McAfee Secure

Select the components required to meet a given network specification

Exam: Cisco 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)

A network consists of many components such as hosts, peripherals, networking devices, and media. Each has a different role in network communication. The hosts are the end user devices such as personal computers or servers that send and receive information on a network. The peripherals are devices on a network that do not communicate with each other. Rather they are connected to a host computer to perform their functions. The devices such as local printers and scanners are the examples of the peripherals.

The networking devices are the devices on a network that help in managing and controlling the network traffic. The networking devices connect host computers on a network and manage traffic between them. The examples of network devices are hubs, routers, and switches. Another important component on a network is media, which connects two devices on a network. The media can be wires such as fiber optic, copper, or UTP cables or wireless technologies.

All computers that communicate with each other on a network by sending and receiving messages are classified as hosts. The communication between two hosts on a network is performed on different network layers, which make use of the services provided by the adjacent layers. A host communicates with the peer layer of another host with the help of the services provided by the adjacent layers.

The networking layers use the service user, the provider, and the services access point to provide service.

OSI Model Layers and Information Exchange

During information exchange, each network device communicates only with its peer networking layer on the other device. Also the data is encapsulated at each layer by the sending computer before transmitting on the network and is decapsulated at each layer by the receiving computer. The network layers attach PDUs (Protocol Data Units) to the data at the time of encapsulation to manage the control information of the data at each layer.

The PDUs encapsulate data and then attaches themselves to the data as control information for the data either as a header or the trailer. The PDUs are read only can be read by the peer layer on the receiving host.

After reading the PDU, the networking layer strips off the PDU and handover the data to the next layer up. The data travels two ways on a network. While encapsulation, the data travels down and at the time of decapsulation, the data travels up.

For Example, consider an application on System A has to send some information to an application on System B. The process followed for information exchange would be:

  1. System A passes the data to be communicated to its Application layer. The Application layer in System A attaches the control information required by Application layer of System B and passes the data to Presentation Layer. The presentation layer again prepends its own information to the data received by it from the application layer. Similarly the data is forwarded to the Session layer and the Session layer again adds header to the information.In the same way each layer prepends its header/trailer information to the information received by it from the previous layer. The information grows with each layer.
  2. At Transport layer the data is broken into smaller pieces called segments and a transport header is attached to each piece. Each segment is transmitted separately to the network layer for logical addressing or routing.
  3. The data (transport header and upper layer information) called a Packet is appended by IP header and handed over to the data link layer.
  4. The data (Network layer header, Transport layer header, and upper layer information) is encapsulated in to a frame. The frame contains the MAC address of the source and the destination and the LLC information that specifies the protocol to which the data should be passed to at Data link layer. The frame is handed over to the physical layer. The data link layer also appends the FCS (Frame check sequence) to the data, which is used for error checking.
  5. If the destination device is on the remote network then the frame is sent to the router, which sends it to the destination device by routing it through internetwork. The data can be sent as signals in the logical groups of 0s and 1s.
  6. When the data reaches physical layer the entire information is placed on the network medium after converting the frames into a digital signal.
  7. The Application layer in system B receives the information it receives from System A, synchronizes the digital signal and converts the digital signal into frame again through the process of decapsulation. The Application layer then passes the information to the Data link layer of System B.
  8. The Data link layer in System B reads the header appended by the Data link layer of System A and performs a CRC (Cyclic Redundancy Check). If the result of the CRC mach with the FCS field then it assumes that the data is received without errors.
  9. The Data link layer then removes the header from the frame and passes the remaining information to the Network layer of System B.
  10. The network layer of System B reads the header added by the network layer of system A, matches it with its own IP and removes the header if the match is correct. The network layer then passes the remaining data called segment to the Transport layer.
  11. The Transport layer at System B processes the segments and rebuilds the data stream. It then acknowledges that it has received each piece of data to the sender using TCP protocol and then it finally sends the data to the upper layer application.
  12. The application layer of System B reads the information and removes the header information added by the Application layer of System A. The Application B then passes the information to the software program, which requires this information following the same process.

Example Question

Identify the correct sequence of the encapsulation method

  • Data, segment, packet, frames, and bits
  • Bits, frames, packet, segment, and data
  • Segment, packet, frames, bits, and Data
  • Packet, frames, segment, data, and bits

Correct Answer: A

The encapsulation method is data, segment, packet, frames, and bits.