The OSI model has many similarities to the TCP/IP model from a basic conceptual perspective. It has (seven) layers, and each layer defines a set of typical networking functions. As with TCP/IP, the OSI layers each refer to multiple protocols and standards that implement the functions specified by each layer. In other cases, just as for TCP/IP, the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined. For example, the IEEE defines Ethernet standards, so the OSI committees did not waste time specifying a new type of Ethernet; it simply referred to the IEEE Ethernet standards. Today the OSI Model can be used as standard comparison to other networking models. The table below compares the seven-layer OSI model with both the four-layer and five-layer TCP/IP models.

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OSI Layer 

7 Application       

6 Presentation     

5 Session              

4 Transport         

3 Network            

2 Datalink             

1 Physical             

Describing Protocols by Referencing the OSI Layers

Today, networking documents often describe TCP/IP Protocols and standards by referencing OSI layers, both by layer number and name. For example a “Layer 2” switch is referring to OSI layer 2. Another example TCP/IP the original Internet Layer, as implemented mainly by its IP, equates most directly to the OSI network layer. So most people say IP is a network layer protocol or layer 3 protocol.

The OSI Layers and their functions.

Today, because most people happen to be much more familiar with TCP/IP functions than with OSI functions, one of the best ways to learn about the function of different OSI layers is to think about the Functions in TCP/IP model and correlate those with the OSI model.

For the purpose of learning, you can think of the five OSI layers as doing the same five things as the five layers in the OSI model.

Layer 7 – Application Layer

Provides and Interface from the application to the network by supplying a protocol with actions meaningful to the application for example get “webpage”

Layer 6 – Presentation Layer.

This layer negotiates data formats, such as ASCII text or jpeg

Layer 5 – Session Layer

Session layer provides methods to grouple multiple bi-directional messages into a workflow for measier management and easier backout of work that happened if the entire work flow fails

Layer 4 – Transport Layer

In function, much like TCP/IPs transport layer. The layer focuses on data delievery between 2 end point hosts (for example, error recovery).

Layer 3 – Network Layer

Like the TCP/IP network (internet layer). This layer defines logical addressing, routing (forwarding), and the routing protocols used to learn routes.

Layer 2 – Data-Link Layer

Like the TCP/IP data link layert, this layer defines the protocols for delivering data over a particular single type of physcial network (for example), the ethernet protocols).

Layer 1 – Physical Layer

This Layer defines the physical characteristics of the transmission mdium, including connectors, use of pins, electricl currents, encoding, light modulation, and so on.

The table below lists a sampling of devices and protocols and their comparable OSI layers. Note that many network devices must actually understand the protocols at multiple OSI layers, so the layers listed below actually refers to the highest layer that the device normally thinks about when performing its core work. For example routers need to think about layer 3 concepts, but must support both layers 1 and 2.

Layer name

Application: presentation, Telnet, HTTP, FTP, SMTP Hosts / Firewall

Session layers: (Layer 5-7)POP3, VoIP, SNMP

Transport: (Layer 4) TCP, UDP Hosts / Firewalls

Network Layer: (Layer 3) IPs Router

Data Link: (Layer 2) Ethernet (IEEE 802.3, HDLC) LAN Switch, wireless

AP, Cable modem, DSL modem. Physical (Layer1) RJ-45, Ethernet (IEEE (802.3) LAN hub, LAN repeater, cables Functions in the TCP/IP model and to correlate those with the OSI model.

To memorize the the layers:

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Less complex: Compared to not using a layered model, network models break the concepts into smaller parts.

Standard interfaces: The standard interface definitions between each layer allow multiple vendors to create products that fill a particular role, with all the benefits of open competition.

Easier to learn: Humans can more easily discuss and learn about the many details of a protocol specification.

Easier to develop: Reduced complexity allows easier program changes and faster product development.

Multivendor interoperability: Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network.

Modular engineering: One vendor can write software that implements higher layers—for example, a web browser—and another vendor can write software that implements the lower layers—for example, Microsoft’s built-in TCP/IP software in its operating systems.

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OSI Encapsulation Terminology

Like TCP/IP, each OSI layer asks for services from the next lower layer. To provide the services, each layer makes use of a header and possibly a trailer. The lower layer encapsulates the higher layer’s data behind a header.

OSI uses a more generic term to refer to messages, rather than frame, packet, and segment. OSI uses the term protocol data unit (PDU). A PDU represents the bits that include the headers and trailers for that layer, as well as the encapsulated data. For example, an IP packet, as shown in Figure 1-14, using OSI terminology, is a PDU, more specifically a Layer 3 PDU (abbreviated L3PDU) because IP is a

Layer 3 protocol.

OSI simply refers to the Layer x PDU (LxPDU), with x referring to the number of the layer being discussed, as shown below

L#H Layer # Header

L#T Layer # Trailer

L7HData L7PDU

L6H Data L6PDU

L5H Data L5PDU

L4H Data L4PDU

L3HData L3PDU

L2H Data L2PDU L2T (Trailer)

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page 25 – Table 1-3 Provides definations of same-layer interaction and adjacent Layer interaction

page 28 – Figure 1-10 Shows the general concept of IP routing

Page 29 – Figure 1-11 Depicts the data link services provided to IP for the purpose of delivering IP 

packets from host to host

page 31 – Figure 1-13 Five steps to encapuslate data on the sending host

page 32 – Figure 1-14 Shows the meaning of the terms segment, packet and frame

page 33 – Figure 1-15 Compares the OSI model and the TCP/IP model models

page 35 – List Lists the benefits of using a layered networking model

page 36 – Figure 1-16 Terminology related to encapsulation

Key Terms you should know

Adjacent-layer interaction

de-encapsulation

Pearson CCENT Questions and Answers

Q1 IP functions at what layer of the OSI Reference Model?

A1 Transport Layer – Layer 4

Q2 Which layer of the OSI Reference Model defines end-to-end delivery of packets?

A2 Network Layer – Layer 3

Q3 Which of the following describe a MAC address?

A3 Layer 2 and 48bits

Q4 —– Diagram question Q5

Q5 Which of the following are common internet access links

A5 Leased line, DSL, Cable.

Q6 How does the receiver of an HDLC frame know what has been encapsulated within that frame?

A6— not sure must check — type of field in the header.

Q7 When PC1 on an Ethernet network sends a packet to a remote PC2 that is on another Ethernet network, and the packet is routed by exactly two routers, connected by an HDLC WAN connection, how many different Layer 2 headers (with different addresses in the headers) will be created to encapsulate the IP packet as it travels from PC1 to PC2?

A7 2 headers will be encapsualted ???

Q8 What is the purpose of the FCS field in an HDLC frame?

A8  This allows the data to be checked, to ensure the correct data has been recieved only when this FCS has been sent can the next frame be sent.

Q9 Which protocol finds an unknown Layer 2 MAC address from a known IP address?

A9 Arp ResolutioN