Packet-switching 1840s The telegraph by Cooke and Wheatstone, revolutionised

 Packet-switching (PS) was laternetworks developed (1962-1968). The Internet relies on packets to transferdata. Data is split into tiny packets that may take different routes to adestination. The origin is military: for utmost security in transferringinformation of networks (no single outage point). More than one route available– if one route goes down another may be followed.

Networks can withstand largescale destruction (Nuclear attack – This was the time of the Cold War).In relation to how theInternet has managed to become what it is today their ways three main key partsthat allowed its success. The three major players in Internet history aregovernment, education and commercial.

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1840s The telegraph by Cooke andWheatstone, revolutionised human (tele)communications. Morse code is a seriesof dots and dashes used to communicated between humans. This is similar to howcomputers communicate via binary 0/1 data today. In 1858-1866 transatlanticcable was created which allowed direct instantaneous communications across theAtlantic. Today, cables connect all continents and are still a main hub oftelecommunications. In 1876 Alexander Graham Bell created the telephone. Telephonesexchanges provide the backbone of Internet connections today. Modems provideDigital to Audio conversions to allow computers to connect over the telephonenetwork.

The Internetis a network of networks, it is able to join governments from and around theworld including universities, and private computers together. IT provides aninfrastructure for the use of E-mail, bulletin boards, file archives, hypertextdocuments, databases and other computational resources. The collection ofcomputer networks which form and act as a single huge network for transport ofdata and messages across distances which can be anywhere from the same officeto anywhere in the world. It is the largest network of networks in the world.

Uses TCP/IP protocols and packet switching running on any communicationssubstrate. 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  Advantagesfor packet switching: Line efficiency, single node-to-node linkcan 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 thelocal node at its own speed. In circuit-switching, a connectioncould be blocked if there lack free resources.

On a packet-switching network,even with heavy traffic, packets are still accepted, by delivery delayincreases. Priorities can be used, on each node, packets with higher priority can be forwarded first. Theywill experience less delay than lower-priority packets.Problems that occur is circuitswitching are: designedfor voice service, resources dedicated to a particular call, data transmission, much of thetime the connection is idle (say, web browsing), and data rate is fixed. Packet switchingis designed to address these problems. Data are transmitted in short packets, t ypicallyat the order of 1000 bytes, longer messages are split into series of packets. Each packetcontains a portion of user data plus some control info.

Control informationcontains at least routing (addressing) info, so as to be routed to the intendeddestination 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: Thereis a dedicated communication path between two stations (end-to-end).

The pathis a connected sequence of links between network nodes. On each physical link,a logical channel is dedicated to the connection. Communicationvia 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 howto allocate bandwidth (channel) to establish a connection. Its propertiesinclude: 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: circuitestablishment takes time and Low data delay: after the circuit establishment, information istransmitted at a fixed data rate with no delay other than the propagationdelay.

The delay at each node is negligible. Developed for voice traffic (publictelephone network) but can also applied to data traffic. Forvoice connections, the resulting circuit will enjoy a high percentage ofutilization because most of the time one party or the other is talking.Circuitswitching contrasts with packet switching which divides the data to betransmitted into packets transmitted through the network independently. In packet switching, instead ofbeing dedicated to one communication session at a time, network links areshared by packets from multiple competing communication sessions, resulting inthe loss of the quality of service guarantees that are provided by circuitswitching.  Circuit-switching systems areideal for communications that require data to be transmitted in real-time.

Packet-switching networks are more efficient if some amount of delay isacceptable. Packet switching vs circuit switching  (Figure 5)The internet is similar to a road network in that it hasrules (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 godown the same road you get congestion (reduced bandwidth).  When two devices send messages to each otherit is called handshaking – the client requests access, theserver then grants it, and the protocols are agreed. Once the handshakingprocess is complete, the data transfer can begin. Protocols establish how twocomputers send and receive a message. Data packets travel between source anddestination from one router to the next.

The process of exchanging data packetsis known as packet switching. When transferring data withpackets the TCP establishes a connection. It then assembles the packets at thereceiving end to make sure they are in the correct order.

It then requestsre-transmission of lost packets and will then regulate the speed of thetransmission to make sure the receiver has enough buffer space to accept whatit’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 theNetwork access layer (e.

g., Ethernet, FDDI, ATM)A data packetis a unit of data made into a single package that travels along a given networkpath. Data packets are used in Internet Protocol (IP) transmissions for datathat navigates the Web, and in other kinds of networks.

