Network+ Online Course
Lesson 2 - Communication Standards
C. Internetworking devices
Internetworking devices
- Illustrates layer functionality
- Repeaters
- Bridges
- Routers
- Gateways
Internetworking devices
Internetworking devices are a good example of how the functions assigned to the different layers are used by various devices.
Figure 17: Internetworking devices and the OSI Reference model
Repeaters and the Physical layer
Repeaters are devices that extend the cable distance on a network. This is useful if the cable isn’t long enough to reach a remote device or if the signal has deteriorated by the time it reaches it. The repeater regenerates the signal by making a copy of the original. To do so, it only takes into account the electrical characteristics of the signal and makes no attempt to understand the meaning. Because of this, it operates at the Physical layer of the OSI model and is ignorant of all upper layers. As long as the electrical signal is the same, a repeater can be used. For example, under Ethernet, a repeater can be used to connect coaxial cable to unshielded twisted pair because the signal is the same, even if the cable isn’t. On the other hand, a repeater can not be used to connect an Ethernet to a Token Ring because the electrical signal is different.Bridges and the Data Link layer
Bridges are used to connect two segments of the same network together. The chief function of bridges is to increase network performance by segmenting the network and keeping traffic local wherever possible. Bridges connect LANs with compatible protocols at the Data Link layer. The bridge depends on the Data Link layer for the MAC (Media Access Control layer) address which it uses to determine if a frame should be forwarded to the next segment. The MAC address is typically the unique number given to a network adapter when it is manufactured.Normally a bridge has the same protocol on both sides, for example Ethernet to Ethernet. It is possible, however, to create a bridge with different protocols on either side, for example, Ethernet to ISDN. Because the connection is just under the layers which define transportation protocols (such as XNS, IPX/SPX, TCP/IP), bridges are protocol insensitive.
Routers and the Network layer
Routers connect different networks together. In a similar fashion to bridges, they examine the packets on the network and decide if they should be forwarded to the remote network. For this they depend on the network address which is assigned to the computer. The network address is a function of the network protocol. For example, the machine might have an IP or IPX address, depending on which it was using. Routers connect networks which use the same network protocol. For these reasons, then, routers operate at the Network layer.Gateways and any layer
Gateways provide a communication path between two networks using different LAN types and protocols. They perform protocol conversion for all seven layers of the OSI model. Gateways are application-specific. Here are some typical uses for gateways.- Connecting totally different kinds of systems — a gateway can connect a PC LAN and an SNA mainframe, for example.
- Connecting different kinds of electronic mail systems — an X.400 mail system to ccMail or Microsoft mail.
- Converting protocols — converting between IP and IPX for example. A router does not perform this conversion, it connects two networks with the same protocol!
- Voice systems — transmitting a telephone conversation between a digital Voice over IP system and the analog public telephone system requires a voice gateway.
Figure 18: Repeaters, bridges, routers and gateways
Exercise 2-2: Internetworking devices
Answer the following questions
A. A large university has seventeen buildings on its campus and cable runs between all of them. In fact, each building is directly connected to at least three, if not more, other buildings. Each building has several local area networks and they all use the same communications protocol. Which device can be used for efficient communication between all of them?B. A large organization has a mainframe computer. It has replaced its dumb terminals with workstations on a local area network. The employees still need to access information on the mainframe, even from their personal computers. What device does the organization need to enable this?
C. A LAN administrator is finding that the performance on his network is suffering because he added too many new workstations. He decides to alleviate the problem by dividing the network into two equal segments. The network was Ethernet before and each segment will continue to be Ethernet. What device does he need in order that each computer on one segment can continue to communicate with every other computer on the other segment?D. The cable installer was pulling cable to connect the computer at the back of the warehouse. Some unexpected obstacles forced him to use more cable than intended and the extra cable length exceeded the specification allowed. What device will help him resolve this problem?
E. A small business uses Groupwise for its internal mail. The employees are now demanding access to standard Internet mail as well. What device would allow Groupwise to exchange mail with standard Internet mail servers?Exercise 2-3: Networking devices and the OSI model
On the following diagram of the OSI model, write in the device name at the correct layer(s). Some devices are listed that have not been discussed yet. Click on them in order to research the device.- Repeater
- Hub
- Bridge
- Network Interface Card (NIC)
- Switch
- Wireless Access Point (WAP)
- Router
- Gateway
| OSI Seven Layer Model | |
|---|---|
| Application | . |
| Presentation | . |
| Session | . |
| Transport | . |
| Network | . |
| Data Link | . |
| Physical | . |
[Top of page][On to next section]
