The physical later is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a 0 bit. Typical questions here are how many volts should be used to represent a 1 and how many for a 0, how many microseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established and how it is torn down when both sides are finished, and how many pins the network connector has and what each pin is used for. The design issues here deal largely with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below the physical layer. Physical layer design can properly be considered to be within the domain of the electrical engineer.
Analysis of the behavior of the signal mathematically

1. Fourier Analysis

2. Bandwidth-Limited Signals
Hertz (cycles/sec.) = Amplitudes can transmit undiminished from 0 to some frequency, which is measured in Hertz.
Baud = One signal change per second, a measure of data transmission speed. Named after the French engineer and telegrapher Jean-Maurice-Emile Baudot and originally used to measure the transmission speed of telegraph equipment, the term now most commonly refers to the data transmission speed of a modem.
Baud Rate = The speed at which a modem can transmit data. The baud rate is the number of events, or signal changes, that occur in one second--not the number of bits per second (bps) transmitted. In high-speed digital communications, one event can actually encode more than one bit, and modems are more accurately described in terms of bits per second than baud rate. For example, a so-called 9,600-baud modem actually operates at 2,400 baud but transmits 9,600 bits per second by encoding 4 bits per event (2,400 × 4 = 9,600) and thus is a 9,600-bps modem.

3. Maximum data rate of a channel

Nyquist’s Theorem
(For noiseless channels)
Maximum data rate = 2H log2 V bits / sec where, the signal consists of V discrete levels.

Amount of Noise is measured by Signal-to-noise ratio. If S is signal power and N is the noise power than signal-to-noise ratio = 10 log 10 S/N (measured in [DB] decibels)

Claude Shanon’s
Maximum number of bits/sec = H log 2 (1+S/N)

Transmission Media

1. Magnetic Media
Tapes, Hard disks and Floppies

2. Twisted Pair
Oldest and the most common method of data transmission
Problems of crosstalk and low speed compared to other mediums
Coaxial Cable are available in 2 varieties 50 – ohm (Digital) and 75 – ohm (Analog).

3. Baseband Coaxial Cable (Digital)
Consists of a stiff wire as the core surrounded by an insulating material, encased by a closely woven braided mesh, surrounded by a protective plastic.
Size = 50 – ohm (Digital)
Hider speed (1 km cable data rate of 1 to 2 Gbps) and less crosstalk

4. Broadband Coaxial Cable (Analog)
Used also for cable TV
Size = 75 – ohm (Analog)
Used for a large areas so they require Analog amplifiers to strengthen the signal periodically. These amplifiers can transmit signals in only one direction, so dual cable systems are used. If single cable systems are used than different frequencies for inbound and outbound communication is used.

Technically Broadband is inferior to Baseband but the advantage is that it is already in place as it has been widely used by cable TV and Telephone Cos.

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5. Fiber Optics
There are 3 components : Light Source, Transmission medium and detector. (A pulse of light indicates a 1 bit and absence of light indicates a 0 bit)

The transmission medium is of Fiber optic cable made of a center glass core , surrounded by a glass cladding and protected by a plastic jacket (The glass cladding has a lower index of refraction than the core so that the light from the core is not leaked)

Normally a transmission medium would leak light and transmission would not be possible but due to the Physics of refraction the light bounces back into the silica.

A multimode fiber will have different rays of light bouncing around at different angles. In case of a Single-mode fiber the fiber’s diameter is reduced to a few wave lengths of light and the light is propagated in a straight line only. This kind of fiber is faster but more expensive. Attenuation (weakening of transmitted power) in decibels = 10 log 10 transmitted power received power

Fibers can be connected in 3 ways :

1. Connectors are plugged into Fiber sockets (Easy but Connectors lose 10% to 20% light)
2. Spliced Mechanically (10% loss of light)
3. Two fibers can be fused(melted) to form a solid connection (Very small attenuation of light)

But in all three methods at the point of the join the refracted energy can interfere with the signal.
There are two types of light that can pass through a fiber :

LED light (date rate, distance and cost low --- life long --– Multimode --- Not temperature sensitive)
Laser light (date rate, distance and cost high --- Life short –-- Multimode and single mode --- Temperature sensitive)

Advantages of Fiber Optic Cables :
1. Higher Bandwidth
2. Low attenuation (Repeaters needed every 30 kms compared to every 5 kms in case of copper cables)
3. Not affected by power surges, electromagnetic interference or power failures
4. Not affected by corrosive chemicals in the air, ideal for harsh factory environment.
5. They don’t leak light hence quite difficult to tap, giving excellent security.
6. Electrons in case of a copper wire are effected by one another and also the stray electrons outside, while in case of Fiber optic cables Protons are not affected by one another (as they have no electric charge) and also by stray protons outside.

