First of a series of Cheat Sheets on Computer Networks. This cheat sheet breaks down the Physical Layer aspect of a network, the various forms of transmission, the different forms of modulation, and more.
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Equation expressing maximum data rate for a finite bandwidth noiseless channel
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If signal is run through a low-pass filter of bandwidth B
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The filtered signal can be completely reconstructed by making only 2B samples per second
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Sampling faster than 2B x per second is pointless
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Higher frequency components have already been filtered out.
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A form of transmission media
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Two insulated copper wires (1mm thick)
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Twisted in helical form (like DNA)
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Wires are twisted so the waves cancel out
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Used for transmitting analog or digital information
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Bandwidth depends on wire thickness and distance traveled
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Several megabits/sec for a few kilometers
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Widely used
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Adequate performance and cheaper
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UTP
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Unshielded Twisted Pair
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Cat 5 UTP cable, mostly in office buildings:
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4 pairs of twisted insulated wires in a single plastic sheath.
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Full-Duplex
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Links can be used in both directions at the same time, like a two-way road
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Half-Duplex
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Link can be used in either direction, only one way at a time
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Simplex
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Links that allow traffic in only one direction.
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Process of converting between bits and signals
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To send digital information, we must devise analog signals to represent bits
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Signals occupies frequency from zero up to a maximum
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The maximum frequency depends on the signaling rate.
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The process of extracting timing information from a data stream for the receiver to decode
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To encode bits into symbols, receiver must know when one symbol ends and the next begins
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Receiver needs to reference a clock of the same frequency
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Accurate clocks are expensive
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Another strategy must be used
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Overhead is any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to perform a specific task.
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Non-return to zero
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Simplest, literal line code
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-V for 0
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+V for 1
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A long run of 0's or 1's leaves the signal unchanged
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Differentiating between bits become difficult. A long line of 15 0's looks like 16 without a very accurate clock
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For NRZ, it moves between + and - levels every 2 bits
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Requires bandwidth of at least B/2 when the bit rate is B bits/sec
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This limits the speed, as more bandwidth is required to run faster.
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Using more than two signalling levels, the limited bandwidth can be used for efficiently
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e.g. using 4 voltages, 2 bits can be sent at once, as a single symbol
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Effective only if the receiver can distinguish the 4 levels
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The signal change rate is now half the bit rate, thus reducing the required bandwidth.
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The rate the signal changes is the symbol rate
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the bit rate is the symbol rate multiplied by the number of bits per symbol
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The inverted vesion of NRZ
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transition for 1
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no transition for 0
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Used by USB
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Universal Serial Bus
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Used for classic Ethernet
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low to high = 0
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high to low = 1
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Requires twice as much bandwidth as NRZ because of the clock
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Adds 100% overhead
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Guarantees clock recovery and balanced signal because:
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-Each bit is modulated in a balanced signal
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0 = logical zero
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Encodes 0's with a zero-signal
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1 = + or -
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Encodes 1 with positive or negative level
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In telephone networks, it is know as Alternate Mark Inversion(AMI), where "mark" is 1 and "space" is 0.
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Guarantees a balanced signal because:
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-1's are encoded in alternating +V, - V signal levels
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Base-band signal averages zero
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As much + voltage as -, even after short period of time
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No DC electrical components
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Advantangeous, as some channels (coaxial or lines with transformers) attenuate a DC component due to their physical properties
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DC component filtered out
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Avoids energy waste
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Provides better clock recovery
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Through transitions, due to mix of + and - volatages.
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Allows measuring the signal average
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For error detection and receiver calibration. Impossible with an unbalanced signal
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Instances for possible link failure:
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Sequence used for scrambling could be the same as the signal
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Transmitting all 0's, constituting a link failure
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Method of connecting the reciever to the channel.
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Passes only the AC portion of the signal.
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A form of line code
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Maps groups of 4 bits of data onto groups of 5 bits for transmission
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Used to prevent more than 3 consecutive 0's
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Every data (4B) has a fixed codeword(5) that it is translated to
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This scheme adds 25% overhead
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Better than the 100% overhead of Manchester encoding
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Non-data codes can represent physical layer control signals
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e.g: "11111" - idle line
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"11000" = start of a frame
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Produces at least two transitions per 5 bits of output code, regardless of input data.
