Lecture 05: Application and Layered Architectures The 7-Layer OSI ...

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Oct 23, 2013 (3 years and 8 months ago)

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EE 400: Communication Networks (101) Dr. Wajih A. Abu-Al-Saud
Ref: A. Leon Garcia and I. Widjaja, Communication Networks, 2
nd
Ed. McGraw Hill, 2006
Latest update of this lecture was on 29­09­2010  
 
Lecture 05: Application and Layered Architectures 

 

Referring to Question 24 on Lecture 1, it was clear that the principle of layers in building network
protocols significantly simplifies the issue of designing these protocols in addition to making the
interaction between these layers very clear.
Some of the first pieces of network equipment manufactured in the 1970s ware not compatible with each
other and could not be connected together because of the lack of unifying standards. In 1970s, the
International Standards Organization (ISO) developed its 7-Layer reference open model for networking
called Open Systems Interconnection (OSI) in the hope that this would make equipment manufactured by
different manufacturers compatible.
The 7­Layer OSI Reference Model 

The concept here was that ISO was going to develop a reference model for networks that is based on
having 7 layers and that the protocols in each layer would be developed later. The OSI model divided the
communication process from source to destination into 7 layers. Information would have to pass over
each layer from top to bottom in the source machine, gets transmitted by the source machine over the
physical channel, and received by the destination machine where the information would go up the same
layers as in the source machine.
The figure below shows the OSI architecture with its 7 layers from bottom to top being:
1. Physical Layer: This layer deals directly with the transfer of bit over the physical channel such
as the copper wire, coaxial wire, optical fiber, or over the air. It specifies parameters related to the
transmission of bits such as bit duration, voltages used for logic 1 and logic 0, and frequencies in
modulated transmissions. It also controls the initiation and release of the physical connection and
mechanical aspect of the link such as socket type and number of pins.

2. Data Link Layer: This layer deals with the transfer of complete frames over a single link that
directly connects two machines. This layer inserts framing information to identify the start and
end of the each frame, inserts address information in the header of the frame to identify the next
destination of the frame, and inserts error control and flow control information in the transmitted
frame. In LANs, this layer is responsible for the transfer of frames from the source to the
destination, which is done by broadcasting the frame on the network. This results in the need for a
Medium Access Control (MAC) method. The address used by the Data Link Layer is actually
called MAC address. The MAC address is unique to each Network Interface Card (NIC), which is
the card that connects a computer to the network.

3. Network Layer: This layer is responsible for the overall routing of packets (not frames) over the
whole network usually across several nodes of the network. Each of these nodes is responsible for
routing its traffic towards a node that is closer to the destination of each packet. In the Internet,
the network layer uses the IP address for routing the packets (and not the MAC address). The
addressing used by this layer is a hierarchical address to ease the routing. This layer is also
EE 400: Communication Networks (101) Dr. Wajih A. Abu-Al-Saud
Ref: A. Leon Garcia and I. Widjaja, Communication Networks, 2
nd
Ed. McGraw Hill, 2006
Latest update of this lecture was on 29­09­2010  
 
Lecture 05: Application and Layered Architectures 

 
responsible for dealing with congestion that may occur at different nodes around the network.
Because of the complexity of the routing procedures, the network layer is the most complicated
layer in the OSI model. In case of gateways that route traffic between different non-similar
networks, an internetwork sub-layer of network layer is responsible for routing traffic across the
non-compatible networks hiding from upper layers any differences in addresses in the underlying
networks.

Note:
Each node in the network implements either the lowest layer (Physical Layer only),
lowest 2 layers (Physical and Data Link Layers only), or at most lowest 3 layers (Physical, Data
Link, and Network Layers only) while the upper four layers (Transport, Session, Presentation,
and Application Layers) are implemented only in the end point (source and destination).
Therefore, the processes in the lower three layers are not peer processes while the processes in the
upper four layers are peer processes.

