Thandra Aswinikumar Bharathkumar Kaushik
What is a Network Protocol?
A network protocol is a set of rules for communicating between
computers. Protocols govern format, timing, sequencing, and error control.
Without these rules, the computer cannot make sense of the stream of incoming
Is That All?
There is more than just basic communication. Suppose you plan to
send a file from one computer to another, you could simply send it all in one
single string of data. Unfortunately, that would stop others from using the
network for the entire time it takes to send the message. The other users would
not appreciate this. If an error occurred during the transmission, the entire
file would have to be sent again. To resolve both of these problems, the file is
broken into small pieces called packets and the packets are
grouped in a certain fashion. This means that information must be added to tell
the receiver where each group belongs in relation to others, but this is a minor
issue. To further improve transmission reliability, timing information and error
correcting information are added.
of this complexity, computer communication is broken down into steps. Each step
has its own rules of operation and, consequently, its own protocol. These steps
must be executed in a certain order, from the top down on transmission and from
the bottom up on reception. Because of this hierarchical arrangement, the term
protocol stack is often used to describe these steps. A protocol stack,
therefore, is a set of rules for communication, and each step in the sequence
has its own subset of rules.
this protocol is actually placed?
software that resides either in a computer's memory or in the memory of a
transmission device, like a network interface card. When data is ready for
transmission, this software is executed. The software prepares data for
transmission and sets the transmission in motion. At the receiving end, the
software takes the data off the wire and prepares it for the computer by taking
off all the information added by the transmitting end.
types of protocols.
are many protocols, and this often leads to confusion. A Novell network’s IPX/SPX,
Microsoft’s NetBEUI, DEC’s DECnet,
and IBM’s NetBIOS. Since the transmitter and the receiver
have to “speak” the same protocol, these four systems cannot talk directly
to each other. And even if they could directly communicate, there is no
guarantee the data would be usable once it was communicated.
who's ever wanted to transfer data from an IBM-compatible personal computer to
an Apple Macintosh computer realizes that what should be a simple procedure is
anything but. These two popular computers use widely differing-and
incompatible-file systems. That makes exchanging information between them
impossible, unless you have translation software or a LAN. Even with a network,
file transfer between these two types of computers isn't always transparent.
types of personal computers can't communicate easily, imagine the problems
occurring between PCs and mainframe computers, which operate in vastly different
environments and usually under their own proprietary operating software and
protocols. For example, the original IBM PC's peripheral interface-known as a
bus-transmitted data eight bits at a time. The newer 386, 486, and Pentium PCs
have 32-bit buses, and mainframes have even wider buses. This means that
peripherals designed to operate with one bus are incompatible with another bus,
and this includes network interface cards (NICs). Similar incompatibilities also
exist with software. For instance, Unix-based applications (and often the data
generated with them) cannot be used on PCs operating under Windows or MS-DOS.
Resolving some of these incompatibilities is where protocol standards
is protocol standards?
protocol standard is a set of rules for computer communication that has been
widely agreed upon and implemented by many vendors, users, and standards bodies.
Ideally, a protocol standard should allow computers to talk to each other, even
if they are from different vendors. Computers don't have to use an
industry-standard protocol to communicate, but if they use a proprietary
protocol then they can only communicate with equipment of their own kind.
are many standard protocols, none of which could be called universal, but the
successful ones can be characterized with something called the OSI model. The
standards and protocols associated with the OSI reference model are a
cornerstone of the open systems concept for linking the literally dozens of
dissimilar computers found in offices throughout the world.
THE OSI MODEL
Open System Interconnection (OSI) model includes a set of protocols that attempt
to define and standardize the data communications process. The International
Organization defined the OSI protocols for Standardization ISO. The OSI
protocols have received the support of most major computer and network vendors,
many large customers, and most governments, including the United States.
OSI model is a concept that describes how data communications should take place.
It divides the process into seven groups, called layers. Into these layers are
fitted the protocol standards developed by the ISO and other standards bodies,
including the Institute of Electrical and Electronic Engineers (IEEE), American
National Standards Institute (ANSI), and the International Telecommunications
Union (ITU), formerly known as the CCITT (Comité Consultatif International Téléphonique
OSI model is not a single definition of how data communications actually takes
place in the real world. Numerous protocols may exist at each layer. The OSI
model states how the process should be divided and what protocols should be used
at each layer. If a network vendor implements one of the protocols at each
layer, its network components should work with other vendors' offerings.
