FACULTY OF ELECTRIC ENGINEERING AND INFORMATION TECHNOLOGY Protocoale de Telecomunicații Telecommunications Protocols Professor Prof. Reiz… [625173]

UNIVERSITATEA DIN ORADEA
ROMANIA
FACULTATEA DE INGINERIE ELECTRICĂ ȘI TEHNOLOGIA
INFORMAȚIEI
FACULTY OF ELECTRIC ENGINEERING AND INFORMATION TECHNOLOGY
Protocoale de Telecomunicații
Telecommunications Protocols
Professor
Prof. Reiz RomolusAuthor
Mr. Corrado Cantoni

1
Communication Protocols
What are and why do we need them
Data communications and networking are changing the way we do business and the way
we live. Business decisions have to be made ever more quickly, and the decision makers
require immediate access to accurate information . Why wait a week for that report
from Germany to arrive by mail when it could appear almost instantaneously through
computer networks? Businesses today rely on computer networks and internet works.
But before we ask how quickly we can get hooked up, we need to know how networks
operate, what types of technologies are available, and which design best fills which set
of needs.
The development of the personal
computer brought about tremendous
changes for
business, industry, science, and
education. A similar revolution is
occurring in data
communications and networking.
Technological advances are making it
possible for
communications links to carry more and
faster signals. As a result, services are
evolving
to allow use of this expanded capacity.
Research in data communications and networking has resulted in new technologies. One goal is to
be able to exchange data such as text, audio, and video from all
points in the world. We want to access the Internet to download and upload information
quickly and accurately and at any time .
When we communicate, we are sharing information . This sharing can be local or
remote. Between individuals, local communication usually occurs face to face, while
remote communication takes place over distance. The term telecommunication, which includes
telephony, telegraphy, and television, means communication at a distance (tele is Greek for "far").
The word data refers to information presented in whatever form is agreed upon by
the parties creating and using the data.
Data communications are the exchange of data between two devices via some
form of transmission medium such as a wire cable.
For data communications to occur, the communicating devices must be part of a communication
system made up of a combination of hardware (physical equipment) and software (programs) .

2
The OSI reference model
The Structure and the layers
The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes
and standardizes the communication functions of a telecommunication or computing system
without regard to their underlying internal structure and technology. Its goal is the interoperability
of diverse communication systems with standard protocols. The model partitions a communication
system into abstraction layers. The original version of the model defined seven layers.
A layer serves the layer above it and is served by the layer below it . 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 comprise the contents of
that path. Two instances at the same layer are visualized as connected by a horizontal connection in
that layer.
The model is a product of the Open Systems Interconnection project at the International
Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.

Layer 1: Physical Layer
The physical layer defines the electrical and physical specifications of the data connection . It
defines the relationship between a device and a physical transmission medium (e.g., a copper or
fiber optical cable, radio frequency). This includes the layout of pins, voltages, line impedance,
cable specifications, signal timing and similar characteristics for connected devices and frequency
(5 GHz or 2.4 GHz etc.) for wireless devices. It is responsible for transmission and reception of
unstructured raw data in a physical medium. It may define transmission mode as simplex, half
duplex, and full duplex. It defines the network topology as bus, mesh, or ring being some of the
most common.
The physical layer of Parallel SCSI operates in this layer, as do the physical layers of Ethernet and
other local-area networks, such as token ring, FDDI, ITU-T G.hn, and IEEE 802.11 (Wi-Fi), as well
as personal area networks such as Bluetooth and IEEE 802.15.4.
The physical layer is the layer of low-level networking equipment, such as some hubs, cabling, and
repeaters. The physical layer is never concerned with protocols or other such higher-layer items.
Layer 2: Data Link Layer
The data link layer provides node-to-node data transfer—a link between two directly connected
nodes. It detects and possibly corrects errors that may occur in the physical layer . It, among
other things, defines the protocol to establish and terminate a connection between two
physically connected devices . It also defines the protocol for flow control between them.
IEEE 802 divides the data link layer into two sublayers:
Media Access Control (MAC) layer – responsible for controlling how devices in a network gain
access to medium and permission to transmit it.
Logical Link Control (LLC) layer – responsible for identifying Network layer protocols and then
encapsulating them and controls error checking and frame synchronization.
The MAC and LLC layers of IEEE 802 networks such as 802.3 Ethernet, 802.11 Wi-Fi, and
802.15.4 ZigBee, operate at the data link layer.
Layer 3: Network Layer
The network layer provides the functional and procedural means of transferring variable length
data sequences (called datagrams) from one node to another connected to the same
"network". A network is a medium to which many nodes can be connected, on which every node
has an address and which permits nodes connected to it to transfer messages to other nodes
connected to it by merely providing the content of a message and the address of the destination
node and letting the network find the way to deliver the message to the destination node, possibly
routing it through intermediate nodes. If the message is too large to be transmitted from one node to
another on the data link layer between those nodes, the network may implement message delivery