4. Transport Layer: This layer is responsible for end-to-end transfer of messages from the source
machine to the destination machine. This layer is responsible for receiving messages from the
upper layer in the source machine and segmenting them into smaller pieces that are either called
(segments) or (datagrams) depending on their structure and for reassembling these segments or
datagrams to form the original messages at the destination machine. This layer can provide a
connection-oriented error-free, sequential, communication and flow control, may provide un-
reliable connectionless communication, or something in between depending on the desired quality
of service. This layer is responsible for setting up and releasing connections, for multiplexing
multiple transport layer connections together over a single network layer connection for efficient
data transfer or splitting a single transport layer connection over multiple network layer
connections for faster speed.

5. Session Layer: This layer controls the manner of information exchange (half or full duplex
transmission). It is also responsible for inserting synchronization points to mark progress of
interaction that can be used as points of error recovery such as when transferring long files on a
network with relatively small failure periods.

6. Presentation Layer: This layer provides the upper layers with independence from differences in
representation of data (machine dependent data of Application A is converted to machine
independent data that is transported over the net and then converted to machine dependent data
for Application B at the destination).

7. Application Layer: This layer provides services frequently required by applications like HTTP
(which determines how a client application such as a web-browser requests a website, and how
the server responds to that request), FTP, virtual terminal (remote login), SMTP. These protocols
define how clients and servers interact with each other.


EE 400: Communication Networks (101) Dr. Wajih A. Abu-Al-Saud
Ref: A. Leon Garcia and I. Widjaja, Communication Networks, 2
nd
Ed. McGraw Hill, 2006
Latest update of this lecture was on 29­09­2010  
 
Lecture 05: Application and Layered Architectures 

 
Communication Network
Presentation
Layer
Application
Layer
Transport
Layer
Session
Layer
Data Link
Layer
Network
Layer
Physical
Layer
Presentation
Layer
Application
Layer
Transport
Layer
Session
Layer
Data Link
Layer
Network
Layer
Physical
Layer
Data Link
Layer
Network
Layer
Physical
Layer
Data Link
Layer
Network
Layer
Physical
Layer
Application
A
Application
B
Provides upper layer with independence from differences in representation of data
(machine dependent data of Application A is converted to machine independent data
that is transported over the net and then converted to machine dependent data for
Application B)
Provides services frequently required by applications like HTTP, FTP, virtual terminal
(remote login), SMTP
Responsible for end-to-end transfer of messages. Accepts messages from upper
layer and prepares datagrams. It may provide a connection-oriented error-free,
sequential, communication and flow control, may provide un-reliable connectionless
communication, or something in between. It is also responsible for segmenting long
messages at the source and reassembling them at the destination.
Controls manner of information exchange (half or full duplex transmission). It is also
responsible for inserting synchronization points to mark progress of interaction that
can be used as points of error recovery (such as when transferring long files on a
network with relatively small failure periods)
Deals with transfer of frames (blocks of information) across a transmission link that
directly connects two nodes in the network. It inserts framing information, inserts
control and address information in a header, and inserts check bits to allow for error
correction/detection, and is responsible for flow control.
Responsible for transmission of information as packets across the net. It has a
hierarchical address system. It is responsible for routing of packets (through several
transient links and nodes) Routing protocols run in this layer. It also deals with
congestion. [MOST COMPLICATED LAYER]
Deals with transmission of bits over the communication channel (such as twisted
pair, coaxial cable, optical fiber). It deals with parameters such as voltages of
signals, signal shapes duration, socket types number of pins
Layer
7
6
5
4
3
2
1


Network Devices 

The box in the middle of the above figure represents one of several possible network devices. These
devices are classified based on the number of layers they contain. For example, we can consider a
network cable to be a passive network device with no layers (Layer-0 device). An obsolete network
device is the Hub. Hubs are layer-1 devices since they go up to the Physical Layer only and do not have
upper layers. A switch is a layer-2 device which means that it contains the lower two layers (Physical and
Data Link). This is why a switch reads the MAC addresses but not IP addresses since IP addresses are
layer-3 addresses. A router is a layer-3 device. There are more devices that contain more operate at higher
layers but we will not discuss these in this course.