Describe the purpose and basic operation of the protocols in the OSIand TCP models

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

The OSI and TCP models allow standardization of network data transfer. The standards used on each network layer help you to troubleshoot the network errors and find out exactly where the network is failing.

The basic purpose of all the protocols in the OSI and TCP models is to reduce the duplication of effort, control errors, control the flow of data, control connections, encapsulation, and manage transmissions. Let us understand the basic operations of protocols in each layer of TCP and OSI models.

Physical - 1

The physical layer is the first layer of OSI model that defines how code bits would travel as electrical signals in a wire from one device to another. For example, it defines how to activate physical links between end systems using procedural, electrical, and functional requirements. It defines how interface cards, hubs, cables and connectors send and receive bits.

The protocols used in this layer define:

  • How to establish and terminate a connection between two devices connected to each other.
  • How to control the flow of data transmission through flow control process.
  • How to provision a reliable or an unreliable connection between two directly connected nodes.
  • How to define data modulation and represent digital data in equipment and their corresponding physical communications channel.

The important standards in this layer are IEEE 802.3 standard for cabling system, POTs (Plain old Telephone System) - standard for telephone systems, SONET - a fiber connection often used in Metro-Ethernet systems, 802.11 - a Wireless standard, USB, Bluetooth, and even Hubs.

Data - 2

The data link layer or data layer is the layer where the data packets or the information provided by the layers above this layer are encapsulated into frames with a header and footer information. The header contains the MAC address of the machine to which the information is to be sent and footer contains the frame check sequence field to ensure that the data packet is not corrupted at the physical layer.

Switches operate on this layer. This layer is responsible for identifying each network device on the LAN. The Data Link layer has two sub layers, Media Access Control (MAC) 802.3, and Logical Link Control (LLC) 802.2. The MAC layer defines how packets should be placed on the media. It defines the physical addressing and logical topologies. The LLC is responsible for identifying network layer protocols and encapsulate them.

The PPP (Point-to-Point) protocol is an example of data link layer standard, which defines how the Internet can be provided over POTs with modems. The standards for serial links are ATM and Frame Relay, 802.2 or LLC, and SDLC for PPP serial links.

Network - 3

The network layer is the routing layer that uses logical addressing or IP addresses to identify a host on the network. This layer allows many nodes to be connected on a network. However the datagram delivery to various nodes on the network is not reliable on this layer. The protocols used in this network layer are management protocols such as routing protocols, network- layer address assignment protocols, multicast group management protocols, and network-layer information and assignment protocols.

The network layer uses two types of data packets, Data packets and Route update packets. The Data packets transport user data through internetwork and Route update packets update neighboringrouters about the other networks on the Internetwork. The protocols used to support data packets are called routed protocols. This includes protocols such as IP, IPV4, and IPV6. The protocols that send route update packets are called Routing protocols, which build routing tables on each router. The examples of Routing protocol are RIP, RIPv2, EIGRP, and OSPF.

Transport - 4

The transport layer separates the data it receives from the upper layers, segments and reassembles it and ensures that they are delivered to the right place reliably. This layer handles the flow control and retransmission of lost packets.

The protocols used in this layer are TCP and UDP protocols. These protocols are state and connection oriented protocols that keep track of the data packets that transmit through the network and ensures the successful transmission of data through acknowledgement of successful data transmission. In addition the protocols used in this layer ensure retransmission of the data packets that fail.

The OSI model defines five types of protocols for this layer TP0, TP1, TP2, TP3, and TP4. The TP0 protocol type has fewest features and is used for error free connections and TP4 protocol type is used for less reliable networks that are more prone to errors.

Session - 5

The session layer defines how to manage a session between two computers. It defines when and how to start, monitor, and end a session.

The protocols used in this layer are responsible for controlling the connections between the connections through full duplex, half duplex, and simplex operations. The protocols such as Transmission Control Protocol, used in this layer are responsible for establishing check pointing, adjournment, termination, and restarting procedures.

Presentation - 6

The presentation layer defines the data formats to be used. The encryption, compression, and encoding of information is done in this layer. The data units in this layer are encapsulated into session protocol units and passed to TCP/IP stack. The OSI model defines protocols standards that define how data should be formatted.

Application - 7

The application layer defines how an application communicates with the other applications on other computers. The user authentication is performed in this layer.

The protocols used in this layer determine the resource availability to which the data needs to be transmitted. The protocols used in OSI model are FTAM (File Transfer and Access Management Protocol), X.400 Mail, and Common Management Information Protocol (CMIP).

The protocols used in TCP/IP model are: HTTP, FTP, SNMP, and SMTP.

Example question

Which of the following OSI layer is responsible for segmenting the data stream?

  • Physical Layer
  • Data Link Layer
  • Network Layer
  • Transport Layer

Answer: D

The transport layer separates the data it receives from the upper layers, segments and reassembles it and ensures that it is delivered to the right place reliably.


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