OSI model is modular. Each successive layer of the OSI model works with the one
above and below it. At least in theory, you may substitute one protocol for
another at the same layer without affecting the operation of layers above or
below. For example, Token Ring or Ethernet hardware should operate with multiple
upper-layer services, including the transport protocols, network operating
system, inter-network protocols, and applications interfaces. However, for this
interoperability to work, vendors must create products that meet the OSI model's
each layer of the OSI model provides its own set of functions, it is possible,
to group the layers into two distinct categories. The first four layers-
physical, data link, network, and transport-provide the end-to-end services
necessary for the transfer of data between two systems. These layers provide the
protocols associated with the communications network used to link two computers
top three layers-the application, presentation, and session layers-provide the
application services required for the exchange of information. That is, they
allow two applications, each running on a different node of the network, to
interact with each other through the services provided by their respective
graphical illustration of the OSI model is shown above. The following is a
description of just what each layer does.
protocols in the real world do not conform precisely to these neat definitions.
Some network products and architectures combine layers. Others leave layers out.
Still others break the layers apart. But no matter how they do it, all working
network products achieve the same result-getting data from here to there. The
question is, do they do it in a way that is compatible with networks in the rest
of the world?
discussing the OSI reference model it is important to understand what the model
does not specify as well as what it actually spells out. The ISO created the OSI
reference model solely to describe the external behavior of electronics systems,
not their internal functions.
reference model does not determine programming or operating system functions,
nor does it specify an application-programming interface (API). Neither does it
dictate the end-user interface-that is, the command-line and/or icon-based
prompts a user uses to interact with a computer system.
OSI standards merely describe what is placed on a network cable and when and how
it will be placed there. It does not state how vendors must build their
computers, only the kinds of behavior these systems may exhibit while performing
certain communications operations.
OSI standards are distinct from the OSI suite of protocols. This concept permits
a vendor to develop network elements that are more or less ignorant of the other
components on the network. They are said to be ignorant in that they may need to
know that other network components exist, but not the specific details about
their operating systems or interface buses. One of the primary benefits of this
concept is that vendors can change the internal design of their network
components without affecting their network functionality, as long as they
maintain the OSI-prescribed external attributes. The figure below shows the
protocols in the OSI suite.
OSI protocol suite is inherently connection-oriented, but the services each OSI
layer provides can either be connection-oriented, or connectionless. In the
three-step connection-oriented mode operation (the steps are connection
establishment, data transfer, and connection release), an explicit binding
between two systems takes place.
connectionless operation, no such explicit link occurs; data transfer takes
place with no specified connection and disconnection function occurring between
the two communicating systems. Connectionless communication is also known as
the physical layer
compare some real protocols to the OSI model. The best known physical layer
standards of the OSI model are those from the IEEE. That is, the ISO adopted
some of the IEEE's physical network standards as part of its OSI model,
including IEEE 802.3 or Ethernet, IEEE 802.4 or token-passing bus, and IEEE
802.5 or Token Ring. ISO has changed the numbering scheme, however, so 802.3
networks are referred to as ISO 8802-3, 802.4 networks are ISO 8802-4, and 802.5
networks are ISO 8802-5.
physical layer standard defines the network's physical characteristics and how
to get raw data from one place to another. They also define how multiple
computers can simultaneously use the network without interfering with each
other. (Technically, this last part is a job for the data-link layer, but we'll
deal with that later.)
802.3 defines a network that can transmit data at 10Mbps and uses a logical bus
(or a straight line) layout. (Physically, the network can be configured as a bus
or a star.) Data is simultaneously visible to all machines on the network and is
nondirectional on the cable. All machines receive every frame, but only those
meant to receive the data will process the frame and pass it to the next layer
of the stack. Network access is determined by a protocol called Carrier Sense
Multiple Access/Collision Detection (CSMA/CD). CSMA/CD lets any computer send
data whenever the cable is free of traffic. If the data collides with another
data packet, both computers “back off,” or wait a random time, then try
again to send the data until access is permitted. Thus, once there is a high
level of traffic, the more users there are, the more crowded and slower the
network will become. Ethernet has found wide acceptance in office automation
802.4 defines a physical network that has a bus layout. Like 802.3, Token Bus is
a shared medium network. All machines receive all data but do not respond unless
data is addressed to them. But unlike 802.3, network access is determined by a
token that moves around the network. The token is visible to every device but
only the device that is next in line for the token gets it. Once a device has
the token it may transmit data. The Manufacturing Automation Protocol (MAP) and
Technical Office Protocol (TOP) standards use an 802.4 physical layer. Token Bus
has had little success outside of factory automation networks.
