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 Packet-switching (PS) was later
networks developed (1962-1968). The Internet relies on packets to transfer
data. Data is split into tiny packets that may take different routes to a
destination. The origin is military: for utmost security in transferring
information of networks (no single outage point). More than one route available
— if one route goes down another may be followed. Networks can withstand large
scale destruction (Nuclear attack – This was the time of the Cold War).

In relation to how the
Internet has managed to become what it is today their ways three main key parts
that allowed its success. The three major players in Internet history are
government, education and commercial. 1840s The telegraph by Cooke and
Wheatstone, revolutionised human (tele)communications. Morse code is a series
of dots and dashes used to communicated between humans. This is similar to how
computers communicate via binary 0/1 data today. In 1858-1866 transatlantic
cable was created which allowed direct instantaneous communications across the
Atlantic. Today, cables connect all continents and are still a main hub of
telecommunications. In 1876 Alexander Graham Bell created the telephone. Telephones
exchanges provide the backbone of Internet connections today. Modems provide
Digital to Audio conversions to allow computers to connect over the telephone
network.

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The Internet
is a network of networks, it is able to join governments from and around the
world including universities, and private computers together. IT provides an
infrastructure for the use of E-mail, bulletin boards, file archives, hypertext
documents, databases and other computational resources. The collection of
computer networks which form and act as a single huge network for transport of
data and messages across distances which can be anywhere from the same office
to anywhere in the world. It is the largest network of networks in the world.
Uses TCP/IP protocols and packet switching running on any communications
substrate.

How the internet came about

 

 

Circuit
switching

Packet
switching (datagram/virtual packet switching)

Orientation

Connection orientated

Connectionless

purpose

Initially designed for Voice
communication

Initially designed for Data
Transmission

Flexibility

Inflexible, because once a path is set
all parts of a transmission follows the same path

Flexible, because a route is created
for each packet to travel to the destination

Orders

Message is received in the order, sent
from the source

Packets of a message are received out
of order and assembled at the destination

Layers

Circuit Switching is implemented at
Physical Layer

Packet Switching is implemented at
Network Layer

Path

Dedicated transmission path

No dedicated path

Data

Continuous transmission on data

Transmission on packets

Messages

Messages are not stored

Packets maybe be stored until
delivered

bandwidth

Fixed bandwidth

Dynamic bandwidth

Bits

No overhead bits after call setup

Overhead bits in each packet

Signal

Busy signal is called party busy

Sender notified of conversation denial

blocks

Overload may block call setup: no
delay for established calls

Overload increases packet delay/ may
block call setup: increase packet delay

 

Advantages
for packet switching: Line efficiency, single node-to-node link
can be dynamically shared by many packets over time and packets are queued up and transmitted as fast as possible. Data rate conversion each station connects to the
local node at its own speed. In circuit-switching, a connection
could be blocked if there lack free resources. On a packet-switching network,
even with heavy traffic, packets are still accepted, by delivery delay
increases. Priorities can be used, on each node, packets with higher priority can be forwarded first. They
will experience less delay than lower-priority packets.

Problems that occur is circuit
switching are: designed
for voice service, resources dedicated to a particular call, data transmission, much of the
time the connection is idle (say, web browsing), and data rate is fixed. Packet switching
is designed to address these problems. Data are transmitted in short packets, t ypically
at the order of 1000 bytes, longer messages are split into series of packets. Each packet
contains a portion of user data plus some control info. Control information
contains at least routing (addressing) info, so as to be routed to the intended
destination and re-call the content of an IP header. Store and forward on each switching node, packets are received,
stored briefly (buffered) and passed on to the next node.

In circuit-switching Circuit switching: There
is a dedicated communication path between two stations (end-to-end). The path
is a connected sequence of links between network nodes. On each physical link,
a logical channel is dedicated to the connection. Communication
via circuit switching has three phases: Circuit establishment (link by link), Routing &
resource allocation (FDM or TDM), Data transfer, and Circuit disconnect. The switches must know how to find the route to the destination and how
to allocate bandwidth (channel) to establish a connection. Its properties
include: Inefficiency, Channel capacity is dedicated for the whole duration of a connection, If no data,
capacity is wasted. Secondly, Delay which is the Long initial delay: circuit
establishment takes time and Low data delay: after the circuit establishment, information is
transmitted at a fixed data rate with no delay other than the propagation
delay. The delay at each node is negligible. Developed for voice traffic (public
telephone network) but can also applied to data traffic. For
voice connections, the resulting circuit will enjoy a high percentage of
utilization because most of the time one party or the other is talking.

