NETWORK STANDARDS
WHAT ARE NETWORKING STANDARDS?
A networking standard is a document
that's been developed to provide technical requirements, specifications, and
guidelines that must be employed consistently to ensure devices, equipment, and
software which govern networking are fit for their intended purpose.
NETWORK STANDARDS AND DEVICES
All networks are built on the same standards;
networking stardards govern the hardware and software that uses them and define
the rules for data communications that are needed for interoperability between
devices, official organizations that publish regulated standards are:
Ø International Standards Organization (ISO)
Ø International Telecommunication Union (ITU)
Ø Institute of Electronics and Electrical Engineers
(IEEE)
Ø American National Standards Institute (ANSI)
Ø Internet Research Task Force (IETF)
Ø Electronic Industries Association (EIA)
During
data communication different standards can be used at the same time at
different layers, the commonly used are:
Application
layer − HTTP, HTML, POP, H.323, IMAP
Transport layer − TCP, SPX
Network layer −IP, IPX
Data link layer − Ethernet IEEE 802.3, X.25, Frame Relay
Physical layer −RS-232C (cable), V.92 (modem)
WHAT’S THE OSI MODEL?
The Open
Systems Interconnection Model (short:OSI
model) was designed by the International Organization for
Standardization (IOS) as a reference model for open communication through various
technical systems. This makes sense when you think about the early
days of the internet: at the end of the 1970s, all of the leading manufacturers
for network technology were dealing with the problems brought on by proprietary
network infrastructures. Following this, devices from ‘manufacturer A’ could only be used within that
same company’s own network, as these products were not compatible with the
networks of its competitors. At the time, there simply was no
incentive in place to encourage engineers to design their products and systems
with those of rival companies in mind. The dawn of the internet completely
changed this paradigm, since common standards were needed in order to
facilitate shared communication and network access.
The OSI model is the result of such
standardization attempts and, as conceptual framework, offers a design basis
for creating communication standards independently of manufacturers. In order
to achieve this, the ISO OSI model subdivides the complex process of network communication
into seven levels, also referred to as layers. This is where the
term, OSI layer model comes from. When communicating between two systems, specific tasks
have to be carried out on each layer. Such tasks include communication control,
addressing target systems, or translating data packages into physical signals.
This only works, however, when all systems involved with communication adhere
to certain rules, which are determined in protocols.
The ISO reference model, on the other
hand, is not a concrete network standard. Instead, it describes, in abstract
form, which processes need to be regulated so that network communication is
able to take place.
The
Open System Interconnection (OSI) Model is a seven layer model developed by
International Organization for Standardization (ISO) in the late 1970s. It is a layered, abstract definition for
communications protocol and computer network protocol design. Every layer has
certain functionalities which provide services to the layer immediately above
it and below it. The seven layers are, from Top to Bottom, Application, Presentation,
Session, Transport, Network, Data link and Physical layer. The OSI
Layered Model defines the following seven layers:
7 - Application Layer
6 - Presentation layer
5 - Session layer
4 - Transport layer
3 - Network layer
2 - Data Link layer
1 - Physical layer
The OSI layers are often referred to by
their layer number, for example, "IP is a layer 3 protocol".
It is important to note that the OSI
model has long been retired as a real practical use stack in favor of the
TCP/IP stack. However, for educational purposes and for a more complete
breakdown of how intercommunication works, the OSI model is used for that
purpose.
Application Layer
The Application layer provides network
services to the user's applications. In essence, this does not refer to the
actual application itself (ie. Email client, Web browser) but the actual
protocols that it utilizes such as: HTTP, POP3, IMAP, SMTP, DHCP, DNS and many
others in that same category. These higher level protocols are what the
application uses to present the information to you.
Presentation Layer
The Presentation Layer exists to make
sure that the Application Layer of the sender sends information that can be
read by the Application Layer of the receiver. Included in this layer is data
conversion, data compression, and data encryption.
In typical real world scenarios this
layer is not always used so don't be confused and figure out why sometimes it
doesn't make a lot of sense that you can't include this in most
intercommunication facets.
Session Layer
The Session Layer is responsible for the
establishment, management, and termination of communication sessions between
two hosts software applications. This may seem confusing as you might believe
that this is the job of TCP to establish, maintain and terminate communication
between hosts.
