The History Of The Network Layer

Published Date: 02 Nov 2017

Brad Randalson

Bellevue University

CIS340A-H369

Helene Hector

April 18th, 2013

Abstract

Although not the most widely used model, the OSI model is still very important in networking. The paper will explain the 7 different layers. The layers being Physical, Data Link, Network, Transport, Session, Presentation, and Application. The paper will have explanations and examples for each model. The paper will also explain the TCP/IP model which is the most widely used model today. It will go into detail with examples of the Internet and Transport layers of this model.

OSI Model

Physical Layer

The first layer of the OSI model is the Physical Layer. The Physical Layer is responsible for the transmission of data over network communication media. The data are in the form of bits that are sent from the Physical Layer of the sending source and received at the Physical Layer of the receiving source. According to inetdaemon.com Physical Layer is responsible for communication with the Data Link Layer, fragmentation of data into frames, reassembly of frames into Data Link Protocol Data Units, transmission to the physical media, and receiving from the physical media (InetDaemon, 2013). An example of the Physical Layer is sending a signal from one host to another host.

Data Link Layer

The second layer of the OSI model is the Data Link layer. Accord to inetdaemon.com the Data Link layer how four primary functions Communication with the Network Layer, Segmentation and Reassembly, Bit Ordering, and Communication with the Physical Layer. (InetDaemon, 2013).

The Data Link Layer organizes data bits into frames before transmission. Start and stop bits, bit order, parity, and other functions are handled here. This layer provides reliable data across a physical link. According to inetdaemon.com the Data Link Layer is concerned with Physical Addressing, Network Topology, Physical Link Management, Error Notification, Order Delivery of Frames, and Flow Control (InetDaemon, 2013). As an example the Data Link layer will place its own MAC address into the frame followed by the destination MAC address which it would like to communicate with.

Network Layer

The Network Layer is layer 3 is the OSI model. The network layer adds routing above the Data Link Layer. A frame is examined when it reach the Network Layer for the source and destination MAC addresses. This will tell if the Network Layer if the frame has reached its final destination. Primary functions of the Network Layer according to inetdaemin.com are Communication with the Transport Layer, Encapsulation of Transport data into Network Layer Protocol Data Units, Management of connectivity and routing between hosts or networks, and communication with the Data Link Layer below it (InetDaemon, 2013). When you ping another IP address this is done on the Network Layer. The ping sends a frame to the address which in turns returns a frame.

Transport Layer

Layer 4 of the OSI model is the Transport Layer. The Transport layer is responsible for end to end communication over a network. It is also responsible for providing quality and reliability to the end user. This is done by allowing the host to send and receive error corrected data, packets or messages over a network. According to techopedia.com the Transport Layer can provide some of the following services Connection-Oriented Communication, Same Order Delivery, Data Integrity, Flow Control, Traffic Control, Multiplexing, and Byte Operation.

With connection-oriented communication devices at the end-point of a network communication ensures a connection is robust before data is exchanged. A weakness with this method is that each delivered message needs an acknowledgement. This added more network load then with self-correcting error packets.

Same order delivery ensures packets arrive in the correct order. The Transport Layer can fix and discrepancies in sequence. So if packets 1, 2 and 4 show up before packet 3, the Transport Layer will fix this so they come in order.

Data integrity uses checksums to guarantee that the data transmitted in the same as a data received. This is done by making repeat attempts made by other layers to have missing data resent.

Flow control is used to make sure there is no communication breakdown. Devices do not know how much data the other devices can receive quickly. This can cause a reduction is network performance if one device is sending packets faster than the receive device can read the packets.

Networks are subject to bandwidth and processing speed restrictions. This can cause a huge amount of congestion on the network. The Transport Layer can notice the symptoms of overloaded nodes and reduce the flow rate.

The Transport Layer uses dedicated mechanisms which control the transmission of multiple packets from unrelated applications or other sources across the network which is called multiplexing. Multiplexing allows the use of simultaneous applications over a network.

Some application prefers to receive byte streams instead of packets. The Transport Layer allows for this type of transmission if the data streams require it. This is called byte-orientation.

Session Layer

Layer 5 of the OSI model is the Session Layer. The Session Layer manages connections between applications. This layer sets up, coordinates, and terminates conversations, exchanges and dialogues between applications at each end. The Session Layer supports full-duplex and half-duplex operations and creates procedures for check pointing, adjournment, restart and termination. Synchronizing information from different sources is also the responsibility of the Session layer. According to inetdaemon.com the Session Layer is responsible for Communication with the Presentation Layer, Organize and manage one or more connections per application between hosts, Communication with the Transport Layer (InetDaemon, 2013).