802.5 defines a network that transmits data at 4Mbps or 16Mbps and uses a
logical ring layout, but is physically configured as a star. Data moves around
the ring from station to station, and each station regenerates the signal. It
does not support simultaneous multiple access as Ethernet does. The network
access protocol is token-passing. The token and data move about in a ring,
rather than over a bus as they do in Token Bus. Token Ring has found moderate
acceptance in office automation networks and a greater degree of support in
For Other Layers
are other physical and data-link layer standards, some that conform to the OSI
model and others that don't. ARCnet is a well-known one that only became
standardized in 1998, long after the time when it had any commercial
significance. It uses a token-passing bus access method, but not the same, as
does IEEE 802.4. LocalTalk is Apple's proprietary network that transmits data at
230.4Kbps and uses CSMA/CA (Collision Avoidance). Fiber Distributed Data
Interface (FDDI) is an ANSI and OSI standard for a fiber-optic LAN that uses a
token-passing protocol to transmit data at 100Mbps on a ring.
International Standards Organization, based in Geneva, Switzerland, is a
multinational body of representatives from the standards-setting agencies of
about 90 countries. These agencies include the American National Standards
Institute (ANSI) and British Standards Institute (BSI).
of the multinational nature of Europe, and its critical need for intersystem
communication, the market for OSI-based products is particularly strong there.
As a result, the European Computer Manufacturers' Association (ECMA) has played
a major role in developing the OSI standards. In fact, before the Internet's
Transmission Control Protocol/Internet Protocol (TCP/IP) began to dominate
international networks, European networking vendors and users were generally
further advanced in network standards, based on OSI implementations, than were
their American counterparts, who relied principally on proprietary solutions
such as IBM's Systems Network Architecture (SNA) or TCP/IP.
the OSI standards was a long, drawn-out process: The ISO began work on OSI
protocols in the late 1970s, finally releasing its seven-layer architecture in
1984. It wasn't until 1988 that the five-step standards-setting process finally
resulted in stabilized protocols for the upper layers of the OSI reference
The Data-Link layer (the second OSI layer) is often divided into
two sublayers; the Logical Link Control (LLC) and the Medium Access Control
(MAC). The IEEE also defines standards at the data-link layer. The ISO standards
for the MAC layer, or lower half of the data-link layer, were taken directly
from the IEEE 802.x standards.
Access Control, as its name suggests, is the protocol that determines which
computer gets to use the cable (the transmission medium) when several computers
are trying. For example, 802.3 allows packets to collide with each other,
forcing the computers to retry a transmission until it is sent successfully.
802.4 and 802.5 limit conversation to the computer with the token. Remember,
this is done in fractions of a second, so even when the network is busy, users
don't wait very long for access on any of these three network types.
upper half of the data-link layer, the LLC, provides reliable data transfer over
the physical link. In essence, it manages the physical link.
IEEE splits the data-link layer in half because the layer has two jobs to do.
The first is to coordinate the physical transfer of data. The second is to
manage access to the physical medium. Dividing the layer allows more modularity
and therefore more flexibility. The type of medium access control has more to do
with the physical requirements of the network than the actual management of data
transfer. In other words, the MAC layer is closer to the physical layer than the
LLC layer. By dividing the layer, a number of MAC layers can be created, each
corresponding to a different physical layer, but just one LLC layer can handle
them all. This increases flexibility and gives the LLC an important role in
providing an interface between the various MAC layers and the higher-layer
protocols. The role of the data-link's upper layer is so crucial, the IEEE gave
it a standard of its own: 802.2 LLC.
802.2, other protocols can perform the LLC functions. High-level Data-Link
Control (HDLC) is a protocol from ISO, which also conforms to the OSI model.
IBM's Synchronous Data-Link Control (SDLC) does not conform to the OSI model but
performs functions similar to the data-link layer. Digital Equipment's DDCMP or
Digital Data Communications Protocol provides similar functions.
ISO has established protocol standards for the middle layers of the OSI model.