The session layer deals more with how
two software applications establish, maintain and terminate communication among
themselves completely separate from the actual practical intercommunication of
data packets. The capabilities of the session layer are almost always found in
API's (Application Programming Interfaces). Examples of session layer
"protocols" are: NetBIOS, TCP/IP Socket, RPC and Unix Sockets.
Transport Layer
In the most basic sense the Transport
Layer controls the segmentation and reassembly of the message. This layer is
also responsible for error control and flow control. This layer is primarily
known for two protocols that you will encounter in real world scenarios: TCP
and UDP.
Transmission Control Protocol is the
famous TCP in TCP/IP. Transmission Control Protocol is a session/connection
oriented protocol. UDP (Uniform Datagram Protocol) is a connectionless oriented
protocol that provides no error recovery. It does everything else that TCP does
minus that one important aspect. This is useful for communications that do not
need reliable communication such as: DNS and VOIP.
TCP and UDP are responsible for taking
many connections coming into a server and forwarding the data correctly based
upon port numbers. Thus, TCP and UDP are responsible for port numbering for
applications. This is best explained if you have ONE server that is running
these services: Web Server (80), DNS (53), SMTP (25), IMAP (143) and POP (110).
Many people can connect to this server for several reasons. Some people just
want to view a web page, thus, the TCP header will include the destination port
as 80 and they will get their web page and if they want to relay mail the TCP
header includes 25 and the SMTP service answers forwarding the mail.
TCP, unlike UDP, offers error recovery
through a process called windowing using SYN and ACK controls. Essentially, a
computer send a SYN (Synchronize) to a server and the server responds with an
ACK (Acknowledgement) and in that the server itself also includes it's own SYN flag
as well for the ACK that will follow from the host and it also includes the
size of the window (the amount of unconfirmed packets that can be sent at once
before an ACK is sent back). Windowing allows a certain amount of packets to
arrive while not receiving an ACK. Once this window is full, in theory, the
server responds with an ACK and if that ACK number is in sequence with the next
SYN to be sent the sending host knows it received all the packets. If it
doesn't get a response it sends all the packets over again and if the ACK from
the server is a numeric value LESS than what was sent the host resend the
packets that were after that and waits for the correctly numbered ACK.
Network Layer
The Network Layer is responsible for the
transport of a packet from one network to another. Logical addressing is used
to achieve this. This is not required if two computers are directly connected
to one another such as in a LAN setting where MAC addressing can be used to
communicate among two computers.
What exists at this layer is most
famously the IP in TCP/IP (Internet Protcol v4 and v6). This layer deals with
IP addresses to provide "logical" addressing of systems both in a LAN
and WAN environments. At this layer routing also takes place to route data
between two different subnets. The network layer is also responsible for
fragmentation of the packets if they are to be sent down a Layer 2 link that
has a smaller MTU.
Most importantly IP is used for routing
and logical addressing of machines.
Data Link Layer
The Data Link Layer is responsible for
the transport of ‘frames’ on the same network. Physical addressing is used to
achieve this. This layer implements access control to determine which devices
are able to transmit on a network with multiple devices using such processes as
CSMA/CD (Carrier Sense Multiple Access / Collision Detection) on networks that
utilize full duplex communication.
This layer refers to the physical
addressing (MAC addresses) that are burned into the each NIC (Network Interface
Card) and is unique among all cards in the world, although the MAC address of
certain network adapters can be changed, such as SpeedDemon network
adapters.
Physical Layer
The Physical Layer controls the
transmission and reception of the bit stream over a physical medium. This layer
defines mediums such as UTP/STP, Fiber, Coax etc. It defines cabling pin outs,
electrical conductivity, light amplification (fiber), cabling distance and the
other physical features.
Goals of the OSI Model
The goal of the OSI Model was to produce an open
and standardized network model that would allow vendor-independent
communication between networked devices. This was to offer an
alternative to the many proprietary protocols developed by companies at the
time which had effectively tied customers into buying from one main provider.
Alongside the TCP/IP Model, the OSI Model also helps to teach students of networking
by breaking down network transmissions into easily understandable modules so it
is not uncommon to come across either of the models in a learning environment.
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