By: Dr. Imane Fahmy

Multiplexing  Multiple (many) links on 1 physical line  Common on long-haul, high capacity, links  Techniques: FDM, TDM, STDM

Frequency Division Multiplexing (FDM)

Synchronous Time Division Multiplexing (STDM)

Error Detection  Parity bit: even – odd parity (on 1s)  check bits: Error detecting-code (appended to k-bits data block to produce n-bits frame called “codeword”  Cyclic Redundancy Check (CRC): for block of k-bits data, the transmitter generates an n-bit frame called “Frame Check Sequence” (FCS) by the encoder/decoder

Error Correction  Usually requires re-transmission after detection.  Not suitable for wireless transmission since its data rates are very high.

Error Control Error Control techniques use: 1) Error detection 2) Positive acknowledgment 3) Retransmission after timeout 4) Negative acknowledgement & retransmission

Data into Signals Transformations  Signal encoding techniques:  Digital data Digital signal  Digital data Analog signal  Analog dataAnalog signal  Analog dataDigital signal PCM Encoding Modulation Digitization

Digital data Digital signal Encoding

Non Return to Zero-Level (NRZ-L)  Uses two different voltages for 0 and 1 bits  Voltage is constant during bit interval  No transition means no return to zero voltage  0: Absence of voltage  1: Constant positive voltage

Non Return to Zero Inverted (NRZ-I)  Non return to zero inverted on 1s  Constant voltage pulse for duration of bit  Data encoded as presence or absence of signal transition at beginning of bit time  1: transition (low to high or high to low)  0: no transition  Example of differential encoding since we have:  data represented by changes rather than levels  more reliable detection of transition rather than level

Multilevel Binary Bipolar-AMI  Uses more than two levels  Bipolar-AMI (Alternate Mark Inversion)  0: No line signal  1: Positive or negative pulse  one pulses alternate in polarity

Multilevel Binary Pseudoternary  1:Absence of line signal  0: Alternating positive and negative  No advantage or disadvantage over bipolar-AMI

Manchester Encoding  Has transition in middle of each bit period  Transition serves as clock and data  1: low to high  0: high to low represents zero

Differential Manchester Encoding  Mid-bit transition is clocking only  0: Transition at start of bit period  1: No transition at start of bit period

Encoding Schemes

Encoding Schemes Pros & Cons ProsCons NRZ (L or I) 1.Easy to engineer 2.Make good use of bandwidth 1.Has dc component 2.lack of synchronization capability (No synchronization) 3.Constant voltage over long string of 0s or 1s Bipolar (AMI or pseudoternary) 1.Synchronization 2.No net dc component 3.easy error detection 1. long runs of zeros still problem 2. lower bandwidth Biphase (Manchester or Differential Manchester) 1.Synchronization on mid bit transition (self clocking) 2.Has no dc component 3.Has error detection 1.At least one transition per bit time and possibly two 2.Maximum modulation rate is twice NRZ 3.Requires more bandwidth

Digital / Analog data Analog signal Modulation

Modem (digital/analog data->analog signal)

Modulation Techniques

Communication Model

Analog/ digital data Digital signal Digitization

Digitization(analog data->digital signal)

Digital Signals

Transmission Terminology

no intermediate devices Direct link direct link only 2 devices share link Point-to-point more than two devices share the link Multi-point

TransmissionTerminology SSimplex ssignals transmitted in one direction eeg. Television HHalf duplex bboth stations transmit, but only one at a time eeg. police radio FFull duplex ssimultaneous transmissions eeg. telephone

Guided Transmission Media

Guided (Wired) Transmission Media Unshielded Twisted Pair(UTP or TP) Shielded Twisted Pair(STP) Coaxial Cable (Coax) Optical Fiber (OF) CostCheapestCheapExpensiveMore expensive InstallationEasiestHarder (thick & heavy) HarderEasier: Light weight and small size BandwidthLimited bandwidth, distance and data rates Higher bandwidth and data rate Superior bandwidth and data rates Impairments Noise & interference Less interference Attenuation & Noise No impairments ApplicationsTelephone Cables (PBX) and LANs TV, long dist telephone & LANs Trunks, subscriber loops & LANs

Unguided Transmission Media

Unguided (Wireless) Transmission Media Terrestrial Microwave Satellite Microwave Radio BroadcastInfrared Applications 1- Long hauls Telecoms(substi -tuting coax or O.F. cables) 2- Short PTP 3- closed circuit TV or betn LANs 1- Long distance Telephone 2- Satellite TV broadcast 3- PTP Private business NWs 1- FM Radio transmission 2- UHF & VHF TV 1-TV remotes 2- IRD ports Trans. Requirements / Equipments parabolic dish (directional) Parabolic dish (directional) and relay stations Line of sight Antennas (omnidirectional) Line of sight I mpairments 1- Attenuation (by distance & rainfall) 2- Interference Multipath interference (by refraction) Blocked by walls (reflection)

Transmission Impairments 1. Attenuation and attenuation distortion 2. Delay distortion 3. Noise: a) Thermal noise (white noise) b) Intermodulation c) Crosstalk d) Impulse noise

TCP/IP Encapsulation

OSI Layers

OSI (7 layers) Vs TCP/IP (5 layers)

Internet Architecture

WAN Switching Techniques 1. Circuit switching (dedicated link) 2. Packet switching (data block divided into packets) 3. Frame relay (data block divided into variable size frames) 4. Asynchronous Transfer Mode (ATM): (data block divided into fixed size cells)