Circuit
switching contrasts with packet switching which divides the data to be
transmitted into packets transmitted through the network independently. In packet switching, instead of
being dedicated to one communication session at a time, network links are
shared by packets from multiple competing communication sessions, resulting in
the loss of the quality of service guarantees that are provided by circuit
switching.  Circuit-switching systems are
ideal for communications that require data to be transmitted in real-time.
Packet-switching networks are more efficient if some amount of delay is
acceptable.

Packet switching vs circuit switching

 

(Figure 5)

The internet is similar to a road network in that it has
rules (protocols) that you need to follow and only a certain number of vehicles
(data) can get through at a time (bandwidth). If too many vehicles try to go
down the same road you get congestion (reduced bandwidth).  When two devices send messages to each other
it is called handshaking – the client requests access, the
server then grants it, and the protocols are agreed. Once the handshaking
process is complete, the data transfer can begin. Protocols establish how two
computers send and receive a message. Data packets travel between source and
destination from one router to the next. The process of exchanging data packets
is known as packet switching. When transferring data with
packets the TCP establishes a connection. It then assembles the packets at the
receiving end to make sure they are in the correct order. It then requests
re-transmission of lost packets and will then regulate the speed of the
transmission to make sure the receiver has enough buffer space to accept what
it’s sent. TCP layer is made up of the following: Application layer (e.g., FTP,
Telnet, HTTP), Transport layer (TCP or UDP), Internet layer (IP) and the
Network access layer (e.g., Ethernet, FDDI, ATM)

A data packet
is a unit of data made into a single package that travels along a given network
path. Data packets are used in Internet Protocol (IP) transmissions for data
that navigates the Web, and in other kinds of networks.

Transferring data with
packets

The receiving device adds up the 1s
in the payload and compares the result to the value stored in the trailer. If
the values match, the packet is good. But if the values do not match, the
receiving device sends a request to the originating
device to resend the packet.

The trailer is known to be the last
component which is also more commonly known as the footer. The small amount of
bits that it has inside works together to tell the device that is device receiving
the packets that it has reached the end of its destination. It will also be
doing some checking to see if any errors have taken place. Cyclic redundancy
check which is also “CRC” is once way of checking for errors. What it does is
it will take out all the 1’s that are consistent in the payload and them add
them together. Once the result is gathered it will be kept in the trailer,
stored as a hexadecimal number. To know if the packet is good or not what is
done it will check all the results in the trailer comparing them, if there is a
match then we know the packet it is a good one. Similarly, if the values end up
not matching then we know something has gone wrong, what is then done is a
request is send to the devices which had originally sent the packet. The
request will ask the device to resend the packet so the same method can be
played out.

The second component of a packet is
the payload which can also be referred to as the body or data of a packet, the
packet that ends up at the destination is the actual original packet. To make
the packet the correct size what must be done is padding of the payload, this
will help fill out blank information. The reason that this happens in the first
place is because the packet is known to be “fixed-length”.

The packets are made up of three
components. The first component is the header this contains the instructions
about the packets which is being carried in the packet. The instructions includes
the length of the packet, some networks have fixed-length packets, while others
rely on the header to contain this information. Synchronization which includes
a few bits that aid the packet to match up to the network. The packet number
this tells the user which packet that is in a sequence of packets being sent.
The protocol which are on the networks that carry multiple types of
information, the protocol defines what type of packet is being transmitted: e-mail,
Web page, streaming video etc. The destination address which tells the user
where the packet is going. And the originating address which is where the
packet came from.

Each packet contains the necessary
information required to aid it to its destination this would be the senders IP address,
the receivers IP address as this will ensure the network knows how many packets
the email has been broken down into. The packets then carry the data using the
following protocols: Transmission Control Protocol/Internet Protocol (TCP/IP).
The packets are individually sent to the destination on the best available
route thus making it a more efficient process. This process enables to balance
the amount which is sent across at a given time.

As mentioned previously anything that
involves transferring data using the internet requires packets. An example of
this would be sending and receiving an e-mail, once sent a series of packets
have been transferred in order for the end user to receive the message at its
final destination. The network enables the data to be shipped across in small
packets referred to as packet switched networks. The network breaks up an email
into parts these are referred to as bytes.