Transferring data withpackets The receiving device adds up the 1sin the payload and compares the result to the value stored in the trailer. Ifthe values match, the packet is good. But if the values do not match, thereceiving device sends a request to the originatingdevice to resend the packet.The trailer is known to be the lastcomponent which is also more commonly known as the footer. The small amount ofbits that it has inside works together to tell the device that is device receivingthe packets that it has reached the end of its destination. It will also bedoing some checking to see if any errors have taken place. Cyclic redundancycheck which is also “CRC” is once way of checking for errors. What it does isit will take out all the 1’s that are consistent in the payload and them addthem 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 isdone it will check all the results in the trailer comparing them, if there is amatch then we know the packet it is a good one. Similarly, if the values end upnot matching then we know something has gone wrong, what is then done is arequest is send to the devices which had originally sent the packet. Therequest will ask the device to resend the packet so the same method can beplayed out. The second component of a packet isthe payload which can also be referred to as the body or data of a packet, thepacket that ends up at the destination is the actual original packet.

To makethe packet the correct size what must be done is padding of the payload, thiswill help fill out blank information. The reason that this happens in the firstplace is because the packet is known to be “fixed-length”. The packets are made up of threecomponents. The first component is the header this contains the instructionsabout the packets which is being carried in the packet. The instructions includesthe length of the packet, some networks have fixed-length packets, while othersrely on the header to contain this information. Synchronization which includesa few bits that aid the packet to match up to the network.

The packet numberthis 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 ofinformation, the protocol defines what type of packet is being transmitted: e-mail,Web page, streaming video etc. The destination address which tells the userwhere the packet is going. And the originating address which is where thepacket came from. Each packet contains the necessaryinformation 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 packetsthe email has been broken down into. The packets then carry the data using thefollowing protocols: Transmission Control Protocol/Internet Protocol (TCP/IP).

The packets are individually sent to the destination on the best availableroute thus making it a more efficient process. This process enables to balancethe amount which is sent across at a given time. As mentioned previously anything thatinvolves transferring data using the internet requires packets. An example ofthis would be sending and receiving an e-mail, once sent a series of packetshave been transferred in order for the end user to receive the message at itsfinal destination. The network enables the data to be shipped across in smallpackets referred to as packet switched networks. The network breaks up an emailinto parts these are referred to as bytes. How packets are used   (Figure 8) (Virtual-circuit network)   Avirtual-circuit network is a cross between a circuit-switched network and adatagram network.

The virtual-circuit shares characteristics of both. Thepackets for a virtual circuit network are known as frames. A virtual-circuitnetwork uses a series of special temporary addresses known as virtual circuitidentifiers (VCI). The VCI at each switch, is used to advance the frame towardsits final destination.

The VCN behaves like a circuitswitched net because there is a setup phase to establish the VCI entries in theswitch table. There is also a data transfer phase and teardown phase. In a datagramnetwork, each packet is treated independently of all others. Even if a packetis part of a multi-packet transmission, the network treats packets as thoughthey existed alone. Packets in this approach are referred to as datagrams.

Packetsusing this approach are referred to as datagrams. Each packet of one messagecan travel a different route towards their final destination. Allpackets have a destination address in the header. The destination address foreach datagram is used at a router to forward the message towards its finaldestination. A circuit switched network does not require a header ordestination address for the data transfer stage, the link is dedicated! Thepacket header contains a sequence number in the header so it can be ordered atthe destination.

A circuit-switchednetwork creates a dedicated path to complete a link between the sender andreceiver. In actual communication a circuit-switched network requires threephases: connection setup, data transfer, and connection teardown. It can beargued that circuit-switched networks are not as efficient as the other twotypes of networks because resources are allocated duringthe entire duration of the connection. These resources are unavailable to otherconnections. In a telephone network, people normally terminate thecommunication when they have finished their conversation. During data transferthe data are not delayed at each switch; the resources are allocated for theduration of the connection.

The two most common methodsof switching that we have covered is circuit switching and packet switching.Packets switching can further divide into two subcategories, virtual-circuitapproach, 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 switchesconnected by physical links.  (Figure 7)             Network connections replyon switches. Switched operate at the physical layer, data link layer andnetwork layer.

Below is an example of a switched network. Switching techniques Virtualcircuits network can providesequencing (packets arrive at the same order) and error control (retransmissionbetween two nodes). Packets are forwarded morequickly, 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. Invirtual 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 ofdestination address, and each node on the pre-established route knows where toforward such packets. A route between stations is set up prior to datatransfer. All the data packets then follow the same rout but there are nodedicated resources reserved for the virtual circuit! Packets need to bestored-and-forwarded.