The transport layer, at layer four, ensures that data is reliably transferred
among transport services and users. Layer five, the session layer, is
responsible for process-to-process communication. The line between the session
and transport layers is often blurred.
transport or session layer has been implemented on a widespread basis, nor has
the complete OSI protocol stack been established. To make matters more
confusing, most middle-layer protocols on the market today do not fit neatly
into the OSI model's transport and session layers, since many were created
before the ISO began work on the OSI model.
good news is many existing protocols are being incorporated into the OSI model.
Where existing protocols are not incorporated, interfaces to the OSI model are
being implemented. This is the case for TCP/IP, and IPX, which are the major
middle-layer protocols available today.
PC LAN environment, NetBIOS has
been an important protocol. IBM developed NetBIOS (or Network Basic Input/Output
System) as an input/output system for networks. NetBIOS can be considered a
session-layer protocol that acts as an application interface to the network. It
provides the tools for a program to establish a session with another program
over the network. Many programs have been written to this interface.
does not obey the rules of the OSI model in that it does not talk only to the
layers above and below it. Programs can talk directly to NetBIOS, skipping the
application and presentation layers. This doesn't keep NetBIOS from doing its
job; it just makes it incompatible with the OSI model. The main drawback of
NetBIOS is that it is limited to working on a single network.
or Transmission Control Protocol/Internet Protocol is actually several
protocols. TCP is a transport protocol. IP operates on the network layer. TCP/IP
traditionally enjoyed enormous support in government, scientific, and academic
internetworks and in recent years has dominated the commercial networking
environment, too. Part of the explanation is that corporate networks began to
approach the size of networks found in the government and in universities, which
drove corporations to look for internetworking protocol standards. They found
TCP/IP to be progressively more useful as it became more widespread. Many people
once viewed TCP/IP as an interim solution until OSI could be deployed, but no
one seriously believes that the OSI protocols will ever have more than a niche
role in the future.
when TCP/IP is discussed, the subjects of SMTP, FTP, Telnet, and SNMP are also
raised. These are application protocols developed specifically for TCP/IP. SMTP
or the Simple Mail Transfer Protocol is the electronic mail relay standard. FTP
stands for File Transfer Protocol and is used to exchange files among computers
running TCP/IP. Telnet is remote log-in and terminal emulation software. SNMP or
the Simple Network Management Protocol is the most widely implemented network
management protocol. The figure shows the protocols of TCP/IP.
traditionally used IPX/SPX as its native transport protocols, though the company
introduced a “native” implementation of TCP/IP in place of IPX/SPX.
Internetwork Packet Exchange (IPX) and Sequenced Packet Exchange (SPX) are both
variants of Xerox's XNS protocol. IPX provides network layer services, while SPX
is somewhat rarely employed by applications that need transport layer services.
Because IPX implementations prior to the introduction of NetWare Link Services
Protocol (NLSP) in NetWare 4 caused a great deal of broadcast traffic and
required frequent transmission acknowledgements, which can cause problems in a
WAN, Novell also supported TCP/IP with gateways prior to its native TCP/IP
transport layer protocols include XNS and NETBEUI. XNS or Xerox Network System
was one of the first local area network protocols used on a wide basis, mainly
for Ethernet networks. 3Com's 3+ used a version of it. NetBEUI is IBM's
transport protocol for its PC networking products.
number of available protocols seems like senseless confusion, it is and it
isn't. Certain protocols have different advantages in specific environments. No
single protocol stack will work better than every other in every setting.
NetBIOS works well in small PC networks but is practically useless for
communicating with WANs; APPC works well in peer-to-peer mainframe environments;
TCP/IP excels in internetworks and heterogeneous environments.
other hand, much more is made about the differences in protocols than is
warranted. Proprietary protocols can be perfect solutions in many cases.
Besides, if proprietary protocols are sufficiently widespread, they become de
facto standards, and gateways to other protocols are built. These include DEC's
protocol suite, Sun Microsystems' Network Filing System and other protocols, and
Apple's AppleTalk protocols. While these enjoy widespread use, that use is based
on the computers these companies sell and not the proliferation of the protocols
throughout the networking industry.
it's a proprietary or standard protocol, users are faced with difficult choices.
These choices are made slightly easier by the shakeout and standardization that
has occurred at the physical and data-link layers. There are three choices:
Token Ring Ethernet or FDDI . At the transport layers, IPX/SPX and TCP/IP
emerged as the dominant protocols.