How packets are used

 

 

(Figure 8)

 

(Virtual-circuit network)

 

 

A
virtual-circuit network is a cross between a circuit-switched network and a
datagram network. The virtual-circuit shares characteristics of both. The
packets for a virtual circuit network are known as frames. A virtual-circuit
network uses a series of special temporary addresses known as virtual circuit
identifiers (VCI). The VCI at each switch, is used to advance the frame towards
its final destination. The VCN behaves like a circuit
switched net because there is a setup phase to establish the VCI entries in the
switch table. There is also a data transfer phase and teardown phase.

In a datagram
network, each packet is treated independently of all others. Even if a packet
is part of a multi-packet transmission, the network treats packets as though
they existed alone. Packets in this approach are referred to as datagrams. Packets
using this approach are referred to as datagrams. Each packet of one message
can travel a different route towards their final destination. All
packets have a destination address in the header. The destination address for
each datagram is used at a router to forward the message towards its final
destination. A circuit switched network does not require a header or
destination address for the data transfer stage, the link is dedicated! The
packet header contains a sequence number in the header so it can be ordered at
the destination.

A circuit-switched
network creates a dedicated path to complete a link between the sender and
receiver. In actual communication a circuit-switched network requires three
phases: connection setup, data transfer, and connection teardown. It can be
argued that circuit-switched networks are not as efficient as the other two
types of networks because resources are allocated during
the entire duration of the connection. These resources are unavailable to other
connections. In a telephone network, people normally terminate the
communication when they have finished their conversation. During data transfer
the data are not delayed at each switch; the resources are allocated for the
duration of the connection.

The two most common methods
of switching that we have covered is circuit switching and packet switching.
Packets switching can further divide into two subcategories, virtual-circuit
approach, and datagram approach. Virtual-circuit network operates at the Data-Link layer. Datagram network operates at the Network layer. Circuit switched network operates at the Physical layer it consists of a set of switches
connected by physical links.

 

(Figure 7)

 

 

 

 

 

 

 

 

 

 

 

 

 

Network connections reply
on switches. Switched operate at the physical layer, data link layer and
network layer. Below is an example of a switched network.

Switching techniques

Virtual
circuits network can provide
sequencing (packets arrive at the same order) and error control (retransmission
between two nodes). Packets are forwarded more
quickly, based on the virtual circuit identifier, no routing decisions to make. It is also less reliable If a node fails, all virtual circuits that pass through that node fail. Datagram no call setup phase is good for bursty data, such as Web applications. It is more flexible, if a node fails, packets may find an alternate route, routing can be used to avoid congested parts of the network.

In
virtual circuit, a preplanned route is established before any packets are sent,
then all packets follow the same route. Each packet contains a virtual circuit identifier instead of
destination address, and each node on the pre-established route knows where to
forward such packets. A route between stations is set up prior to data
transfer. All the data packets then follow the same rout but there are no
dedicated resources reserved for the virtual circuit! Packets need to be
stored-and-forwarded.

In
datagram each packet is treated independently, with no reference to packets
that have gone before. Each node chooses the next
node on a packet’s path. Packets can take any
possible route and once arrived at the receiver maybe be out of order, packets
can also go missing during this process. It is up to the receiver to re-order
packets and recover from missing packets.

The
techniques of packet switching and how is work is a station breaks long message
into packets, packets are sent out to the
network sequentially, one at a time. The network handles this steam of packets
and attempts to route them through and deliver them to the intended destination
which the datagram approach or the virtual circuit approach.

 

(Figure 6)

The packet contains the following: A source which enables the user to
know which computer the message came from. The destination which is where the
message should go. The packet sequence which is the order the message data
should be re-assembled. The data which is the data of the message being sent.
Finally, the error checks this checks to see that the message has been sent
correctly.

Data is transmitted in
blocks, called packets. A packet is the unit of data that is routed between an
origin and a destination on the Internet or any other packet-switched network.
Before sending, the message is broken into a series of packets. Packets consists of a portion of data
plus a packet header that includes control information. At each node end route,
the packet is received, stored briefly and passed to the next node.

A packet-switched network divides the data into
packets of fixed or variable size. The governing protocols will help you to
identify the size of the packet. Packet switched networks are classified as
Datagram Networks and
Virtual Circuit Networks.

