Download Layers in OSI Model – Transport layer

Lecture 1
The OSI Model
Reference: TCP/IP Protocol Suite, 4th Edition (chapter 2)
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Outline
• To discuss the OSI model and its layer architecture and to
show the interface between the layers.
• To briefly discuss the functions of each layer in the OSI model.
2 THE OSI MODEL
• Introduced in late 1970s, Open Systems Interconnection (OSI)
model is an ISO standard covers all aspects of network
communications.
– OSI is not a protocol, but a model for designing a network architecture
and the basis for the protocols creation in OSI stack.
• OSI model is a layered framework that allows communication
between all types of computer systems without requiring
changes to the logic of underlying software and hardware.
– Consists of seven separated but related layers.
– Each defines a part of the process of moving information across a
network.
Figure 3
The OSI model
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• The OSI model consists of 7 ordered layers.
• The designers of the OSI model distilled the process of
transmitting data to its most fundamental elements.
– They identify which networking functions had related uses and
collected those functions into discrete groups that become the layers.
• Each layer defines a family of functions distinct from those of
the other layers.
• By localizing functionality in this fashion, this architecture is
both comprehensive and flexible.
– Allows complete interoperability between incompatible systems.
Figure 4
OSI layers
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Layer-to-layer communication
• Interfaces between layers
– Define what information and services a layer must provide for the
layer above it.
– Provide modularity to a network.
• As long as a layer provides expected services to the layer above it, specific
implementation of its functions can be replaced without changes to the
surrounding layers.
• Organization of layers
– Network support layers: 1, 2, 3; deals with physical aspect (electrical
spec, physical connections and addressing, transport timing), mostly
hardware Impl..
– User support layers: 5, 6, 7; allow interoperability among unrelated
software systems. (software Impl.)
– Transport layer: 4; links two sub-groups.
Figure 5
An exchange using the OSI model
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Layers in OSI Model – physical layer
• It coordinates functions required to carry a bit stream over a
physical medium.
– Deals with mechanical and electrical specifications of interface and
transmission media.
– Defines procedures and functions that physical devices and interfaces
have to perform for transmission to occur.
• It also concerns with
– Representation of bits: the type of encoding (bits -> signals, electrical
or optical)
– Data rate: i.e., transmission rate – number of bits sent per second, or
duration of a bit, which is how long it lasts.
– Synchronization of bits: sender and receiver clocks must be
synchronized.
– Line configuration, i.e., connection of devices to media.
• In a p2p configuration, two devices are connected together through a
dedicated link. In a multipoint configuration, a link is shared between
several devices.
– Physical topology: it defines how devices are connected to
make a network.
• mesh topology (every device connected to every other device), a star
topology (devices connected through a central device), a ring topology
(each device is connected to the next, forming a ring), a bus topology
(every device on a common link)
– Transmission mode: the direction of transmission.
• simplex (one device sends, the other only receives, a one-way
communication), half-duplex (two devices send and receive, but not at
the same time), or full-duplex (two devices send and receive at the same
time).
The physical layer is responsible for moving
individual bits from one (node) to the next.
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Layers in OSI Model – Data Link layer
• It transforms the physical layer, a raw transmission facility to a
reliable link.
– Makes physical layer appear error-free to upper layer (network layer)
• It is also responsible for
– Framing, it divides the stream of bits received from the network layer
into manageable data units called frames.
– Physical addressing: if frames are to be distributed to different systems
on the network, it adds a header to the frame to define sender and/or
receiver of the frame; if the frame is intended for a system outside the
sender’s network, the receiver address is the address of the
connecting device that connects the network to the next on.
– Flow control: it imposes a flow control mechanism to prevent
overwhelming the receiver.
• Data link layer is also responsible for (continued)
– Error control, it adds reliability to the physical layer by adding
mechanisms to detect and retransmit damaged or lost frames. It also
uses a mechanism to recognize duplicate frames. It is normally
achieved through a trailer added to the end of frame.
– Access control, when two or more devices are connected to the same
link, data link layer protocols are necessary to determine which device
has control over the link at any given time.
Layers in OSI Model – Network layer
• It is responsible for source-to-destination delivery of a packet,
possibly across multiple networks.
– If two systems are connected to the same link, there is usually no need
for a network layer.
• Other responsibilities
– Logical addressing: if a packet passes the network boundary, another
addressing system is needed to help distinguish the source and
destination systems. This layer adds a header to a packet coming from
the upper layer, including logical addresses of sender and receiver.
– Routing: connecting devices (routers or switches) route packets to
their final destination when independent networks or links are
connected together to create internetworks or a large network.
Layers in OSI Model – Transport layer
• It provides the process-to-process delivery of entire message.
– A process is an application program running on the host.
• It ensures the whole message arrives intact and in order;
oversees both error control and flow control at the source-todestination level.
• Other responsibilities
– Service-point addressing: It gets the entire message to the correct
process on the computer.
– Segmentation and reassembly: a message is divided into transmitted
segments, with each segment containing a sequence number. These
numbers enable this layer to reassemble the message correctly upon
arriving at the destination, and identify and replace packets that were
lost in transmission.
• Other responsibilities (more)
– Connection control: it can be connectionless or connection-oriented.
A connectionless transport layer treats each segment as an independent
packet and delivers it to the transport layer at the destination machine.
A connection-oriented one makes a connection with the transport layer
at the destination first before delivering packets. After all data are
transferred, the connection is terminated.
– Flow control: flow control is performed end to end rather than across
a single link.
– Error control: error control is performed process-to-process rather
than across a single link. The sending transport layer makes sure that
entire message arrives at the receiving transport layer without error.
Error connection is usually achieved through retransmission.
Layers in OSI Model – Session layer
• It is network dialog controller, establish, maintains and
synchronizes the interaction between communicating
systems.
– Dialog control: it allows two systems to enter into a dialog; allows the
communication between two processes to take place in either halfduplex or full-duplex mode.
– Synchronization: It allow a process to add checkpoints
(synchronization points) into a stream of data.
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Layers in OSI Model – Presentation layer
• It is concerned with the syntax and semantics of the
information exchange between two systems.
– Translation: the processes in two systems are usually exchanging
information in the form of character strings, numbers, which should
be changed to bit streams before being transmitted.
It is responsible for interoperability between different encoding methods:
sender-dependent format -> common format -> receiver-dependent
format
– Encryption: it is responsible to assure privacy to carry sensitive
information.
– Compression: it reduces the number of bits contained in the
information, for example, in transmission of multimedia.
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Layers in OSI Model – Application layer
• It provides user interfaces and support for services such as
email, remote file access and transfer, shared database
management, and other types of distributed information
services.
– Network virtual terminal: a software version of a physical terminal and
allows a user to log on to a remote host.
– File transfer, access and management (FTAM). It allows a user to
access files in a remote host, to retrieve files from a remote computer
to the local computer, etc.
– Email services: email forwarding and storage.
– Directory services: provides distributed database sources and access
for global information about various objects and services.
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Figure 6 Summary of OSI Layers
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