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Maps 8 bits input onto 10 bits output
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80% efficient
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Achieves DC signal balance, never far from balanced
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At most 2 bit disparity
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8 bits of data are transmitted as a 10-bit entity called a symbol
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Low 5 bits are encoded into a 6 bit group
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5b/6b portion
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Top 3 bits encoded into a 4-bit group
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3b/4b portion
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These groups are concatenated together to form a 10-bit symbol that is transmitted.
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Standards also define up to 12 symbols that can be sent in place of a data symbol
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These indicate start-of-frame, end-of-frame, link idle, etc.
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Helps clock recovery, never more than 5 consecuive 1s or 0s
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Signals that are shifted to occupy a higher range of frequencies, (all wireless transmissions)
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Scheme that regulates the amplitude, phase or frequency of a carrier signal to convey bits.
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Occupies a band of frequencies around the frequency of the carrier signal.
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Common for wireless and optical channels.
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Regulatory constraints and intereference prevention dictates choice of frequencies.
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Modulating the amplitude, frequency/phase of a carrier signal sends bits in a (non-zero) frequency range
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Amplitude Shift Keying (ASK)
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-Two different amplitudes represent 0 and 1
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-More levels can represent more sumbols
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Frequency Shift Keying (FSK)
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-Two or more tones used
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Phase Shift Keying(PSK)
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- Carrier wave systematicaly shifted 0 or 180 dgrees at each symbol period.
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Two frequencies used
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one symbol for 0, another for 1
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Only Phase is modulated through time
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to identify points on the plane
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Amplitude stays constant, not modulated
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Each point corresponds to one of four symbols.
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2 bits per symbol transmitted
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Shorthand to capture the amplitude and phase modulations symbols
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The points give the legal amplitude and phase combinations of each symbol.
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The phase of a dot is indicated by the angle a line from it to the origin makes with the positive x-axis
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The amplitude of a dot is the distance from the origin
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QAM
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Quadrature Amplitude Modulation
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16 combinations of amplitudes and phase used
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Can transmit 4 bits per symbol
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Assigns(maps) bits to symbols so that adjacent symbols differ in only 1 bit position
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If a receiver decodes the symbol in error
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It will make only a single bit in error
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Channels shared by multiple signals
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More convenient than using a single wire to carry several signals than to install a wire for every signal.
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Divides the spectrum into frequency bands. e.g. AM radio
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Shares the channel by placing users on different frequencies
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Frequencies are allocated different logical channels, with interchannel separation great enough to prevent interference
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Subcarriers are coordinated to be tightly packed
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Filters limit the useable bandwidth to 3100hz p/voice-grade channel.
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Many channels multiplexed together, 4000hz allocated per channel
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Different frequencies encode different values, while phase and amplitude remain constant.
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Higher frequency is associated to 1 bit, and a lower to 0
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OFDM (Orthogonal FDM) is an efficient FDM technique used for 802.11, 4G cellular and other communications that does not use guard bands.
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The channel is divided into many subcarriers that independently send data.
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Subcarriers are tightly packed in the frequency domain.
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Frequency response of each subcarrieris zero at the center of adjacent subcarriers, therefore subcarriersbe sampled at their center frequencies without interference from neighbours
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Guard time required to repeat ports of symbol signals so that they have the desired frequency
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Shares a channel over time
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Users take turns on a fixed schedule
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Not packet switching
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Each gets the entire badwidth for a little burst of time
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Bits from each input stream are taken in a fixed time slot and output to the aggregate stream.
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This stream runs the sum rate of individual streams.
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Streams must be synchronized in time.
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Widely used in telephone/cellular systems
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Shares the channel by giving users a code
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Codes are orthogonal
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Can be sent at the same time
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Widely used as part of 3G networks
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Scalar Product (example):
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A ● B = (a₁, a₂, a₃) ● (b₁, b₂, b₃) = a₁b₁ + a₂b₂ + a₃b₃
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A form of spread spectrum communication
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Narrowband signal spread out over a wider frequency band
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Tolerant of interference.
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Allows multiple signals from different users to share the same frequency band.
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CDMA shares the channel by giving users a code
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Codes are orthogonal;
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Can be sent at the same time
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Widely used as part of 3G networks
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Each station can transmit over the entire frequency spectrum all the time
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Cheatography n. The art of mapping complex subjects into clear, instantly navigable reference guides.