by splitting the message into several fragments at one node, sending the fragments independently,
and reassembling the fragments at another node. It may, but need not, report delivery errors.
Message delivery at the network layer is not necessarily guaranteed to be reliable ; a network
layer protocol may provide reliable message delivery, but it need not do so.
Layer 4: Transport Layer
The transport layer provides the functional and procedural means of transferring variable-
length data sequences from a source to a destination host via one or more networks , while
maintaining the quality of service functions.
An example of a transport-layer protocol in the standard Internet stack is Transmission Control
Protocol (TCP), usually built on top of the Internet Protocol (IP) .
The transport layer controls the reliability of a given link through flow control,
segmentation/desegmentation, and error control . Some protocols are state- and connection-
oriented. This means that the transport layer can keep track of the segments and retransmit those
that fail. The transport layer also provides the acknowledgement of the successful data transmission
and sends the next data if no errors occurred. The transport layer creates packets out of the
message received from the application layer. Packetizing is a process of dividing the long
message into smaller messages .
Layer 5: Session Layer
The session layer controls the dialogues (connections) between computers . It establishes,
manages and terminates the connections between the local and remote application . It provides for
full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment,
termination, and restart procedures. The OSI model made this layer responsible for graceful close of
sessions, which is a property of the Transmission Control Protocol, and also for session
checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session
layer is commonly implemented explicitly in application environments that use remote procedure
calls.
Layer 6: Presentation Layer
The presentation layer establishes context between application-layer entities , in which the
application-layer entities may use different syntax and semantics if the presentation service provides
a mapping between them. If a mapping is available, presentation service data units are encapsulated
into session protocol data units, and passed down the protocol stack.
This layer provides independence from data representation (e.g., encryption) by translating between
application and network formats. The presentation layer transforms data into the form that the
application accepts . This layer formats and encrypts data to be sent across a network. It is
sometimes called the syntax layer.

Layer 7: Application Layer
The application layer is the OSI layer closest to the end user, which means both the OSI
application layer and the user interact directly with the software application . This layer
interacts with software applications that implement a communicating component. Such application
programs fall outside the scope of the OSI model. Application-layer functions typically include
identifying communication partners, determining resource availability, and synchronizing
communication. When identifying communication partners, the application layer determines the
identity and availability of communication partners for an application with data to transmit. When
determining resource availability, the application layer must decide whether sufficient network
resources for the requested communication exist. In synchronizing communication, all
communication between applications requires cooperation that is managed by the application layer.
This layer supports application and end-user processes. Communication partners are identified,
quality of service is identified, user authentication and privacy are considered, and any constraints
on data syntax are identified. Everything at this layer is application-specific.
3
The TCP/IP reference model
The Structure and the layers
The Internet protocol suite is the conceptual model and set of communications protocols used on
the Internet and similar computer networks . It is commonly known as TCP/IP because the
original protocols in the suite are the Transmission Control Protocol ( TCP) and the Internet
Protocol (IP).
The Internet protocol suite provides end-to-end data communication specifying how data should
be packetized, addressed, transmitted, routed and received. This functionality is organized into four
abstraction layers which are used to sort all related protocols according to the scope of networking
involved. From lowest to highest, the layers are the link layer, containing communication methods
for data that remains within a single network segment (link); the internet layer, connecting
independent networks, thus providing internetworking; the transport layer handling host-to-host
communication; and the application layer, which provides process-to-process data exchange for
applications.
Link layer
The link layer has the networking scope of the local network connection to which a host is
attached. This regime is called the link in TCP/IP literature. It is the lowest component layer of the
Internet protocols, as TCP/IP is designed to be hardware independent. As a result, TCP/IP may be
implemented on top of virtually any hardware networking technology.