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

Thetechniques of packet switching and how is work is a station breaks long messageinto packets, packets are sent out to thenetwork sequentially, one at a time. The network handles this steam of packetsand attempts to route them through and deliver them to the intended destinationwhich the datagram approach or the virtual circuit approach.  (Figure 6)The packet contains the following: A source which enables the user toknow which computer the message came from. The destination which is where themessage should go. The packet sequence which is the order the message datashould 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 sentcorrectly.

Data is transmitted inblocks, called packets. A packet is the unit of data that is routed between anorigin 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 dataplus 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 intopackets of fixed or variable size. The governing protocols will help you toidentify the size of the packet. Packet switched networks are classified asDatagram Networks andVirtual Circuit Networks.

Packet switching A circuitswitched network is not what it used to be, technology is finding better newcheaper ways to be able to the task which circuit switching does. In comparisonto modern switching/ routing making calls is proving to be very expensive this especiallyis you are making international calls or evening using a line rental. The circuitswitched network whilst on a call will also not alert you if someone has triedto call you if you were currently already on a call. Mobile phones and betterdevices on the other will let you know if someone has tried to contact you if youwas or wasn’t on a call at the time. As the connection is dedicated it cannot be used to transmitany other data even if the channel is free.

It is inefficient in terms ofutilization of system resources. As resources are allocated for the entireduration of connection, these are not available to other connections. Packet switching telephones systems for this reason will be more likely tohave congestion issues as supposed to circuit switching. Negatives of circuit switching The dedicated path/circuitestablished between sender and receiver provides a guaranteed data rate.

Oncethe circuit is established, data is transmitted without any delay as there isno waiting time at each switch. Since a dedicated continuous transmission pathis established, the method is suitable for long continuous transmission.Not having any interruptions whilstbeing on a current call is important when it comes to a traditional switchingsystem. It focuses on this being important so much so that once a call is madeit doesn’t allow any other calls that may come through to interrupt the currentone. This a positive as it means the calls duration will be able to stay clearwith a good type of quality for the users. Local calls will have even betterresults which a good amount of clear consistency. Positives of circuit switching This approach shows a timesequence of snapshots of the progress of three packets through the network.Each node chooses the next node and the packet path taking into accountinformation received from neighbouring nodes on traffic, line failure and soon.

So the packet with the same destination address does not all to follow thesame route, and they may drive out of sequence at the exits point. In thevirtual circuit approach, route is established before any packets are sent.Once the route is established all the packets between a pair of communicatingparties follow the same route through the network. The route is fixed for theduration of the logical connection it is somewhat similar to a circuit in acircuit switching network and it is referred to a virtual circuit. Each node onthe pre-established route knows where to direct such packets no routingdecisions are required. Each station can have more than one virtual circuit toany other station and can have virtual circuits to more than one station.If a station has a messageto send through a packet switching network that is a greater length than themaximum packet size that is permitted to be sent through without causing thesystem to crash, it breaks the message up into packets. There are two networksthat 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, theyare analysed and check before send or once received, this is done to eachpacket one by one in an order. Circuit switchingtelecommunications networks is commonly used to handle voice traffic. The keycharacteristic of circuit switching networks is that resources within thenetwork are dedicated to a particular call. The circuit switching network beganto be used increasingly for data connections. In a Circuit-Switching networkthe connection provides for transmission at a constant data rate. Each of thetwo devices that are connected must transmit and receive at the same data rateas the other in the system. This limits the utility of the networks ininterconnecting a variety of host computers and work stations. Packet switchingaddresses these problems the data is transmitted in short packets.

A typicalupper bound on packet length is 1000 octets. If the source has a longer messageto send, the message is broken into a series of packets. Each packet contains aportion of the user’s data plus some control information.          (Figure 5)  Circuit switching in a telephonenetwork: Many paths are possible but only one path is selected per call.

Thecircuit stays active even if no one is speaking. Once a call is established allthe 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. Theconnection path is established before the transmission begins. The channelcapacity must be reserved between the source and destination throughout thenetwork.

Each node must have available internal switching capacity to handlethe requested connection. The Switching nodes must have the intelligence tomake proper allocations and to establish a route through the network. Circuit Concept is a technology ofcircuit switching that is between operations of a single circuit- switchingnode. A network built around a single circuit-switchingnode consists of a collection of stations attached to a central switching unit.

The central switch establishes a dedicated path between any two devices thatwish to communicate. The function of the digital switch is to provide atransparent signal path between any pair of attached devices. Theconnection allows full-duplex transmission, the path is self is transparent, itmakes sure that there is an attachment within the devices that keeps themconnected.                                               (figure 4) Circuit Switching is when resourcesare dedicated to a particular call for the call duration.

In data traffic someof the time, no data is sent and the data rate is pre-arranged to ensure thatboth ends must operate at the same rate. This is demonstrated in figure 4 below