Computer Networks

Characteristics of Data communications system:

Delivery- The system must deliver data to the correct destination. Data must be received
by the intended device or user and only by that device or user.
Accuracy- The system must deliver the data accurately. Data that have been altered in
transmission and left uncorrected are unusable.
Timeliness- The system must deliver data in a timely manner. Data delivered late are
useless. In the case of video and audio, timely delivery means delivering data as they are
produced, in the same order that they are produced, and without significant delay. This
kind of delivery is called real-time transmission.
Jitter- Jitter refers to the variation in the packet arrival time. It is the uneven delay in the
delivery of audio or video packets. For example, let us assume that video packets are sent
every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms
delay, an uneven quality in the video is the result

Data communications system has five components:

Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio, and video.
Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera, and so on.
Receiver. The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television, and so on.
Transmission medium. The transmission medium is the physical path by which a
message travels from sender to receiver. Some examples of transmission media include
twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
Protocol. A protocol is a set of rules that govern data communications. It represents an
agreement between the communicating devices. Without a protocol, two devices may be
connected but not communicating, just as a person speaking French cannot be understood
by a person who speaks only Japanese

Communication between two devices

Simplex- The communication is unidirectional, as on a one-way street. Only one of the
two devices on a link can transmit; the other can only receive

Half-duplex- Each station can both transmit and receive, but not at the same time. When
one device is sending, the other can only receive, and vice versa

Full-duplex (Duplex)- Communication in both directions is required all the time.

Networks
A node can be a computer, printer, or any other device capable of sending and/or
receiving data generated by other nodes on the network.
Distributed Processing:
A task is divided among multiple computers instead of one single large machine.

Network Criteria:
A network must be able to meet a certain number of criteria. The most important of these
are performance, reliability, and security.

Performance:
Performance can be measured in many ways, including transit time and response time.

Transit time:
Is the amount of time required for a message to travel from one device to another.
Response time: Is the elapsed time between an inquiry and a response.

The performance of a network depends on a number of factors, including the number of

users, the type of transmission medium, the capabilities of the connected hardware, and
the efficiency of the software.

Performance is often evaluated by two networking metrics: Throughput and Delay.

We often need more throughput and less delay

Reliability:
The frequency of failure, the time it takes a link to recover from a failure, and the
network's robustness in a catastrophe.

Security:
Protecting data from unauthorized access, protecting data from damage and development,
and implementing policies and procedures for recovery from breaches and data losses

Physical Structures
Type of Connection
A network is two or more devices connected through links. A link is a communications
pathway that transfers data from one device to another.

There are two possible types of connections: point-to-point and multipoint.

Point-to-point
The entire capacity of the link is reserved for transmission between those two devices.
Most point-to-point connections use an actual length of wire or cable to connect the two
ends.

Multipoint
A multipoint (also called multidrop) connection is one in which more than two specific
devices share a single link. If several devices can use the link simultaneously, it is a
spatially shared connection. If users must take turns, it is a timeshared connection.
Physical Topology
Two or more devices connect to a link; two or more links form a topology. The topology
of a network is the geometric representation of the relationship of all the links and linking
devices (usually called nodes) to one another There are four basic topologies possible:
mesh, star, bus, and ring

Mesh
Mesh In a mesh topology, every device has a dedicated point-to-point link to every other
device. In a mesh topology, we need n(n -1) /2 duplex-mode links.
Advantages:
Privacy and Security.
Each connection can carry its own load, thus eliminating the traffic problems.
Point-to-point links make fault identification and fault isolation easy.
Disadvantages:
The amount of cabling and the number of I/O ports required, installation and
reconnection are difficult.
The hardware required to connect each link (I/O ports and cable) can be prohibitively
expensive.

Star Topology
Each device has a dedicated point-to-point link only to a central controller, usually called
a hub. The devices are not directly linked to one another. A star topology is less
expensive than a mesh topology.
Advantages:
Each device needs only one link and one I/O port to connect it to any number of others.
Far less cabling needs to be housed.
Robustness.
Disadvantages
Is the dependency of the whole topology on one single point, the hub. If the hub goes
down, the whole system is dead.

Bus Topology
A bus topology, on the other hand, is multipoint. One long cable act as a backbone to link
all the devices in a network. A drop line is a connection running between the device and
the main cable.
Advantages:
Ease of installation
Less cabling than mesh or star topologies.
Disadvantages
Difficult reconnection and fault isolation.

Ring Topology
Each device has a dedicated point-to-point connection with only the two devices on either
side of it. A signal is passed along the ring in one direction, from device to device, until it
reaches its destination.
Disadvantages
Unidirectional traffic

Hybrid Topology:
A network can be hybrid. For example, we can have a main star topology with each
branch connecting several stations in a bus.

Network Models
The two best-known standards are the OSI model and the Internet model. The OSI (Open
Systems Interconnection) model defines a seven-layer network; the Internet model
defines a five-layer network.

