Application layer
Application layer
The application layer enables the user, whether human or software, to access the network. Application layer provides user interfaces and support for services such as electronic mail. Remote file access and transfer, share database management and other types of distributed information services. Specific services provided by the application layer include the following:
Ø Network virtual terminal. A network virtual terminal is a software version of physical terminal and allow user to log on to a remote host. To do the application creates a software emulation of a terminal at the remote host. The user's computer talk to the software terminal, which in turn talks to the host, vice versa. The remote host believes it is communicating with one of its own terminals and allows you to log on.
Ø File transfer, access and management (FTAM). This application allows a user to access files in remote host (to make change or read data), to retrieve files form a remote computer for use in the local computer, and to manage or control files in a remote computer locally.
Ø E-mail services. This application provides the basis for e-mail forwarding and storage.
Ø Directory services. The application provides distributed database sources and access for global information about various objects and services.
Presentation layer
Presentation layer
The presentation layer is concerned with the syntax and semantics of the information exchange between two systems. Specific responsibilities of the presentation layer include the following:
Ø Translation. The process(running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information should be changed to bit streams before being transmitted. Because different computers use different encoding system, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent into a common format. The presentation layer at the receiving machine changes the common format into its receiver. Dependent format.
Ø Encryption. To carry sensitive information a system must be able to assure privacy. Encryption means that the sender transformer the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.
Ø Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio and video.
Session layer
Session layer
The services provided by the first four layers (physical, data link, network and transport) are not sufficient for some processes. The session layer is the network dialogue controller. It establishes, and synchronizes the interaction between communicating system. Specific responsibilities of the session layer include the following:
Ø Dialogue control. The session layer allows two system to enter into a dialogue. It allows the communication between two processes to take place in either half-duplex (one way at a time) or full-duplex (two way at a time) mode.
Ø Synchronization. The session layer allows a process to add check points (synchronization points) into a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert check point after every 100 pages to ensure that each 100-page unit is received and acknowledged independently In this case, if a crash happens during the transmission of page 515, the only pages that need be resent after system recovery are pages 475 to. Pages previous to 475 need not be resent.
Transport layer
Transport layer
The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on the host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belong to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Other responsibilities of the transport layer include the following:
Ø Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.
Ø Segmentation and reassembling. A message is divided into transmittable segments, with each segment containing sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.
Ø Connection control. The transport layer can be either 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 computer. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets after all the data is transferred, the connection is terminated.
Ø Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.
Ø Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than to across a single link. The sending transport layer make sure that the entire message arrives at the receiving transport layer without error (damage, loss or duplication). Error correction is usually achieved through transmission.
Network layer
Network Layer
The network layer is responsible for the source-to destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two system on the same network (link), the network layer ensures that each packet gets from its point if origin to its final destination.
If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached two different networks (link) with connecting devices between the networks (link), there is often a need for the network layer to accomplish source-to-destination delivery. Other responsibilities of the network layer include the following:
Ø Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system distinguish the source and destination system. The network layer adds a header to the packet coming from the upper layer that, among other thing, includes the logical addresses of the sender and receiver.
Ø Routing. When independent networks or links are connected together to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the function of the network layer is to provides this mechanism.
Data link layer
Data link layer
The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer(network layer). Other responsibilities of the data link layer include the following:
Ø Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frame.
Ø Physical addressing. If frames are to be distributed to different system on the network, the data link layers add a header to the frame to define the 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 one.
Ø Flow control. If the rate at which the data is absorbed by the receiver is less than the rate produced at the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.
Ø Error control. The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize a duplicate frame. Error control is normally achieved through a trailer added to the end of the frame.
Ø 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.
Physical Lyer
Layers in the OSI model
Physical layer
The physical layer coordinates the function required to carry a bit stream over a physical medium. Physical Layer deals with the mechanical and electrical specifications of the interface and transmission media Physical Layer also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.
The physical layer is responsible for moving individual bits from one
(NODE) to the next.
