Hardware Building Blocks and Encoding COM211 Communications and Networks CDA College Theodoros Christophides

Performance Bandwidth: number of bits per time unit. We can talk about bandwidth at the physical level, but we can also talk about logical process- to-process bandwidth. Latency: time taken for a message to travel from one end of the network to the other.

Latency

Delay and Bandwidth This product is analogous to the volume of a pipe or the number of bits it holds. It corresponds to how many bits the sender must transmit before the first bit arrives at the receiver. Delay may be thought of as one-way latency or round-trip time (RTT) depending on the context.

Throughput (effective end-to-end throughput) We often think of throughput as measured performance. Implementation inefficiencies may cause the achievable bit rate to be less than the bandwidth for which the networks was designed. Throughput and Transfer Time Example: A user fetches a 1-MB file across a 1-G pbs network with a round-trip time of 100 ms. Compute the transfer time.

Shannon’s Theorem Real communication have some measure of noise. This theorem tells us the limits to a channel’s capacity (in bits per second) in the presence of noise. Shannon’s theorem uses the notion of signal-to-noise ratio (S/N), which is usually expressed in decibels (dB): In a typical analog system, e.g. analog telephone system, dB = 30, which gives 30 = 10 * log 10 (S/N) S/N = 1000 for a typical analog system, including plain old telephone systems

Shannon’s Theorem Question: How come you get more than this with your modem? Shannon’s Theorem: C: achievable channel rate (bps) B: channel bandwidth For POTS, bandwidth is 3000 Hz (upper limit of 3300 Hz and lower limit of 300 Hz), S/N = 1000 Information: Frequency audible to human ears: 20-20KHz

Jitter Network Interpacket gap Packet source Packet sink Jitter is a variation (somewhat random) of the latency from packet to packet. Jitter is most often observed when packets traverse multiple hops from source to destination.

Building Blocks Networks nodes/ End Devices Links  Dedicated cables  Leased lines  Last-mile links  Wireless

Network Node/End Device Memory: getting larger and larger as we can see the last years, but never enough! Processor: Moore’s law still holds for speed On a typical networked application, one must keep in mind the computation to communication ratio.

Links and Signals Links: Twisted pair, coax, optical fiber, the ether; half-duplex or full-duplex. Signals: Waveforms that travel on some medium (frequency) (period)

Frequency & Wavelength Wavelength: the distance between a pair of adjacent Maxima or minima of a wave, denoted as λ. λ *f = c, c is the speed of light in a given medium. Example: take c = 300 M meters/second, f = 100 M Hz, its wavelength λ = 3 meters

Spectrum

Encoding – NRZ (Non Return to Zero) Signals that maintain constant voltage levels with no signal transitions (non return to a zero voltage level) during a bit interval.

4B/5B Encoding Insert extra bits into the stream to break up long sequences of 0s and 1s. Doesn’t allow more than one leading 0 and no more than two trailing 0s. 4 bits5 bits f

4B/5B Encoding 4-bit Data Symbol5-bit Code codes are “left over” and some can be used for purposes other than encoding data. For instance: = idle line = dead line = halt 7 codes violate the “one leading 0, two trailing 0s rule”.