Categories of Networks
Two primary categories: Local-area networks and wide-area networks. A LAN normally
covers an area less than 2 mi; a WAN can be worldwide. Networks of a size in between
are normally referred to as metropolitan area networks and span tens of miles.

Local Area Network

Local area network (LAN) is usually privately owned and links the devices in a single
office, building, or campus. LAN size is limited to a few kilometers. LAN will use only
one type of transmission medium. The most common LAN topologies are bus, ring, and
star.

Wide Area Network

A wide area network (WAN) provides long-distance transmission of data, image, audio,
and video information over large geographic areas that may comprise a country. We
normally refer to the first as a switched WAN and to the second as a point-to-point
WAN.

Metropolitan Area Networks

A network with a size between a LAN and a WAN. It normally covers the area inside a
town or a city. Another example is the cable TV network that originally was designed for
cable TV, but today can also be used for high-speed data connection to the Internet.

Interconnection of Networks: Internetwork

When two or more networks are connected, they become an internetwork, or internet.

PROTOCOLS AND STANDARDS

we define protocol, which is synonymous with rule. Then we discuss standards, which
are agreed-upon rules. An entity is anything capable of sending or receiving information.
For communication to occur, the entities must agree on a protocol. A protocol is a set of
rules that govern data communications. A protocol defines what is communicated, how it
is communicated, and when it is communicated. The key elements of a protocol are
syntax, semantics, and timing.
Syntax: The term syntax refers to the structure or format of the data, meaning the order in
which they are presented.

Semantics: The word semantics refers to the meaning of each section of bits. How is a
particular pattern to be interpreted.
Timing: when data should be sent and how fast they can be sent. For example, if a sender
produces data at 100 Mbps but the receiver can process.

Standards Creation Committees:

International Organization for Standardization (ISO)
International Telecommunication Union-Telecommunication Standards Sector (ITU-T)
American National Standards Institute (ANSI)
Institute of Electrical and Electronics Engineers (IEEE)
Electronic Industries Association (EIA)

Forums:
Telecommunications technology development is moving faster than the ability of
standards committees to ratify standards.

Internet Standards:
An Internet draft is a working document (a work in progress) with no official status and a
6-month lifetime. Upon recommendation from the Internet authorities, a draft may be
published as a Request for Comment (RFC). Each RFC is edited, assigned a number, and
made available to all interested parties. RFCs go through maturity levels and are
categorized according to their requirement level.

Layered task:
We use the concept of layers in our daily life. As an example, let us consider two friends
who communicate through postal mail. The process of sending a letter to a friend would
be complex if there were no services available from the post office. Topics discussed in
this section: Sender, Receiver, and Carrier Hierarchy

we have a sender, a receiver, and a carrier that transports the letter. There is a hierarchy
of tasks.

At the Sender Site

The activities that take place at the sender site :

Higher layer : The sender writes the letter, inserts the letter in an envelope, writes the
sender and receiver addresses, and drops the letter in a mailbox.
Middle layer : The letter is picked up by a letter carrier and delivered to the post office.
Lower layer : The letter is sorted at the post office; a carrier transports the letter
On the Way

The letter is then on its way to the recipient. On the way to the recipient's local post
office, the letter may actually go through a central office. In addition, it may be
transported by truck, train, airplane, boat, or a combination of these.

At the Receiver Site

Lower layer : The carrier transports the letter to the post office.
Middle layer : The letter is sorted and delivered to the recipient's mailbox.
Higher layer : The receiver picks up the letter, opens the envelope, and reads it.

THE OSI MODEL

The OSI model is a layered framework for the design of network systems 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

An open system is a set of protocols that allows any two different systems to
communicate regardless of their underlying architecture. The purpose of the OSI model is
to show how to facilitate communication between different systems without requiring
changes to the logic of the underlying hardware and software. The OSI model is not a
protocol; it is a model for understanding and designing a network architecture that is
flexible, robust, and interoperable.

LAYERED ARCHITECTURE

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 functions into discrete groups 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.

The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2),
network (layer 3), transport (layer 4), session (layer 5), presentation (layer 6), and
application (layer 7).

Peer to Peer Processes

Interfaces Between Layers

The passing of the data and network information down through the layers of the sending
device and back up through the layers of the receiving device is made possible by
an interface between each pair of adjacent layers. Each interface defines the 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 replaced without requiring changes to the surrounding layers.

Organization of the Layers

The seven layers can be thought of as belonging to three subgroups. Layers I, 2, and
3-physical, data link, and network-are the network support layers; they deal with the
physical aspects of moving data from one device to another. Layers 5, 6, and 7-session,
presentation, and application-can be thought of as the user support layers; they allow
interoperability among unrelated 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.

Encapsulation
The data portion of a packet at level N - 1 carries the whole packet from level N. The
concept is called encapsulation. Level N - 1 is not aware of which part of the encapsulated
packet is data and which part is the header or trailer. For level N - 1, the whole packet
coming from level N is treated as one integral unit.