Physical layer is concerned with the following
Ø Physical characteristics of interfaces and media. The physical layer defines the characteristics of the interface between the devices and the transmission media. It also defines the type of transmission media.
Ø Representation of bits. The physical layer data consists of a stream of bits (sequence of 0s and 1s ) with no interpretation. To be transmitted bits must be encoded into signals (electrical or optical). The physical layer defines the type of encoding (how 0s and 1s are changed to signals).
Ø Data rate. The transmission rate – the number of bits sent each second –is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts.
Ø Synchronization The sender and receiver must not only use the same bit rate but must also be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.
Ø Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration a link is shared between several devices .
Ø Physical topology. The physical topology define how devices are connected to make a network. Devices can be connected using a mesh topology (every device connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next. Forming a ring), or a bus topology (every device ion a common link)
Ø Transmission mode The physical layer also defines the direction of transmission between two devices: simplex, half duplex. In the simplex mode, only one device can send; the other can only receive. The simplex mode is a one way communication. In the half duplex mode, two devices can send and receive, but not at the same time, in a full-duplex (or simple duplex) mode, two devices can send and receive at the same time.
SIP (Session Initiation Protocol)
SIP (Session Initiation Protocol)
SIP is an application layer protocol designed to be independent of the underlying transport layer; it can run on transmission control protocol (TCP), UDP, or SCTP. It is a text based protocol, incorporating many elements of the HTTP and the SMTP.The session Initiation Protocol is a signaling communication protocol, widely used for controlling multimedia communication sessions such as voice and video calls over IP.
The protocol defines the messages that are sent between peers which govern establishment, terminations and other essential element of call. SIP can be used for creating, modifying, and terminating two party (unicast) or multiparty (multicast) sessions consisting one or several media streams. Other SIP applications include video conferencing, streaming multimedia distribution, instant messaging, presence information, file transfer and online games.
SIP work in conjunction with several other application layer protocols that identify and carry the session media identification and negotiation is achieved with session description protocol (SDP). For the transmission of media stream a(voice, video) SIP typically employs the Real-Time transport protocol (RTP), which many be secured with the secure Real-Time transport protocol (RTP). For secure transmissions of SIP message the protocol may be encrypted with transport layer security TLS.
NNTP (Network News Transport Protocol)
NNTP (Network News Transfer Protocol)
The News Transport Protocol (NNTP) is an application protocol used for
transporting Usenet news articles (net news) between news server and for
reading and posting articles by end client application.
Usenet was originally designed based on the UUCP network, with most
article transfer taking place over direct point to point telephone link between
news sever and which were powerful time sharing systems. Readers and posters logged
into this computer reading the article directly from the local disc.
As local area network and internet participation it became describe to
allow news readers to be run on personal computers connected to local networks.
Because distributed files system were not yet widely available, a news protocol
was developed based on the client server model. It resembled the simple Mail Transfer
Protocol (SMTP), but was tailored for exchanging news group article.
A news reader, also known as a news client, is a software application
that reads article on Usenet, either directly from the news server's disks or
via the NNTP. The well-known TCP port 119 is reserved for NNTP. When client
connect to a news server with transport layer security (TLS), TCP port 563 is
used. This is sometimes referred to as NNTPS.
In October 2006, the IETF released RFC 3977 which updates the NNTP protocol
and codifies many of the additions over the years since RFC977.
Domain Name System
DNS (Domain Name System)
The
Domain Name System is a hierarchical distributed naming system for computers,
services, or any resource connected to internet or a private network. It
associates various information with domain names assigned to each of the
participating entities. Most prominently, it translates easily memorized domain
name to the numerical IP addresses needs for the purpose of locating computer
services and devices worldwide. By providing a worldwide, distributed
keyword-based redirection service, the Domain Name System is an essential component
of the functionality of the internet.
