A.2. OSI Reference Model

The OSI reference model is the foundation of all network communications. While a good understanding is certainly required at certification test time, it also has plenty of practical applications. For example, knowledge of the OSI layers greatly helps when troubleshooting a network problem.Table A-8 shows the entire OSI reference model.

Table A-8. OSI reference model

Layer	Name	Data encapsulation representation	Encapsulation with header information
7	Application	Protocol Data Unit	User Data
6	Presentation	Protocol Data Unit	User Data
5	Session	Protocol Data Unit	User Data
4	Transport	Segment	TCP Header + User Data
3	Network	Packets	IP Header + User Data
2	Data Link	Frames	LLC Header + User Data MAC Header + LLC Header + User Data
1	Physical	Bits	101010100000...

When data leaves a node to go out on the network, the data starts at layer 7 (the application layer) and travels all the way down until it finally exits at layer 1 (the physical layer). When data arrives at a node from the network, it arrives at the physical layer and works its way back up the stack until it reaches the application layer. As data travels up and down the OSI model , the network header information is added and removed from the user data, as shown in the last column of the table.

Layers 7 through 4 are considered the upper layers; layers 3 through 1 are the lower layers.

A.2.1. Application Layer

The application layer provides network services and protocols directly to end user applications. For example, an application might make an HTTP call to another computer. The HTTP interface is the network service that exists at this layer, not the application making the call. In other words, your browser doesn't operate at this layer, but the HTTP protocol does. Examples of other protocols that operate at this layer are SMTP, DNS, FTP, Telnet, and SNMP.

A.2.2. Presentation Layer

This layer is concerned with transforming application layer data, which ensures that the data is readable by the application layer of other computers on the network. Some encoding examples are ASCII, MP3, JPEG, and MPEG.

A.2.3. Session Layer

This layer creates, maintains, and terminates the session communication between computers by established rules regarding how the end devices will talk to each other. Methods for this communication include one-way (simplex), taking turns (half-duplex), or talking simultaneously (full-duplex).

A.2.4. Transport Layer

In TCP/IP, this is the layer at which TCP and UDP operate. By segmenting outgoing data and reassembling incoming data, this layer provides transmission of the data. Functions include sequencing, flow control, error detection, multiplexing, and the retransmission of bad or missing data.

A.2.5. Network Layer

By using logical addressing (IP or IPX addresses), this layer defines processes to route the data across network, which is assembled into packets and datagrams. Routers and Layer 3 switches operate at this layer. In other words, IP addressing and IP routing both occur at this layer.

Some show commands that help with debugging at this layer include:

• show ip protocol (addressing)
• show interface (addressing)
• show ip interface brief (addressing)
• show ip route (routing)
• show ip eigrp/ospf/etc (routing)

A.2.6. Data Link Layer

This layer defines how data is moved across a physical media such as Ethernet, FDDI, or Token Ring. Addressing of this layer is controlled through MAC addresses. Two sublayers divide this layer in two: MAC (Media Access Layer) and LLC (Logical Link Control). Of course, here is where layer 2 switches operate.

Some show commands for this layer are:

• show interface
• show cdp neighbor
• show arp

A.2.7. Physical Layer

This layer is the electrical specification for carrying bits over the media, such as signal voltages rates, maximum transmission distances, and other physical characteristics. Hubs and repeaters operate at this layer. For this layer, the show command that's most helpful is show controller.