The link layer is used to move packets between the Internet layer interfaces of two different
hosts on the same link . The processes of transmitting and receiving packets on a given link can be
controlled both in the software device driver for the network card, as well as on firmware or
specialized chipsets. These perform data link functions such as adding a packet header to prepare it
for transmission, then actually transmit the frame over a physical medium. The TCP/IP model
includes specifications of translating the network addressing methods used in the Internet Protocol
to data link addressing, such as Media Access Control (MAC) . All other aspects below that level,
however, are implicitly assumed to exist in the link layer, but are not explicitly defined.
This is also the layer where packets may be selected to be sent over a virtual private network or
other networking tunnel. In this scenario, the link layer data may be considered application data
which traverses another instantiation of the IP stack for transmission or reception over another IP
connection. Such a connection, or virtual link, may be established with a transport protocol or even
an application scope protocol that serves as a tunnel in the link layer of the protocol stack. Thus, the
TCP/IP model does not dictate a strict hierarchical encapsulation sequence.
The TCP/IP model's link layer corresponds to the Open Systems Interconnection (OSI) model
physical and data link layers, layers one and two of the OSI model.
Internet layer
The internet layer has the responsibility of s ending packets across potentially multiple networks .
Internetworking requires sending data from the source network to the destination network. This
process is called routing.
The Internet Protocol performs two basic functions:
Host addressing and identification : This is accomplished with a hierarchical IP addressing
system.
Packet routing: This is the basic task of sending packets of data (datagrams) from source to
destination by forwarding them to the next network router closer to the final destination.
The internet layer is not only agnostic of data structures at the transport layer, but it also does not
distinguish between operation of the various transport layer protocols. IP carries data for a variety
of different upper layer protocols. These protocols are each identified by a unique protocol number:
for example, Internet Control Message Protocol (ICMP) and Internet Group Management
Protocol (IGMP) are protocols 1 and 2, respectively.
Some of the protocols carried by IP, such as ICMP which is used to transmit diagnostic information,
and IGMP which is used to manage IP Multicast data, are layered on top of IP but perform
internetworking functions. This illustrates the differences in the architecture of the TCP/IP stack of
the Internet and the OSI model. The TCP/IP model's internet layer corresponds to layer three of the
Open Systems Interconnection (OSI) model, where it is referred to as the network layer.