The OSI Model

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used
to describe the functions of a networking system. The OSI model characterizes
computing functions into a universal set of rules and requirements in order to support
interoperability between different products and software. In the OSI reference model, the
communications between a computing system are split into seven different abstraction
layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
Created at a time when network computing was in its infancy, the OSI was published in
1984 by the International Organization for Standardization (ISO). Though it does not
always map directly to specific systems, the OSI Model is still used today as a means to
describe Network Architecture.

The 7 Layers of the OSI Model

Physical Layer
The lowest layer of the OSI Model is concerned with electrically or optically transmitting
raw unstructured data bits across the network from the physical layer of the sending
device to the physical layer of the receiving device. It can include specifications such as
voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find
“physical” resources such as network hubs, cabling, repeaters, network adapters or
modems.
Data Link Layer
At the data link layer, directly connected nodes are used to perform node-to-node data
transfer where data is packaged into frames. The data link layer also corrects errors that
may have occurred at the physical layer.
The data link layer encompasses two sub-layers of its own. The first, media access
control (MAC), provides flow control and multiplexing for device transmissions over a
network. The second, the logical link control (LLC), provides flow and error control over
the physical medium as well as identifies line protocols.

Network Layer
The network layer is responsible for receiving frames from the data link layer, and
delivering them to their intended destinations among based on the addresses contained
inside the frame. The network layer finds the destination by using logical addresses, such
as IP (internet protocol). At this layer, routers are a crucial component used to quite
literally route information where it needs to go between networks.

Transport Layer
The transport layer manages the delivery and error checking of data packets. It regulates
the size, sequencing, and ultimately the transfer of data between systems and hosts. One
of the most common examples of the transport layer is TCP or the Transmission Control
Protocol.

Session Layer
The session layer controls the conversations between different computers. A session or
connection between machines is set up, managed, and ermined at layer 5. Session layer
services also include authentication and reconnections.

Presentation Layer
The presentation layer formats or translates data for the application layer based on the
syntax or semantics that the application accepts. Because of this, it at times also called
the syntax layer. This layer can also handle the encryption and decryption required by the
application layer.

Application Layer
At this layer, both the end user and the application layer interact directly with the
software application. This layer sees network services provided to end-user applications
such as a web browser or Office 365. The application layer identifies communication
partners, resource availability, and synchronizes communication.

2.4 TCP/IP PROTOCOL SUITE

The TCPIIP protocol suite was developed prior to the OSI model. Therefore, the layers in
the TCP/IP protocol suite do not exactly match those in the OSI model. The original
TCP/IP protocol suite was defined as having four layers: host-to-network, internet,
transport, and application. However, when TCP/IP is compared to OSI, we can say that
the host-to-network layer is equivalent to the combination of the physical and data link
layers. We assume that the TCPIIP protocol suite is made of five layers: physical, data
link, network, transport, and application. The first four layers provide physical standards,
network interfaces, internetworking, and transport functions that correspond to the first
four layers of the OSI model
TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides
a specific functionality; however, the modules are not necessarily interdependent. The
term hierarchical means that each upper-level protocol is supported by one or more
lower-level protocols
At the transport layer, TCP/IP defines three protocols: Transmission Control Protocol
(TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol
(SCTP). At the network layer, the main protocol defined by TCP/IP is the
Internetworking Protocol (IP); there are also some other protocols that support data
movement in this layer

Physical and Data Link Layers

At the physical and data link layers, TCPIIP does not define any specific protocol. It
supports all the standard and proprietary protocols. A network in a TCPIIP internetwork
can be a local-area network or a wide-area network

Network Layer
At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the
Internetworking Protocol. IP, in turn, uses four supporting protocols: ARP, RARP,
ICMP, and IGMP
The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP
protocols. It is an unreliable and connectionless protocol-a best-effort delivery service.
The Address Resolution Protocol (ARP) is used to associate a logical address with a
physical address.
The Reverse Address Resolution Protocol (RARP) allows a host to discover its Internet
address when it knows only its physical address. It is used when a computer is connected
to a network for the first time or when a diskless computer is booted.
The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and
gateways to send notification of datagram problems back to the sender.
The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous
transmission of a message to a group of recipients.

Transport Layer
Traditionally the transport layer was represented in TCP/IP by two protocols: TCP and
UDP. IP is a host-to-host protocol, meaning that it can deliver a packet from one physical
device to another. UDP and TCP are transport level protocols responsible for delivery of
a message from a process (running program) to another process.
The User Datagram Protocol (UDP) is the simpler of the two standard TCPIIP transport
protocols. It is a process-to-process protocol that adds only port addresses, checksum
error control, and length information to the data from the upper layer.
The Transmission Control Protocol (TCP) provides full transport-layer services to
applications. TCP is a reliable stream transport protocol.
TCP divides a stream of data into smaller units called segments. Each segment includes a
sequence number for reordering after receipt, together with an acknowledgment number
for the segments received.
The Stream Control Transmission Protocol (SCTP) provides support for newer
applications such as voice over the Internet

Application Layer
The application layer in TCPIIP is equivalent to the combined session, presentation, and
application layers in the OSI model. Many protocols are defined at this layer.