An
often-used analogy to explain the Domain Name System is that it serves as the
phone book for the internet by translating human-friendly computer hostnames
into IP addresses. For example the domain name www.example.com
translates to the addresses 192.163.0.10 (IPv4) and 2001:500:88:200::10 (IPv6).
Unlike a phonebook the DNS can be quick updated, allowing a service's location
on the network to change without effecting the end users, who continue to use
the same hostname. Users take advantage of this when they use meaningful
uniform resource locator (URL) and E-mail addresses without having to known how
the computer actually locates the services.
The
Domain Name System distributes the responsibility of assigning domain names and
mapping those names to IP addresses by designating authoritative name serves
for each domain. Authoritative name servers are assigned to be responsible for
their particular domain and in turn can assign other authoritative name servers
for their sub-domains. This mechanism has made the DNS distributed and fault
tolerant and has held avoid the need for a single central register to be
continually consulted and updated. Additionally the responsibility for
maintaining and updating the master record for the domain is spread among many
domain name registers, who compete for the end-user's (the domain-owner's)
business. Domain can be moved from one registrar to other registrar at any time.
The
Domain Name System also specifies the technical functionality of this data base
service. It defines the DNS protocol, a detailed specification of the data
structures and data communication exchanges used in DNS, as part of the
internet protocol suit.
What is Protocol?
Protocols
A
communication protocol is (Networking protocol) is a system of digital message
formats and rules for exchanging messages in or between computing system and in
telecommunications. A protocol may have a formal description. Protocol may include
signaling, authentication and error detection and correction capability.
In
a routing protocol, it specifies that how routers communicate with each other
and with the other types of machines. Protocols are determines and enable the
routes between the nodes on a computer network. Algorithms determine the
specific choice of routing. A router has knowledge only the direct attached
networks and a protocol shares information about the neighbors immediate and
then throughout the network. A router can understand the network topology
through the protocol. So we can say that a protocol is playing very important
role in a network. Although, there are many types of protocols.
Types of protocols
There
are many types of protocols for different purpose in networking.
Routing protocols
IS-IS,
OSPF, IGRP and EIGRP, RIP, BGP,
Internet protocols
Application Layer
DHCP,
DHCPv6, DNS, FTP, HTTP, IMAP, IRC, LDAP, MGCP, NNTP, BGP, NTP, POP, RPC, RTP,
RTSP, RIP, SIP, SMTP, SNMP, SOCKS, SSH, Telnet, TLS/SSL, XMPP.
Transport Layer
TCP, UDP, DCCP, SCTP, RSVP, TP-TCP, NC, MTP
Network Layer
IP(IPv4,IPv6),
ICMP, ICMPv6, ECN, IGMP, IPSec, GGP.
Link Layer
What is subnet mask?
Subnet mask
A network mask is used when a network is not sub netted. The
subnet mask is used to find the first address in the block. However when a mask
is sub netted, the situation is different. We must have a subnet mask. The
subnet has more 1s.
Subnetting increases the length of the netid and decrease
the length of hostid. When we divide a network to s number of subnetworks, each
of number of hosts, we can calculate the subnetid for each subnetwork as in
which n is the length of netid,.
Subnet Address
When a network is subnetted, the first address in the subnet
is the identifier of the subnet and is used by the router to route the packets
destined for that subnet works. Given any address in the subnet, the router can
find the subnet mask using the same procedure we discussed to find the
subnetwork mask. ANDing the given address with the subnet mask.
Super netting
Subnetting couldn’t completely solve address depletion problems
in classful addreeing, because most organization did not want to share their
granted blocks with others. since class C blocks were still available but the
size of block did not meet the requirement of new organization that wanted to
join the internet, one solution was super
netting . in super netting, an organization can combine several class C block
to create a larger range of addresses. In other, words several networks are
combined to create a supernet work. By doing this, an organizarion can apply
for several class C blocks instead of just one. For example, an organization
that needs 1000 addresses can be granted four class C blocks.
What is subnetmask?