Packet switching

A circuit
switched network is not what it used to be, technology is finding better new
cheaper ways to be able to the task which circuit switching does. In comparison
to modern switching/ routing making calls is proving to be very expensive this especially
is you are making international calls or evening using a line rental. The circuit
switched network whilst on a call will also not alert you if someone has tried
to call you if you were currently already on a call. Mobile phones and better
devices on the other will let you know if someone has tried to contact you if you
was or wasn’t on a call at the time. As the connection is dedicated it cannot be used to transmit
any other data even if the channel is free. It is inefficient in terms of
utilization of system resources. As resources are allocated for the entire
duration of connection, these are not available to other connections. Packet switching telephones systems for this reason will be more likely to
have congestion issues as supposed to circuit switching.

Negatives of circuit switching

The dedicated path/circuit
established between sender and receiver provides a guaranteed data rate. Once
the circuit is established, data is transmitted without any delay as there is
no waiting time at each switch. Since a dedicated continuous transmission path
is established, the method is suitable for long continuous transmission.

Not having any interruptions whilst
being on a current call is important when it comes to a traditional switching
system. It focuses on this being important so much so that once a call is made
it doesn’t allow any other calls that may come through to interrupt the current
one. This a positive as it means the calls duration will be able to stay clear
with a good type of quality for the users. Local calls will have even better
results which a good amount of clear consistency.

Positives of circuit switching

This approach shows a time
sequence of snapshots of the progress of three packets through the network.
Each node chooses the next node and the packet path taking into account
information received from neighbouring nodes on traffic, line failure and so
on. So the packet with the same destination address does not all to follow the
same route, and they may drive out of sequence at the exits point. In the
virtual circuit approach, route is established before any packets are sent.
Once the route is established all the packets between a pair of communicating
parties follow the same route through the network. The route is fixed for the
duration of the logical connection it is somewhat similar to a circuit in a
circuit switching network and it is referred to a virtual circuit. Each node on
the pre-established route knows where to direct such packets no routing
decisions are required. Each station can have more than one virtual circuit to
any other station and can have virtual circuits to more than one station.

If a station has a message
to send through a packet switching network that is a greater length than the
maximum packet size that is permitted to be sent through without causing the
system to crash, it breaks the message up into packets. There are two networks
that are used in contemporary networks they are Datagram OR Virtual circuit.
The packets are the first part which gets the full attention in the datagram, they
are analysed and check before send or once received, this is done to each
packet one by one in an order.

Circuit switching
telecommunications networks is commonly used to handle voice traffic. The key
characteristic of circuit switching networks is that resources within the
network are dedicated to a particular call. The circuit switching network began
to be used increasingly for data connections. In a Circuit-Switching network
the connection provides for transmission at a constant data rate. Each of the
two devices that are connected must transmit and receive at the same data rate
as the other in the system. This limits the utility of the networks in
interconnecting a variety of host computers and work stations. Packet switching
addresses these problems the data is transmitted in short packets. A typical
upper bound on packet length is 1000 octets. If the source has a longer message
to send, the message is broken into a series of packets. Each packet contains a
portion of the user’s data plus some control information.

 

 

 

 

 

 

 

 

 

(Figure 5)

 

 

Circuit switching in a telephone
network: Many paths are possible but only one path is selected per call. The
circuit stays active even if no one is speaking. Once a call is established all
the communication takes place on this path or circuit. As shown in (figure 5)
below a circuit is dedicated to this call for the duration of the call. The
connection path is established before the transmission begins. The channel
capacity must be reserved between the source and destination throughout the
network. Each node must have available internal switching capacity to handle
the requested connection. The Switching nodes must have the intelligence to
make proper allocations and to establish a route through the network.

Circuit Concept is a technology of
circuit switching that is between operations of a single circuit- switching
node. A network built around a single circuit-switching
node consists of a collection of stations attached to a central switching unit.
The central switch establishes a dedicated path between any two devices that
wish to communicate. The function of the digital switch is to provide a
transparent signal path between any pair of attached devices. The
connection allows full-duplex transmission, the path is self is transparent, it
makes sure that there is an attachment within the devices that keeps them
connected. 

 

 

 

 

                                         
(figure 4)

Circuit Switching is when resources
are dedicated to a particular call for the call duration. In data traffic some
of the time, no data is sent and the data rate is pre-arranged to ensure that
both ends must operate at the same rate. This is demonstrated in figure 4 below

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