Disadvantages :
1. Requires skilled engineers.
2. Fiber optic cables are unidirectional, hence two cables or two frequency bands are required.
3. Very costly

Wireless Transmission
The Electromagnetic Spectrum (eg. Wireless)
Radio Transmission (eg. Radios)
Microwave Transmission (eg. TV)
Infrared and millimeter waves (eg. TV remote)
Lightwave transmission (uses lasers)


Telephone System
Problems with Transmission mediums
1. Attenuation (loss of energy)
2. Delay distortion
3. Noise

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Modem
A modem modulates outgoing digital signals from a computer or other digital device to analog signals for a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device.
In recent years, the 2400 bits per second modem that could carry e-mail has become obsolete. 14.4 Kbps and 28.8 Kbps modems were temporary landing places on the way to the much higher bandwidth devices and carriers of tomorrow. From early 1998, most new personal computers came with 56 Kbps modems. By comparison, using a digital Integrated Services Digital Network adapter instead of a conventional modem, the same telephone wire can now carry up to 128 Kbps. With Digital Subscriber Line (Digital Subscriber Line) systems, now being deployed in a number of communities, bandwidth on twisted-pair can be in the megabit range.

RS-232-C connector
Multiple xing
1. Frequency Division Multiplexing & Wavelength Division Multiplexing (For Fiber optics) --- Analog
2. Time Division Multiplexing --- Digital Sonet

Switching
1. Circuit Switching, message switching
2. Packet Switching

Switch Hierarchy
1. Crossbar switches
2. Space Division switches
3. Time Division switches

Narrowband ISDN
Integrated Services Digital Network (ISDN) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other media. Home and business users who install an ISDN adapter (in place of a modem) can see highly-graphic Web pages arriving very quickly (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas in the United States and Europe.
There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channels. Each B-channel carries data, voice, and other services. Each D-channel carries control and signaling information.

The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe.

Integrated Services Digital Network in concept is the integration of both analog or voice data together with digital data over the same network. Although the ISDN you can install is integrating these on a medium designed for analog transmission, broadband ISDN (BISDN) will extend the integration of both services throughout the rest of the end-to-end path using fiber optic and radio media. Broadband ISDN will encompass frame relay service for high-speed data that can be sent in large bursts, the Fiber Distributed-Data Interface (FDDI), and the Synchronous Opical Network (SONET). BISDN will support transmission from 2 Mbps up to much higher, but as yet unspecified, rates.

Broadband ISDN – Virtual Circuits
Broadband Integrated Services Digital Network

BISDN is both a concept and a set of services and developing standards for integrating digital transmission services in a broadband network of fiber optic and radio media. BISDN will encompass frame relay service for high-speed data that can be sent in large bursts, the Fiber Distributed-Data Interface (Fiber Distributed-Data Interface), and the Synchronous Optical Network (Synchronous Optical Network). BISDN will support transmission from 2 Mbps up to much higher, but as yet unspecified, rates.
BISDN is the broadband counterpart to Integrated Services Digital Network, which provides digital transmission over ordinary telephone company copper wires on the narrowband local loop.

ATM : asynchronous transfer mode
Asynchronous transfer mode (ATM) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.

Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).

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ATM switches
Knockout switch
Batcher-Banyan switch

Cellular Radio
Paging
Cordless telephones
Analog Cellular telephones
Digital Cellular telephones
Personal communication systems

Communication satellites
1. Geo-synchronous
2. Low orbit

X.21

A digital signaling interface called X.21 was recommended by the CCITT in 1976. The recommendation specifies how the customer's computer, the DTE, sets up and clears calls by exchanging signals with the carrier's equipment, the DCE.

The names and functions of the eight wires defined by X.21 are given in the following figure. The physical connector has 15 pins, but not all of them are used. the DTE uses the T and C lines to transmit data and control information, respectively. The DCE uses the R and I lines for data and control. The S line contains a signal stream emitted by the DCE to provide timing information, so the DTE knows when each bit interval starts and stops. At the carrier's option, a B line may also be provided to group the bits into 8-bit frames. If this option is provided, the DTE must begin each character on a frame boundary. If the option is not provided, both DTE and DCE must begin every control sequence with at least two SYN characters, to enable the other one to deduce the implied frame boundaries.

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