CCNA LESSON 4 - NETWORK REFERENCE MODELS

LESSON 4.Network Reference Models

A network reference model is a conceptual framework for understanding the relationships between different entities within a network. Entities refer to all the software and hardware elements which are part of the networking. The core objective of reference model is to have interoperability among multiple vendors so that the products (both software & hardware) designed by one vendor can communicate or work with the products manufactured by the other.

There are three important networking reference models.


OSI (Open System Interconnection)
TCP/IP Model
Cisco Three-layer Hierarchical ModelOSI Model

• The Open Systems Interconnection (OSI) model is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers.

• The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO).

• OSI Model consist of seven layer

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Seven Layer of OSI Model

Application Layer

The Application layer integrates network functionality into the host operating system, and enables network services. The Application layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network.

These services include:

1. File services: transferring, storing, and updating shared data
2. Print services: enabling network printers to be shared by multiple users
3. Message services: transferring data in many formats (text, audio, video) from one location to another, or from one user to another
4. Application services: sharing application processing throughout the Network and enabling specialized network servers to perform processing tasks
5. Database services: storing, retrieving, and coordinating database information throughout the network

Presentation Layer

The Presentation layer formats or “presents” data into a compatible form for receipt by the Application layer or the destination system. Specifically, the Presentation layer ensures:

Formatting and translation of data between systems
Negotiation of data transfer syntax between systems, through converting character sets to the correct format.
Compatibility with the host
Encapsulation of data into message envelopes by encryption and compression
Restoration of data by decryption and decompression

The Presentation layer formats data for the Application layer. Therefore, it also sets standards for multimedia and other file formats. These include standard file formats such as:

1. JPEG, BMP, TIFF, PICT
2. MPEG, WMV, AVI
3. ASCII, EBCDIC
4. MIDI, WAV

Session Layer

The Session layer’s primary function is managing the sessions in which data is transferred.
Functions at this layer may include:

1. Establishment and maintenance of communication sessions between the network hosts, ensuring that data is transported.
2. Management of multiple sessions (each client connection is called a session). A server can concurrently maintain thousands of sessions.
3. Assignment of the session ID number to each session, which is then used by the Transport layer to properly route the messages.
4. Dialog control: specifying how the network devices coordinate with each other (simplex, half-duplex, and full-duplex).
5. Termination of communication sessions between network hosts upon completion of the data transfer.

Transport Layer

The Transport layer provides a transition between the upper and lower layers of the OSI model, making the upper and lower layers transparent from each other.

Upper layers format and process data without any concern for delivery
Lower layers prepare the data for delivery by fragmenting and attaching transport required information

Transport layer uses the following:

1. Port (or socket) numbers are used to identify distinct applications running onthe same system. This allows each host to provide multiple services.
2. The Transport layer receives large packets of information from higher layers and breaks them into smaller packets called segments.
3. Segmentation is necessary to enable the data to meet network size and format restrictions.
4. The receiving Transport layer uses packet sequence numbers to reassemble segments into the original message. 
5. Connection-oriented protocols perform error detection and correction and identify lost packets for re-transmission.
6. Connection less communications use no error checking, session establishment, or acknowledgements. Connection less protocols allow quick, efficient communication at the risk of data errors and packet loss.

Network Layer

The Network layer describes how data is routed across networks and on to the destination.

Network layer functions include:

1. Maintaining addresses of neighbouring routers.
2. Maintaining a list of known networks.
3. Determining the next network point to which data should be sent. Routers use a routing protocol to take into account various factors such as the number of hops in the path, link speed, and link reliability to select the optimal path for data.
4. Packets forwarded from the Transport to the Network layer become datagrams and network-specific (routing) information is added.
5. Network layer protocols then ensure that the data arrives at the intended destinations.

