Chapter 7 – Data Communications Aims: Outline the history of data communications, especially the main events. Define the main parameters involved in the transmission of data. Outline methods that are used to carrier data. Define method of routing data through networks.

Automated telephone switching Automated telephone switching. In 1889, Almon Strowger, a Kansas City undertaker, patented an automatic switching system. In one of the least catchy advertising slogans, it was advertised as a ‘girl-less, cuss-less, out-of-orderless, wait-less telephone system.’ Radio transmission. One of the few benefits of war (whether it be a real war or a cold war) is the rapid development of science and technology. Radio transmission benefited from this over World War I. Trans-continental cables. After World War II, the first telephone cable across the Atlantic was laid from Oban, in Scotland to Clarenville in Newfoundland. Previously, in 1902, the first Pacific Ocean cable was laid. Satellites. The first artificial satellite was Sputnik 1, which was launched by the USSR in 1957. This was closely followed in the following year by the US satellite, Explorer 1. The great revolution when the ATT-owned Telstar satellite started communicating over large distances using microwave signals.

Digital transmission and coding Digital transmission and coding. Most information transmitted is now transmitted in the form of digital pulses. A standard code for this transmission, called pulse code modulation (PCM), was invented by A.H. Reeves in the 1930s, but was not used until the 1960s . Fibre-optic transmissions. Satellite communications increased the amount of data that could be transmitted over a channel, but in 1965 Charles Kao laid down the future of high-capacity communication with the proof that data could be carried using optical fibres.

Communication types Bandwidth contention, bandwidth sharing or reserved bandwidth. Some communication systems reserve bandwidth for a connection (such as ISDN and ATM), while others allow systems to contend for it (such as Ethernet). Virtual path, dedicated line or datagram. Some communication systems allow for a virtual path to be setup between the two connected systems, while others support a dedicated line between the two systems. Global addressing, local addressing or no addressing. An addressing structure provides for individual data packets to have an associated destination address. Each of the devices involved in the routing of the data read this address and send the data packet off on the optimal path.

Integrated digital network (IDN)

Frequencies and banwidth Octave 1 Octave 2 Octave 3 Octave 4 Octave 5 Octave 6 Octave 7 C 32.70 65.41 130.81 261.63 523.25 1046.50 2093.00 C#,Db 34.65 69.30 138.59 277.18 554.36 1100.73 2217.46 D 36.71 73.42 146.83 293.66 587.33 1174.66 2349.32 D#,Eb 38.89 77.78 155.56 311.13 622.25 1244.51 2489.02 E 41.20 82.41 164.81 329.63 659.26 1318.51 2367.02 F 43.65 87.31 174.61 349.23 698.46 1396.91 2637.02 F#,Gb 46.25 92.45 185.00 369.99 739.99 1474.98 2959.96 G 49.00 98.00 196.00 392.00 783.99 1567.98 3135.96 G#,Ab 51.91 103.83 207.65 415.30 830.61 1661.22 3322.44 A 55.00 110.00 220.00 440.00 880.00 1760.00 3520.00 A#,Bb 58.27 116.54 233.08 466.16 932.33 1664.66 3729.31 B 61.74 123.47 246.94 493.88 987.77 1975.53 3951.07

Digital system Analogue system

Bit capacity of a channel depends on the signal-to- noise ratio Noise Thermal noise. Thermal noise occurs from the random movement of electrons in a con­ductor and is independent of frequency. Cross‑talk. Electrical signals propagate with an electric and a magnetic field. If two conductors are laid beside each other then the magnetic field from one couples into the other. Impulse noise. Impulse noise is any unpredictable electromagnetic disturbance, such as from lightning or from energy radiated from an electric motor. Bit capacity of a channel depends on the signal-to- noise ratio

Modulation Amplitude modulation Frequency modulation

Digital modulation

Phase and amplitude modulation

Frequency modulation

Time-division multiplexing

Electromagnetic waves

Routing of data Circuit switching. This type of switching uses a dedicated line to make the connection between the source and destination, just as a telephone line makes a connection between the caller and the recipient. Packet switching. This type of switching involves splitting data into data packets. Each packet contains the data and a packet header which has the information that is used to route the packet through the network. Datagram. This is where the data packets travel from the source to the destination, and can take any path through the interconnected network. Virtual circuit. This is where all the data packets are routed along the same path. It differs from circuit switching in that there is no dedicated path for the data.

Multirate circuit switching Multirate circuit switching. Traditionally TDM (time division multiplexing) is used to transmit data over a PSN (public switched network). This uses a circuit switching technology with a fixed data rate, and has fixed channels for the data. Frame relay. This method is similar to packet switching, but the data packets (typically known as data frames in frame relays) have a variable length and are not fixed in length. This allows for variable bit rates. Cell relay. This method uses fixed packets (cells), and is a progression of the frame relay and multirate circuit switching.

Circuit-switching v. packet switching

Circuit-switching Packet-switching Investment in equipment Minimal as it uses existing connections Expensive for initial investment Error and flow control None, this must be supplied by the end users. Yes, using the FCS in the data link layer Simultaneous transmissions and connections No Yes, nodes can communicate with many nodes at the same time and over many different routes Allows for data to be sent without first setting up a connection Yes, using datagrams Response time Once the link is setup it provides a good reliable connection with little propagation delay Response time depends on the size of the data packets and the traffic within the network