1.7 Define the communication devices that communicate at each level of the OSI model.
OSI model has 7 layers from the lowest layer (the physical connections) to the highest (user's applications). Each layer is able to communicate with the layer immediately above it or the layer immediately below it.
Application layer provides the interface to the applications related network services. This layer offers the services directly support network related applications such as E-mail, SQL database etc. The main purpose of this layer is defining the protocols to be used between the application programs. Therefore, the applications such as SMTP, FTP and rlogin also operate at the application layer.
Presentation Layer is used for type conversion. For example, you convert the data from one character set to another, compress the data to reduce network traffic.
Sun's XDR (External Data Representation) and ISO's ANS are two protocols corresponding to this layer. XDR is used by some RPC (Remote procedure call) between different computers such as a PC and a SUN workstation for data conversion. If you know the meaning of R- Representation in XDR, you should remember XDR works at presentation layer.
For Networking Essentials exam, you need to remember two things: The first is the terms related to presentation layer such as XDR and ANS, and the second is for type conversion. It is easy to understand that converting the document from Chinese character set (8bit) to English (of course, this is a nonsense conversion, because after that, all the document is unreadable and meaningless) works on presentation layer. However, it is somewhat difficult to understand that data compression also works on presentation layer. But when you think of compression as one kind of type conversionconvert the non-compression format to compression format, you will know that data compression is also works at presentation layer.
Session Layer establishes and disconnects a session. Moreover, it also ensures the communications is accomplished in an orderly manner, making sure the previous request has been fulfilled before the next one is sent.
Session layer places checkpoints in the data stream to synchronize the user tasks. Some people may have no idea about checkpoint. In fact, the system checks successful delivery after a time interval. If at that time the system has no problem transmitting it marks Success on the checkpoint, otherwise, it marks failure. Checkpoint provides an efficient way to only retransmit the data after the failure checkpoint, which can reduce the network traffic and improve the network performance.
Transportation layer makes sure the entire file or message is received. It checks the error package and offers recovery function. In some cases, it repackages messages into small packets.
Network layer translates logical addresses and names into physical addresses. It also determines how to send package from the source address to the destination address (In fact, this is the function performed by a router) and offers some control on traffic.
Routable protocols such as TCP/IP and IPX/SPX can be divided into those that operate at the transportation layer and those that operate at the network layer. TCP operates at Transport layer while IP operates at the network layer.
SPX operates at the transportation layer while IPX operates at the network layer. If you know IP and IPX are used to route the packages, it will enhance your understanding why IP and IPX work at network layer. If you dont know that, just remember the above conclusion.
NetBEUI operates at both the transportation layer and the network layer. NetBEUI is generally referred as a Transportation layer protocol.
Data Link layer establishes the link between two computers, sending data to one computer and waiting for the acknowledgment from the receiving computer. It divides the transmitted bits into frame for the communication.
Physical layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable. It is responsible for the bits transmitted between two computers. However, it doesn't understand any meaning of the bits, but deals with the electrical and mechanical characteristics of the signals.
When one computer communicates with another computer, the software at each layer on one computer assumes it is communicating with the same layer on the other computer. If we install two network communication software in these two computers, one computer pass its information from its highest level to lowest level, then goes to other computer, from the lowest level to the highest level.
Repeater is a network device that strengthens the physical transmission signal. It regenerates the signals to full strength before these signals passing them on. Repeater works at the physical layer.
Bridge is a network device that connects two LAN segments together, which may be of similar or dissimilar types, such as Ethernet and Token Ring. Bridges segments the network to improve the network performance. Bridge is protocol independent while router is protocol dependent. Although a bridge is not as sophistic as a router, it is faster than a router because it doesn't need to determine the transmission path. Bridge works at the data link layer.
Router is a network device that routes data packets from one network to another. Routers work at the network layer. Routable protocol includes OSI, IP, IPX, AppleTalk, DECNET and XNS. Unroutable protocol includes NetBEUI, LAT and DLC. Of the three major protocols in Windows NT and Windows 95: NetBEUI, TCP/IP and IPX/SPX, only NetBEUI is an unroutable protocol. However, it is also the most efficient one of these three protocols.
Gateway is a computer that performs protocol conversion between different types of networks or applications. For example, from IPX/SPX to TCP/IP. Gateway can work at any layer of the OSI model. FTQ 1.7.5 is a typical question regarding the gateway. Many people can answer this question correctly, but when we provide the following scenario, they got confused.
If your PC only have NetBEUI installed, you want it to access IBMs mainframe, what should you use? Of course you can install a specific program to connect IBM mainframe, however, if there are no specific program, gateway is the correct answer.
Hub is a network device wires computers and other network devices together in a star topology. (Figure 1.13) Passive hub just forms a star topology of the network, which has nothing to do with the data passing through them. Active hub, sometimes referred to as multiport repeater, regenerates signals to connect two computer with longer distance. Both passive hub and active hub work at the physical layer such as repeaters. Some intelligent hubs provide more functionality such as routing and bridging. Therefore, an intelligent hub might work at the data link layer or the network layer.
Switch is a network device that directs network packets from one port to another port directly. With a Hub, all the ports share the bandwidth. For example, if you have a 16-ports 10BaseT Hub, all the 16 ports share 10Mbps bandwidth. However with a switch, the bandwidth between each port is 10Mbps. Therefore, totally you might have 16*10Mbps = 160Mbps for a 10BaseT switch. However, in the real world, most 16-port 10baseT switch only have 100Mbps while not 160Mbps as the total bandwidth because of the consideration of the cost. Moreover, in most cases, only several ports could be busy.
Ethernet is a data link protocol (MAC layer protocol) and functions at layers 1 and 2 of the OSI model.
Token Ring is also a data link protocol (MAC layer protocol) and functions at layers 1 and 2 of the OSI model.
SLIP works on the physical Layer of the OSI model while PPP works on both the physical and data link layer. Although SLIP only works on the physical layer, sometimes we still call it a data link protocol. But in case Microsoft lets you drag and drop the SLIP to the corresponding layer of the OSI model, you must put it into the Physical layer.
Application |
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Presentation |
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Session |
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Transport |
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Network |
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Data Link |
PPP |
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Physical |
SLIP |
Figure 1.12 PPP and SLIP
Following diagram shows the relationship between the NetBEUI, TCP/IP, TDI and NetBIOS and where they resident for the OSI model.
Some people may think D is also correct for FTQ 1.7.3. Because the distance between the station and Hub is only 100 meter, the total distance between two stations can be 200 meter. It could be true if you are using the active Hub that has regenerate signal function such as repeater. But for the Networking Essentials exam, please think Hub as a pure Hub. It just forms a star topology for the Ethernet. Therefore, you need to choose C as the correct answer.
FTQ 1.7.4 test you which protocol is routable and which is unroutable. For the Networking Essentials exam, please remember:
For FTQ 1.7.7, a passive hub only regenerate the signal, it has nothing to do with increase the whole bandwidth. Therefore, C is wrong. A is the best answer because smart hub can connect two LANs, in fact, we can also call a smart hub a LAN switch.