Data-Link Layer

Data Link Layer is divided into 2 sub layers
Media Access Control (MAC)
Logical Link Control (LLC)

Media Access Control

The Media Access Control (MAC) layer defines specifications for controlling access to the media.
The MAC sub layer is responsible for:

1. Adding frame start and stop information to the packet.
2. Adding Cyclical Redundancy Check (CRC) for error checking.
3. Converting frames into bits to be sent across the network.
4. Identifying network devices and network topologies in preparation for media transmission.
5. Defining an address (such as the MAC address) for each physical device on the network.
6. Controlling access to the transmission medium.

Logical Link Control

The Logical Link Control (LLC) layer provides an interface between the MAC layer and upper-layer protocols. LLC protocols are defined by the IEEE 802.2 committee.

1. The LLC sub layer is responsible for:
2. Maintaining orderly delivery of frames through sequencing
3. Controlling the flow or rate of transmissions using the following:
4. Acknowledgements, Buffering and Windowing
5. Ensuring error-free reception of messages by re-transmitting
6. Converting data into an acceptable form for the upper layers
7. Removing framing information from the packet and forwarding the message to the Network layer
8. Provide a way for upper layers of the OSI model to use any MAC layer protocol
9. Defining Service Access Points (SAPs) by tracking and managing different protocols

Physical Layer

The Physical layer of the OSI model sets standards for sending and receiving electrical signals between devices. It describes how digital data (bits) are converted to electric pulses, radio waves, or pulses of lights. Devices that operate at the physical layer send and receive a stream of bits.

The physical layer provides the following services:

1. Modulates the process of converting a signal from one form to another so that it can be physically transmitted over a communication channel
2. Bit-by-bit delivery
3. Carrier sensing and collision detection, whereby the physical layer detects carrier availability and avoids the congestion problems caused by undeliverable packets
4. Signal equalization to ensure reliable connections and facilitate multiplexing
5. Forward error correction/channel coding such as error correction code
6. Transmission mode control

Data Encapsulation

• The sending and receiving of data from a source device to the destination device is possible with the help of networking protocols by using data encapsulation.
• The data is encapsulated with protocol information at each layer of the OSI reference model.
• Each layer communicates with its neighbor layer on the destination.
• Each layer uses Protocol Data Units (PDUs) to communicate and exchange information.

Protocol Data Unit (PDU)

• The Protocol Data Units contain the control information attached to the data at each layer.
• The information is attached to the header of the data field but can also be in end of the data field or trailer.
• PDUs are encapsulating by attached them to the data at each layer of the OSI reference model.
• Each Protocol Data Unit has a name depending on the information each header has.
• This PDU information is only read by the neighbor layer on the destination and then is stripped off and the data is handed to the next layer.

De-Encapsulation

On destination side, the receiving devices will synchronize on the digital signal and extract the 1s and 0s from the digital signal. At this point the devices build the frames, run a Cyclic Redundancy Check (CRC), and then check their output against the output in the Frame Check Sequence (FCS) field of the data frame. If the information matches then the packet is pulled from the frame, and the frame is discarded. This process is known as de-encapsulation.

The packet then transfers to the Network layer, where the IP address is checked. If the IP address matches then the segment is pulled from the packet, and the packet is discarded. The data is processed at the Transport layer that rebuilds the data stream and acknowledges to the transmitting
station that it received each piece of segment. It then happily transfers the data stream to the upper layer application.

TCP/IP Model

• The Internet Protocol Suite, TCP/IP, is a suite of protocols used for communication over the internet.
• The TCP/IP model was created after the OSI 7 layer model for two major reasons. First, the foundation of the Internet was built using the TCP/IP suite and through the spread of the World Wide Web and Internet, TCP/IP has been preferred. Second, a project researched by the Department of Defense (DOD) consisted of creating the TCP/IP protocols
• The TCP/IP model, similar to the OSI model, is comprised of layers. The OSI has seven layers and the TCP/IP model has four layers.