Subnetting
In subnetting, a network divides into several smaller
networks with each subnetwork having its
own subnetworks. A portion of IP address is indicating the network (netid), and
a portion indicates the network hostid. This means that there is a sense of hierarchy
in IP addressing. To each a host on the internet, we must first reach the
network using the first portion of the address. Then we must reach the host
itself using the second portion. In the other word, IP address is designed with
two level of hierarchy. However in many cases two levels of hierarchy are not
enough. For example, imagine an organization with the network address
141.14.0.0 (a class B address). The organization has two hierarchical addressing,
but cannot have more than one physical network. One solution of this problem is
subnetting. Further division of a network into smaller networks is called
subnetting.
Three level of hierarchy
Adding subnetworks an intermediate level of hierarchy in the
IP addressing system. Now we have three levels; site, subnet and host. The site
is the first level. The second level is subnet and the tird level is host of
hierarchy, it defines the connection of the host to the subnetwork.
The
routing of an IP datagram now involves steps: delivery to site, delivery to subnet
work and delivery to the hostWhat is subnetmask?
Mask
If the network address is given, we can find the block and
the range of addresses in the block. What
about the reserves? If an address is given, can we find the given address (the
beginning address in the block). This is important because to route a packet to
the correct network, a router needs to extract a network address from the
destination address (a host address) in the packet header.
One way we can find the network address to first find the
class of the address and the net ID. We then set the host ID to zero to find
the network address. For example, if the address is 134.45.78.2 is given, we
can immediately say that the address belong to class B. the net ID is 134.45 (2
bytes) and the network address is
134.45.0.0.
The above method is feasible if we not subnetted the
network; that is, if we have not divided the network into subnetworks. A
general procedure that can be used involves a mask to find the network address
from a given address.
A mask is a 32 bits binary number that gives first address
in the block (the network address). When bit-wise ANDed with an address in the
block.
AND
Operation
Masking uses the bit-wise AND operation defined in computer
science. The operation is applied bit by bit to the address and the mask.
Introduction to IP address
Classes and Blocks
One problem with the classfull addressing is that each class
is dividing into a fixed number of blocks with each block having the fixed
size. Let’s look
Class A
Class A is divided into 128 blocks with each block having a
different net id. First block covers address from 0.0.0.0 to 0.255.255.255 (net
id 0). The second block covers address 1.0.0.0 to 1.255.255.255 (net id 1). The
last block covers address form 127.0.0.0 to 127.255.255.255 (net id 127).
Note: each block of addresses the first byte (net id) is the
same, but the other three bytes (host id) can take any value in the given
range.
The first and last block in this are reserved for special
purpose as we will discuss shortly. In addition one block is used for private
address. The remaining 125 blocks can be assigned to organization. This means
that the total number of organization that can have class A address is only
125. However, each block in this class contains 16,777,216 addresses. This
address is called network address. It defines the network of the organization,
not individual hosts. The organization is not allowed to use last address; it
is reserved for a special purpose. Class A address is design for large
organization with a large number of hosts or routers attached t their network.
Class B
Class B is divided into 16,384 blocks with each block having
a different net id. Sixteen blocks are reserved for private addresses, leaving
it 16,368 blocks for assignment to organization. The first block covers address
form 128.0.0.0 to 128.0.255.255 (net id 128.0). the last block covers address
from 191.255.0.0 to 191.255.255.255 (net id 191.255).
Note: each block of address the first 2 bytes net ID are
same but other 2 bytes are host ID can take any value in the given range.
There are 16,368 blocks that can be assigned. This means
that the total number of organization that can have class B address is 16,368.
However, each block in this class contain 65,536 addresses, the organization
should be large enough to use all of these addresses.
Class B was designed for mid size organization that many
have tens of thousands of hosts or routers attached to their network. However,
the number of addresses in each block 65,536 is larger than the needs of most
midsize organizations.
Class C
Class C is divided into 2097152 blocks with each block
having a net ID. 256 blocks are used for private addresses, leaving 2,096,896
blocks for assignment to organization. The first block covers address from
192.0.0.0 to 192.0.0.255 (net id 192.0.0). the last address covers address from
223.255.255.0 to 223.255.255.255 (net id 223.255.255).