Presentation Layer

The Presentation Layer is layer 6 of the OSI model. This layer ensures that the communications passing through are in the appropriate form for the recipient. Hypertext Transfer Protocol is an example of the Presentation Layer. It forwards user requests passed to the web browser on to a web server elsewhere in the network. The presentation layer communicates with the Application Layer, Translates data conforming to cross-platform standards into formats understood by the local machine, and communicates with the Session Layer (InetDaemon, 2013).

Application Layer

The Application Layer is layer 7 of the OSI Model. This layer is the only layer in the OSI model that directly interacts with the end user. Techopedia.com states that the Application Layer provides many services including SMTP, file transfer, web surfing, web chat, email clients, network data sharing, virtual terminals, various file and data operations (Techopedia).The Application Layer ensures that effective communication with another application program in a network is possible

D. Chandrasekaran has a great example of the OSI model. Figure 1 shows the this example of an email being sent and received.

Figure 1.

E-mail sent from John:

Meet me at Carl's 1:30

John

E-mail received from John:

Meet me at Carl's 1:30

John

http://www.studynotes.net/images/osi_up.jpg

http://www.studynotes.net/images/osi_down.jpg

Identify sender

and intended receiver; is there an e-mail application available?

APPLICATION

layer 7

Identified sender and intended receiver; found e-mail application.

Encode data with X coding key; use ASCII characters.

PRESENTATION

layer 6

Decoded data with X decoding key; used ASCII characters.

Initiate and terminate the session according to X protocol.

SESSION

layer 5

Initiated and terminated the session according to X protocol.

Make sure all data is sent intact.

TRANSPORT

layer 4

Make sure all data has arrived intact.

Keep track of how many hops;

open shortest path First;

Go to IP address 255.65.0.123

NETWORK

layer 3

Keep track of how many hops;

opened the shortest path First;

Went to IP address 255.65.0.123

Is the initial connection set up? Put data into frames according to X standard.

DATA LINK

layer 2

The initial connection set up. Decoded data in frame according to X standard.

Send as electrical signal over the network cable at X voltage, and X Mbps.

PHYSICAL

layer 1

Receive electrical signal over the network cable at X voltage, and X Mbps.

(Chandrasekaran, 2013)

TCP/IP Model

The TCP/IP model can be defined as a 4 layer network. These layers are:

Network

Internet

Transport

Application

Network

The network layer consists of the Physical and Data Link Layers in the OSI model. This layer is where the data is transmitted and received across a physical network. So just like the Physical Layer in the OSI model, the network layer in the TCP/IP model physically connects devices together.

Internet

The Internet layer is the network layer in the OSI model. This layer provides a global logical addressing scheme, a process for packetization of data, another process for routing packets to their destination and for providing connectivity between networks.

Global addressing provides an address for each device attached to the network. This address is used to find the device on the network. Just like buildings have addresses so they can be easily found on a street.

The Internet Layer takes data from the Transport Layer and encapsulates it into packets. After the data is encapsulated into packets it is then sent to the Network Access Layer to be transmitted to other devices on the network. This encapsulating of data is called packetization of data.

This layer also handles the transfer of packets from one local network to the next local network. This is based on the list of destinations and network interfaces contained in the memory of a device. These devices are called routers and gateways.

Transport

The Transport Layer is the same as the Transport Layer of the OSI Model. Since we are talking about the TCP/IP model then we are actually talking about transmission control protocol and user datagram protocol of this layer. These protocols are responsible for transmitting data between two end points running internet protocol. The transport layer does not handle all the hops in between the end points, just the end to end communication itself.

User datagram protocol is an unreliable protocol. This protocol does not perform and connection setup or teardown. More importantly it does not verify that the data got to the destination in one piece. This protocol relies on the application to manage retransmission of the lost data.

Application

The Application Layer is the same as the Application Layer of the OSI model. This is where computer programs capable of network communication reside. According to softpanorama.org the primary functions of this layer are formatting data, presenting data and transporting data ( Softpanorama). Some of the protocols used in this layer are Telenet Protocol, File Transfer Protocol, Simple Network Management Protocol, Simple Mail Protocol, Dynamic Host Configuration Protocol, Domain Name System, Network Information Services, Network File System. Secure Shell, and Secure Copy.