1. Network Access Layer
2. Internet Layer
3. Transport Layer
4. Application Layer

Application Layer

• This layer is comparable to the application, presentation, and session layers of the OSI model all combined into one.
• It provides a way for applications to have access to networked services. This layer also contains the high level protocols.
• The main issue with this layer is the ability to use both TCP and UDP protocols.
• For example TFTP uses UDP because usually on a LAN the physical links are short enough to ensure quick and reliable packet delivery without many errors. SMTP instead uses TCP because of  the error checking capabilities. Since we consider our email important information we would
like to ensure a safe delivery.

Transport Layer

This layer acts as the delivery service used by the application layer. Again the two protocols used are TCP and UDP. The choice is made based on the application’s transmission reliability requirements. The transport layer also handles all error detection and recovery. It uses checksum, acknowledgements, and timeouts to control transmissions and end to end verification. Unlike the OSI model, TCP/IP treats reliability as an end-to-end problem.

Internet Layer

• The routing and delivery of data is the responsibility of this layer and is the key component of this architecture.
• It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes.
• It inject packets into any network and deliver them to the destination independently to one another. Because the path through the network is not predetermined, the packets may be received out of order.
• The upper layers are responsible for the reordering of the data. This layer can be compared to the network layer of the OSI model.
• IP and ARP are the major protocols used at this layer.

Network Access Layer

• This a combination of the Data Link and Physical layers of the OSI model which consists of the actual hardware.
• This includes wires, network interface cards, etc. Other related details within this layer are connectors, signal strength, and wavelength along with various others. It will use the required LAN operating algorithms, such as Carrier Sense Multiple Access with Collision Detect (CMSA/CD) or IBM Token Passing etc. and is responsible for placing the data within a frame.
• The frame format is dependent on the system being used, for example Ethernet LAN, Frame relay, etc. The frame is the package that holds the data, in the same way as an envelope holds a letter. The frame holds the hardware address of the host and checking algorithms for data integrity. This layer has actually not been specified in details because it depends on which technology is being used such as Ethernet.

The Cisco Three-Layer Hierarchical Model

• The Cisco Three-Layer Hierarchical network model make networks more eliable and predictable and level by level design help to understand networking, for example we can use tools like access lists at specific level and can avoid them from others.
• Cisco layer model consist of the following three layers.


1. The Core layer
2. The Distribution layer
3. The Access layer

Core Layer

• Core layer is known as core i.e. backbone of network. It is on the top of the network and is responsible to transmit heavy amount of network traffic in reliable and quick manner.
• Core layer objective is to speed up the network traffic. In core layer the network traffic is same for every users and user data is carried to distribution layers which then forwards requests if it is required.
• If core layer is affected by a fault or failure, every user will be affected that are on network. Fault tolerance is main issue that is to consider on this layer.
• The main responsibility of core layer is to see heavy traffic, so speed and traffic issues are concerned at this layer.


Functions of Core layer
• Protect the network from slowing down the traffic, use of access lists, routing between different Virtual Local Area Networks (VLANs) and Packet Filtering.
• Protect network from workgroup access support.
• Don’t expand core as network grows. Try to overcome performance issue by adding routers and prefer to upgrade devices over expansion.

Distribution Layer

• It is also known as workgroup layer and it is called communication point between access and core layer.
• This layer deals with the routing.
• Basic function of distribution layer is routing, filtering and WAN access and find out the method by which packets can access the core.
• This layer must find out the fastest mechanism to handle network operations like how to handling and forwarding a file to server on request.
• After finding best path, distribution layer forward request towards core layer and then to the right service.
• Policy implementation is done on distribution layer and you can exercise flexibility defining network operations.


Functions of distribution layer

• Implementations of access lists for filtering interesting traffic and blocking uninteresting traffic.
• Security and network policy implementation containing address translation and firewalls.
• Static routing redistribution.
• Enabling routing between all VLANs.
• Defining broadcast and multicast domains.

The Access Layer

• User and workgroup access to network and resources is defined at access layer and this layer is also known as desktop layer.
• DDR (Double Data Rate) and Ethernet switching technology are mainly used in access layer with Static routing.


Functions of Access layers

Manage access control and policy
Create separate collision domains
Connectivity of workgroup through distribution layer