Note: the each block of address the first 3 bytes are the
same but last one byte can take any value in the given range. There are 2096902
blocks that can be assigned have a class C address is 2096902. However, each
block in this class contains 256 addresses, which means the organization should
be small enough to need less than 256 addresses.
Class C was designed for small size organization with a
small number of hosts or routers attached to their network.
Class D
There just one block of class D addresses. It designed for
multicasting. Each address in this class is used to define one group of hosts on
the internet. When a group is assigned an address in this class, every host
that is member of this group will have a multicast address in addition to its
normal (unicast) address.
Class E
There is just one block also in class E address. It was
designed for used as reserved address. The last address in this class
255.255.255.255 is used for a special address.
Network
Addresses
Network addresses play a very important role in classfull
addressing. A classful address has several properties:
1-
The network address is the first address in the
block
2-
The network address defines the network to the
rest of internet.
3-
Given the network address, we can find the class
of the address, the block and the range of the addresses in the block.
We try to understand with the example:
Given the network address 132.210.0.0, class is B because the first byte is between the 128 to 191.
The block has net id of 132.21, the address range from 132.21.0.0 to
132.21.255.255.
Introduction to IP address
Class full IP Addressing
IP addressing,
when started a few decades ago, used the concept of classes. This architecture is
called class full IP addressing. In the mid of 1990s, a new architecture, Called classless IP addressing was introduced that will eventually supersede the original architecture.
However, most of the internet is still using calssfull addressing and the
migration is slow. To understand the classfull IP addressing it is important to
understand classless addressing.
In classfull IP addressing, the IP address is divided into five
classes A,B,C,D and E. each class is occupies some part of the whole address
space, see below the table mansion the class occupation of the address space (approximate).
A
|
|||
B
|
C
|
D
|
E
|
We can see that the class A covers the half of the address
space, a serious design flow. Class B cover ¼ of the whole address space,
another design flow. Class C is cover the 1/8 of the address, and class D and E
each cover the 1/16 of the address space.
Recognize the classes
We can find the class of an address when the address is
given in binary notation or in dotted decimal notation.
Find the class in
binary notation
If the address is given in binary notation, the first bits
can immediately tell us the class of the address.
Find the class in
dotted decimal notation
When the address is given in dotted decimal notation, then
we need to look only at the first byte to determining the class of the address.
Each class has a specific range of number.
Net ID and Host ID
In classfull addressing an IP in classes A,B and C is
divided into net id and host id.
Note: Class D and
E are not divided into net id and host id.
A
|
Net ID
|
Host ID
|
||
B
|
Net ID
|
Host ID
|
||
C
|
Net ID
|
Host ID
|
||
D
|
Reserved For Multicast
|
|||
E
|
Reserved For future use
|
|||
Intriduction to IP address
Classes of IP address
Sr. #
|
Class
|
Range
|
Bits
|
Network/host
|
Subnet mask
|
01
|
A
|
1-126
|
0000
|
8/24
|
255.0.0.0
|
02
|
B
|
128-191
|
1000
|
16/16
|
255.255.0.0
|
03
|
C
|
192-223
|
1100
|
24/8
|
255.255.255.0
|
04
|
D
|
224-239
|
1110
|
N/A
|
N/A
|
05
|
E
|
240-254
|
N/A
|
N/A
|
N/A
|
01. 0 is represent the network
02. 127 is reserved for testing
03. 255 is reserved for broadcast
Introduction to IP address
IP Address
At the network layer, we need to uniquely identify each
device on the internet to allow global communication between all devices. This
is analogous to the telephone system where each telephone subscriber has a
unique telephone number. If we consider the country code and the area code as
part of the identifying scheme
The identifying used in IP layer
of the TCP/IP protocol suit to identify each device connected to the internet
is called the internet address or IP address. An IP address is a 32 bit binary
address that uniquely and universally defines the connection of a host or a
router to the internet. Two devices cannot have the same IP address on the
internet, it is unique address. However if a device has two connection to the
internet via two networks, it has two IP addressesTCP/IP Protocol Suit
Define Protocol in Networking
User Datagram Protocol (UDP)
The UDP is process to process protocol that adds only port
address, check-sum error control and length information to the data from the
upper layer.
