DATA COMMUNICATION & NETWORKING

DR/ABDULMAJEED ABDULRAZAG
STUDENT/ LOAI KHALIFA
ID/IBB058

A RESEARCH IN THE DEPTH OF THE FIRST THREE

LAYERS
OSI model
From Wikipedia, the free encyclopedia that anyone can edit
Jump to: navigation, search

The OSI Model

7. Application Layer

NNTP  · SIP  · SSI  · DNS  · FTP  · Gopher  · HTTP  ·

NFS  · NTP  · SMPP  · SMTP  · SNMP  · Telnet  ·
(more)

6. Presentation Layer

MIME  · XDR  · TLS  · SSL

5. Session Layer

Named Pipes  · NetBIOS  · SAP

4. Transport Layer

TCP  · UDP  · SCTP  · DCCP

3. Network Layer

IP  · ICMP  · IPsec  · IGMP  · IPX  · AppleTalk

2. Data Link Layer

ARP  · CSLIP  · SLIP  · Ethernet  · Frame relay  · ITU-

T G.hn DLL  · L2TP  · PPP  · PPTP

1. Physical Layer

RS-232  · RS-449  · V.35  · V.34  · I.430  · I.431  · T1

 · E1  · POTS  · SONET/SDH  · OTN  · DSL  ·
802.11a/b/g/n PHY  · ITU-T G.hn PHY  · Ethernet  ·
USB  · Bluetooth

This box: view • talk • edit

The O.S.I. (Open System Interconnection) model is a way of sub-dividing a System

into smaller parts (called layers) from the point of view of communications. A layer is
a collection of conceptually similar functions that provide services to the layer above
it and receives services from the layer below it. On each layer an instance provides
services to the instances at the layer above and requests service from the layer below.
For example, a layer that provides error-free communications across a network
provides the path needed by applications above it, while it calls the next lower layer to
send and receive packets that make up the contents of the path. Conceptually two
instances at one layer are connected by a horizontal protocol connection on that layer.

OSI Network Architecture 7 Layers Model

Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984, as a conceptual framework of
standards for communication in the network across different equipment and applications by different vendors. It is now considered the primary architectural model for inter-
computing and inter-networking communications. Most of the network communication protocols used today have a structure based on the OSI model. The OSI model
defines the communications process into 7 layers, dividing the tasks involved with moving information between networked computers into seven smaller, more
manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained, so that the tasks assigned to
each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.

The OSI 7 layers model has clear characteristics at each layer. Basically, layers 7 through 4 deal with end to end communications between data source and destinations,
while layers 3 to 1 deal with communications between network devices. On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers
(layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with application issues and generally are implemented only in software. The
highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer
are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for
placing data on the medium.

The specific description for each layer is as follows:

Layer 7: Application Layer

 Defines interface-to-user processes for communication and data transfer in network

 Provides standardized services such as virtual terminal, file and job transfer and operations

Layer 6: Presentation Layer

 Masks the differences of data formats between dissimilar systems

 Specifies architecture-independent data transfer format

 Encodes and decodes data; encrypts and decrypts data; compresses and decompresses data

Layer 5: Session Layer

 Manages user sessions and dialogues

 Controls establishment and termination of logic links between users

 Reports upper layer errors

Layer 4: Transport Layer

 Manages end-to-end message delivery in network

 Provides reliable and sequential packet delivery through error recovery and flow control mechanisms

 Provides connectionless oriented packet delivery

Layer 3: Network Layer

 Determines how data are transferred between network devices

 Routes packets according to unique network device addresses

 Provides flow and congestion control to prevent network resource depletion

Layer 2: Data Link Layer

 Defines procedures for operating the communication links

 Frames packets

 Detects and corrects packets transmit errors

Layer 1: Physical Layer

 Defines physical means of sending data over network devices

 Interfaces between network medium and devices

 Defines optical, electrical and mechanical characteristics

Application Layer
The Transmission Control Protocol/Internet Protocol (TCP/IP) application layer includes the
functionalities specified in the Open System Interconnection (OSI) application, presentation,
and session layers. The TCP/IP application layer provides applications such as remote file
systems, File Transfer Protocol (FTP) clients, and Web browsers with access to the network
communications subsystem. This layer also includes a number of utility protocols, such as
Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS), which aid in
managing and increasing usability of TCP/IP networks. In addition to Windows components,
the application layer also encompasses custom applications.

2-PHYSICAL LAYER:
The basic hardware needed to set up online communications is:
• a computer
• a modem
• a telephone line.

The overall speed of online communication is governed by the speed of the modem and the
speed of the communications link.
An ordinary telephone line is the cheapest but not the fastest link. An ISDN line has a greater
bandwidth and will send data much faster.
Modem

A modem (MOdulator/DEModulator) converts the data from your computer from digital form
(O’s and l’s) into analogue or wave form so that it can be sent over the telephone line. This is
because the telephone line was originally designed for speech.

Physical media transmit Analog signals

● Modulate/demodulate:
– Encode/decode binary data into signals
– E.g. Nonreturn
to Zero (NRZ)

● 0 as low signal and 1 as high signal

1. analog
o Continuously variable signal.
o Small fluctuations in the signal are meaningful.
o Lot of errors with noise over long distance.
o Used for phone call: A certain number of errors may not matter much
with speech.
2. digital
o Only n values (e.g. just 2 values).
o Small fluctuations in the signal are not meaningful. Analog signal is
quantised. Any signal close to a particular value will be interpreted as
that value.
o Less errors with noise over long distance.
o Used for computer data: A single bit wrong would be disaster
o
 Data Transmission from one point to another is either transmitted in parrallel or in serial.
This is dependant on the type of carrier or media used. Serial transmission only requires a
single “wire” or line where as parallel transmission will require more for example 8.
Following Diagram illustrates the Conversion of Serial data to Parallel

Serial and parallel transmission

In telecommunications, serial transmission is the sequential transmission of signal
elements of a group representing a character or other entity of data. Digital serial
transmissions are bits sent over a single wire, frequency or optical path sequentially.
Because it requires less signal processing and less chances for error than parallel
transmission, the transfer rate of each individual path may be faster. This can be used
over longer distances as a check digit or parity bit can be sent along it easily.

In telecommunications, parallel transmission is the simultaneous transmission of the

signal elements of a character or other entity of data. In digital communications,
parallel transmission is the simultaneous transmission of related signal elements over
two or more separate paths. Multiple electrical wires are used which can transmit
multiple bits simultaneously, which allows for higher data transfer rates than can be
achieved with serial transmission. This method is used internally within the computer,
for example the internal buses, and sometimes externally for such things as printers,
The major issue with this is "skewing" because the wires in parallel data transmission
have slightly different properties (not intentionally) so some bits may arrive before
others, which may corrupt the message. A parity bit can help to reduce this. However,
electrical wire parallel data transmission is therefore less reliable for long distances
because corrupt transmissions are far more likely.

Asynchronous transmission entails one character to be transmitted at a time; each character

is preceded by a start bit and followed by a stop bit. This type of transmission is usually used
by PCs, and is fast and economical for relatively small amounts of data
Communication Satellite allow data/information to be bounced of the satelites in geo-
synchronous orbit over 20,000 miles above earth (geo-synchronous – rotating at the same
speed as earth, hence stationary relative to the earth)

Microwave is similar to radio waves. Due to the earths curvature and as microwaves travel in
straight lines, microwaves stations can not be 30 miles apart. This is one key reason why you
may experience no reception on your cell phones; you may be out of reach from a
transmitter /receiver.

Hence the need for many receivers/transmitters in a small geographical area (Cells)