions, and, as a result, the use of these traditional gateways is becoming more infrequent. In a nutshell, Table 6.1 compares the hardware (devices or interfaces) used to connect LANs.

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