Transport layer
The transport layer establishes basic data channels that applications use for task-specific data
exchange. The layer establishes process-to-process connectivity, meaning it provides end-to-end
services that are independent of the structure of user data and the logistics of exchanging
information for any particular specific purpose. Its responsibility includes end-to-end message
transfer independent of the underlying network, along with error control, segmentation, flow
control, congestion control, and application addressing (port numbers). End-to-end message
transmission or connecting applications at the transport layer can be categorized as either
connection-oriented, implemented in TCP , or connectionless, implemented in UDP .
For the purpose of providing process-specific transmission channels for applications, the layer
establishes the concept of the port. This is a numbered logical construct allocated specifically for
each of the communication channels an application needs. For many types of services, these port
numbers have been standardized so that client computers may address specific services of a server
computer without the involvement of service announcements or directory services.
Because IP provides only a best effort delivery, some transport layer protocols offer reliability .
However, IP can run over a reliable data link protocol such as the High-Level Data Link Control
(HDLC).
The TCP/IP model's transport or host-to-host layer corresponds to the fourth layer in the
Open Systems Interconnection (OSI) model, also called the transport layer.
Application layer
The application layer includes the protocols used by most applications for providing user
services or exchanging application data over the network connections established by the lower
level protocols, but this may include some basic network support services, such as many routing
protocols, and host configuration protocols. Examples of application layer protocols include the
Hypertext Transfer Protocol ( HTTP), the File Transfer Protocol ( FTP), the Simple Mail Transfer
Protocol (SMTP), and the Dynamic Host Configuration Protocol ( DHCP). Data coded according to
application layer protocols are encapsulated into transport layer protocol units (such as TCP or UDP
messages), which in turn use lower layer protocols to effect actual data transfer.
Application layer protocols generally treat the transport layer (and lower) protocols as black boxes
which provide a stable network connection across which to communicate, although the applications
are usually aware of key qualities of the transport layer connection such as the end point IP
addresses and port numbers.
It is also sometimes necessary for network address translator ( NAT) traversal to consider the
application payload.
The application layer in the TCP/IP model is often compared as equivalent to a combination
of the fifth (Session), sixth (Presentation), and the seventh (Application) layers of the Open
Systems Interconnection (OSI) model.

Furthermore, the TCP/IP reference model distinguishes between user protocols and support
protocols. Support protocols provide services to a system. User protocols are used for actual user
applications. For example, FTP is a user protocol and DNS is a support protocol.
4
The Physical Layer
The lowest layer of the OSI Reference Model is layer 1, the physical layer; it is commonly
abbreviated “PHY”. The physical layer is special compared to the other layers of the model,
because it is the only one where data is physically moved across the network interface. All of the
other layers perform useful functions to create messages to be sent, but they must all be transmitted
down the protocol stack to the physical layer, where they are actually sent out over the network.
Note: The physical layer is also “special” in that it is the only layer that really does not apply
specifically to TCP/IP. Even in studying TCP/IP, however, it is still important to understand its
significance and role in relation to the other layers where TCP/IP protocols reside.
Understanding the Role of the Physical Layer
The name “physical layer” can be a bit problematic. Because of that name, and because of what I
just said about the physical layer actually transmitting data, many people who study networking get
the impression that the physical layer is only about actual network hardware. Some people may say
the physical layer is “the network interface cards and cables”. This is not actually the case, however.
The physical layer defines a number of network functions, not just hardware cables and cards.
A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't
strictly accurate. All hardware must have some relation to the physical layer in order to send data
over the network, but hardware devices generally implement multiple layers of the OSI model,
including the physical layer but also others. For example, an Ethernet network interface card
performs functions at both the physical layer and the data link layer.
Physical Layer Functions
The following are the main responsibilities of the physical layer in the OSI Reference Model:
Definition of Hardware Specifications : The details of operation of cables, connectors, wireless
radio transceivers, network interface cards and other hardware devices are generally a function of
the physical layer (although also partially the data link layer; see below).
Encoding and Signaling : The physical layer is responsible for various encoding and signaling
functions that transform the data from bits that reside within a computer or other device into signals
that can be sent over the network.
Data Transmission and Reception : After encoding the data appropriately, the physical layer
actually transmits the data, and of course, receives it. Note that this applies equally to wired and
wireless networks, even if there is no tangible cable in a wireless network!