Transport control protocol (TCP)
The TCP provides full transport layer services to
application. TCP is a reliable stream transport protocol. The term stream in
this context means connection-oriented: a connection must be established
between both ends of a transmission before either can transmit data.
Application Layer
The application layer in TCP/IP is equivalent to the
combined session, presentation and application layer in the OSI model. Many
protocols are defined at this layer.
Network Introduction to TCP/IP Protocol in simple words
User Datagram Protocol (UDP)
The UDP is process to process protocol that adds only port
address, check-sum error control and length information to the data from the
upper layer.
Transport control protocol (TCP)
The TCP provides full transport layer services to
application. TCP is a reliable stream transport protocol. The term stream in
this context means connection-oriented: a connection must be established
between both ends of a transmission before either can transmit data.
Network introduction to TCP/IP Protocol suit in simple words
Transport Layer
The transport layer is represented in TCP/IP by two
protocols, TCP and UDP. The IP is a host-to-host protocol; UDP and TCP are
transport level protocols. IP can deliver a packet form one physical device to
another. UDP and TCP are responsible for delivery of a message from a process
to another process (running programs)
TCP Internet protocol
TCP/IP PRTOCOL SUIT
Physical and Data link
Layer
At the physical and data link layer, TCP/IP does not define
any specific protocol. It support all of the standard and proprietary protocol
which LAN, MAN and WAN.
Network Layer
At the network layer TCP/IP supports the internetworking
protocol which contains four supported protocols: ARP, RARP, ICMP and IGMP
TCP/IP Protocol Suit
Define Protocol In Networking
Internetworking protocol (IP)
The internetworking protocol (IP) is the transmission
mechanism used by TCP/IP protocol. It is un reliable and connectionless
datagram protocol- a best-effort
delivery service. The term best-effort means that IP provides no
error checking or tracking.
Address Resolution Protocol (ARP)
The Address Resolution Protocol (ARP) is used to associate
an IP address with the physical address. ARP is used to find the physical
address of the node, when its internet address is known.
Reverse Address Resolution Protocol (RARP)
The (RARP) allows a host to discover its internet address
when it known only its physical address. It is used when a computer is
connected to the network for the first time or when a diskless computer is
booted.
Internet control message protocol (ICMP)
The ICMP is a mechanism used by hosts and gateways to send
notification datagram problems back to the sender.
Internet Group Message Protocol (IGMP)
The IGMP used to
facilitate the simultaneous transmission of the message to a group of
recipients.
List of Network devices
OSI Referance model
Encapsulation of Data
OSI Referance model
Role of Data Flow layer
OSI Referance model
ROLE OF APPLICATION LAYER IN NETWORK
OSI Reference model overview
OSI reference model overview
OSI Referance model
Network Distribution Layer Characteristics
The Model of Networks
The OSI Model
Established n 1947, the
international standards organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards. Almost three-fourths of countries in the world are represented in
the ISO. An ISO standard that covers all aspect of network communication is the
open system interconnection (OSI) model. It was first introduced in the late
1970s.
An Open System is a set of
protocols that allows any two systems to communicate regardless of their
underlying architecture. The purpose of the OSI model is to show how to
facilitate communication between different system without requiring changes to
the logic of the underlying hardware and software. The OSI model is not a
protocol; it is the model for understanding and designing a network
architecture that is flexible, robust and interoperable. The OSI model was
intended to be the basis for the creation of the protocol in the OSI stack.