Topology and Physical Network Design : The physical layer is also considered the domain of
many hardware-related network design issues, such as LAN and WAN topology.
In general, then, physical layer technologies are ones that are at the very lowest level and deal with
the actual ones and zeroes that are sent over the network. For example, when considering network
interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional
hubs and transceivers. These devices have absolutely no knowledge of the contents of a message.
They just take input bits and send them as output. Devices like switches and routers operate at
higher layers and look at the data they receive as being more than voltage or light pulses that
represent one or zero.
5
The Network Layer
The third-lowest layer of the OSI Reference Model is the network layer. If the data link layer is the
one that basically defines the boundaries of what is considered a network, the network layer is the
one that defines how internetworks (interconnected networks) function. The network layer is the
lowest one in the OSI model that is concerned with actually getting data from one computer to
another even if it is on a remote network; in contrast, the data link layer only deals with devices that
are local to each other.
While all of layers 2 through 6 in the OSI Reference Model serve to act as “fences” between the
layers below them and the layers above them, the network layer is particularly important in this
regard. It is at this layer that the transition really begins from the more abstract functions of the
higher layers—which don't concern themselves as much with data delivery—into the specific tasks
required to get data to its destination. The transport layer, which is related to the network layer in a
number of ways, continues this “abstraction transition” as you go up the OSI protocol stack.
Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
Logical Addressing : Every device that communicates over a network has associated with it a
logical address, sometimes called a layer three address. For example, on the Internet, the Internet
Protocol (IP) is the network layer protocol and every machine has an IP address. Note that
addressing is done at the data link layer as well, but those addresses refer to local physical devices.
In contrast, logical addresses are independent of particular hardware and must be unique across an
entire internetwork.
Routing: Moving data across a series of interconnected networks is probably the defining function
of the network layer. It is the job of the devices and software routines that function at the network
layer to handle incoming packets from various sources, determine their final destination, and then
figure out where they need to be sent to get them where they are supposed to go. I discuss routing in
the OSI model more completely in this topic on the topic on indirect device connection, and show
how it works by way of an OSI model analogy.

Datagram Encapsulation : The network layer normally encapsulates messages received from
higher layers by placing them into datagrams (also called packets) with a network layer header.
Fragmentation and Reassembly : The network layer must send messages down to the data link
layer for transmission. Some data link layer technologies have limits on the length of any message
that can be sent. If the packet that the network layer wants to send is too large, the network layer
must split the packet up, send each piece to the data link layer, and then have pieces reassembled
once they arrive at the network layer on the destination machine. A good example is how this is
done by the Internet Protocol.
Error Handling and Diagnostics : Special protocols are used at the network layer to allow devices
that are logically connected, or that are trying to route traffic, to exchange information about the
status of hosts on the network or the devices themselves.
6
The Application Layer
At the very top of the OSI Reference Model stack of layers, we find layer 7, the application layer.
Continuing the trend that we saw in layers 5 and 6, this one too is named very appropriately: the
application layer is the one that is used by network applications. These programs are what actually
implement the functions performed by users to accomplish various tasks over the network.
It's important to understand that what the OSI model calls an “application” is not exactly the same
as what we normally think of as an “application”. In the OSI model, the application layer provides
services for user applications to employ. For example, when you use your Web browser, that actual
software is an application running on your PC. It doesn't really “reside” at the application layer.
Rather, it makes use of the services offered by a protocol that operates at the application layer,
which is called the Hypertext Transfer Protocol (HTTP). The distinction between the browser and
HTTP is subtle, but important.
The reason for pointing this out is because not all user applications use the application layer of the
network in the same way. Sure, your Web browser does, and so does your e-mail client and your
Usenet news reader. But if you use a text editor to open a file on another machine on your network,
that editor is not using the application layer. In fact, it has no clue that the file you are using is on
the network: it just sees a file addressed with a name that has been mapped to a network somewhere
else. The operating system takes care of redirecting what the editor does, over the network.
Similarly, not all uses of the application layer are by applications. The operating system itself can
(and does) use services directly at the application layer.
That caveat aside, under normal circumstances, whenever you interact with a program on your
computer that is designed specifically for use on a network, you are dealing directly with the
application layer. For example, sending an e-mail, firing up a Web browser, or using an IRC chat
program—all of these involve protocols that reside at the application layer.