The OSI model is a layered
framework for the design of network system that allows communication between
all types of computer systems. It consists of seven separate but related
layers, each of which defines a part of the process of moving information
across a network. Understanding the fundamentals of the OSI model provides a
solid basis for exploring data communication.
The OSI Model
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Application
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Presentation
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Session
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Transport
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Network
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Data
link
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Physical
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Layer 7
Layer6
Layer5
Layer4
Layer3
Layer2
Layer1
Layered Architecture
The OSI model is composed of seven
ordered layers. The layers involved when a message is sent from device A to
Device B. As the message travels from A to B, it may pass through many
intermediate nodes. These intermediate nodes usually involve only the first
three layer of the OSI model.
In developing the model, the
designers distilled the process of transmitting data to its most fundamental
elements. They identified which networking functions had related uses and
collected those function into discrete group that became the layers. Each layer
defines a family of functions distinct from those of the other layers. By
defining and localizing functionality in this fashion, the designers created an
architecture that is both comprehensive and flexible. Most important, the OSI
model allows complete interoperability between otherwise incompatible system.
Within a single machine, each
layer calls upon the services of the layer just below it. Layer 3, for example,
uses the services provided by layer 2 and provides services to the layer 4.
Between machines, layer x on one machine logically communicate with the layer x
on another machine. This communication is governed by an agreed-upon services
of rules and conventions called protocols.
Layer-to-Layer communication
Device A sends to message device
B(through intermediate nodes). At the sending site the message is moved down
from layer 7 to layer 1. At layer 1 the entire package is converted to a form
that can be transferred to the receiving site. At the receiving site, the
message is moved up from layer 1 to layer 7.
Interfaces between layers
The passing of the data and
network information down through the layers of the sending device and backup
through the layers of the receiving device is made possible by an interface
between each pair of adjacent layer. Each Interface defines what information
and services a layer must provide for the Layer above it. Well-defined
interfaces and layer functions provide modularity to a network. As long as a
layer provides the expected services to the layer above it, the specific
implementation of its functions can be modified or replaces without requiring
changes to the surrounding layers.
Organization of the layers
The seven layers can be thought of
as belonging to three subgroup. Layer 1,2, and 3-physical, data link and
network – are the network support layer; they deal with the physical aspects of
moving data from one device to another (such as electrical specifications,
physical connections, physical addressing and transport timing and
reliability). layers 5,6 and 7 – session presentation and application can be
thought of as the user support layers; they allow interoperability among un
related software systems. Layer 4, the transport layer, links the two subgroups
and ensures that what the lower layers have transmitted is in a form that the
upper layers can use. The upper OSI layers are almost always implemented in
software; lower layers are a combination of hardware and software, except for
the physical layer, which is mostly hardware.
In this image, which gives an
overall view of the OSI layers, D& data means the data unit at layer 7, D6
data means the data unit at layer 6, and so on. The process starts at layer 7
(the application layer), then moves from layer to layer in descending,
sequential order. At each layer, a header can be added to the data unit. At
layer 2, a trailer may also be added. When the formatted data unit passes
through the physical layer (layer 1), it is changed into electromagnetic
signals and transported along a physical link.
Upon reaching its destination, the
signals passes into layer 1 and is transformed back into digital form. The data
units then move backup through OSI layers. As each block of data reaches the
next higher layer, the headers and trailers attached to it at the corresponding
sending layer are removed and action appropriate to that layer are taken. By
the time it reaches layer 7, the message is again a form appropriate to the
application and is made available to the recipient.
Encapsulation
Reveals another aspect of data communication
in the OSI model: encapsulation. A packet at level 7 is encapsulated in the
packet at level 6. The whole packet at level 6 is encapsulated in a packet at
layer 5 and so on.
In other words, the
data part of a packet at level N is carrying the whole packet (data and
overhead) from level N+1. The concept is called encapsulation because level N
is not aware what part of the encapsulated packet is data and what part is the
header or trailer. For level N, the whole packet coming from level N+1 is
treated as one integral unit.
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