There are dozens of different application layer protocols that enable various functions at this layer.
Some of the most popular ones include HTTP, FTP, SMTP, DHCP, NFS, Telnet, SNMP, POP3,
NNTP and IRC. Lots of alphabet soup, sorry. J I describe all of these and more in the chapter on
higher-layer protocols and applications.
As the “top of the stack” layer, the application layer is the only one that does not provide any
services to the layer above it in the stack—there isn't one! Instead, it provides services to programs
that want to use the network, and to you, the user. So the responsibilities at this layer are simply to
implement the functions that are needed by users of the network. And, of course, to issue the
appropriate commands to make use of the services provided by the lower layers.
Key Concept: The seventh and highest layer in the OSI Reference Model is the application layer.
Application protocols are defined at this layer, which implement specific user applications and
other high-level functions. Since they are at the top of the stack, application protocols are the only
ones that do not provide services to a higher layer; they make use of services provided by the layers
below.
As we’ve discussed elsewhere, the distinctions between the top layers are not very clear, and this is
largely because of the decision made to not separate out session, presentation and application layer
functions in the important TCP/IP protocol suite. All of the protocols mentioned above are from the
TCP/IP protocol family, and some may cover all three of the top three OSI layers, two of them, or
one; in the TCP/IP model, they are all applications.
7 [Bonus Chapter]
Comparisation of TCP/IP and OSI
The three top layers in the OSI model, i.e. the application layer, the presentation layer and the
session layer, are not distinguished separately in the TCP/IP model which only has an application
layer above the transport layer. While some pure OSI protocol applications, such as X.400, also
combined them, there is no requirement that a TCP/IP protocol stack must impose monolithic
architecture above the transport layer. For example, the NFS application protocol runs over the
eXternal Data Representation (XDR) presentation protocol, which, in turn, runs over a protocol
called Remote Procedure Call (RPC). RPC provides reliable record transmission, so it can safely
use the best-effort UDP transport.
Different authors have interpreted the TCP/IP model differently, and disagree whether the link layer,
or the entire TCP/IP model, covers OSI layer 1 (physical layer) issues, or whether a hardware layer
is assumed below the link layer.
Several authors have attempted to incorporate the OSI model's layers 1 and 2 into the TCP/IP
model, since these are commonly referred to in modern standards (for example, by IEEE and ITU).
This often results in a model with five layers, where the link layer or network access layer is split
into the OSI model's layers 1 and 2.

The IETF protocol development effort is not concerned with strict layering. Some of its protocols
may not fit cleanly into the OSI model, although RFCs sometimes refer to it and often use the old
OSI layer numbers. The IETF has repeatedly stated[citation needed] that Internet protocol and
architecture development is not intended to be OSI-compliant. RFC 3439, addressing Internet
architecture, contains a section entitled: "Layering Considered Harmful".
For example, the session and presentation layers of the OSI suite are considered to be included to
the application layer of the TCP/IP suite. The functionality of the session layer can be found in
protocols like HTTP and SMTP and is more evident in protocols like Telnet and the Session
Initiation Protocol (SIP). Session layer functionality is also realized with the port numbering of the
TCP and UDP protocols, which cover the transport layer in the TCP/IP suite. Functions of the
presentation layer are realized in the TCP/IP applications with the MIME standard in data exchange.
Conflicts are apparent also in the original OSI model, ISO 7498, when not considering the annexes
to this model, e.g., the ISO 7498/4 Management Framework, or the ISO 8648 Internal Organization
of the Network layer (IONL). When the IONL and Management Framework documents are
considered, the ICMP and IGMP are defined as layer management protocols for the network layer.
In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities"
such as ARP and RARP.
IETF protocols can be encapsulated recursively, as demonstrated by tunneling protocols such as
Generic Routing Encapsulation (GRE). GRE uses the same mechanism that OSI uses for tunneling
at the network layer.
List of References
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 – Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights
Reserved.
Computer Networking
Principles Protocols and Practice
© Copyright October 31, 2011 by Oliver Bonaventure, is
licensed under a Creative Commons Attribution (CC BY).
Wikipedia, the free Enciclopedia.
A Wikimedia Foundation.