TTM4105: Access and Transport Networks
#Note:
This is not a complete compendium, the initial goal is to serve as a ctrf+f friendly dictionary of acronyms. If there is something you cant find, please add it as soon as you have learned what it is! Feel free to reorganize as the structure is a bit loose, especially towards the end.
#Access networks
The part of a network that connects directly to the end user or customer. Often wireless, and the bottleneck of a connection. Implements all layers of the OSI model since it interfaces with end-devices.
#Transport-/Core networks
The central part of a network. Typically high speed copper or fiber cables. Main components are switches and routers. Only implements the bottom two or three layers of the OSI model.
#Wireless transmissions
Wireless transmission is the transmission of data between points that are not connected by a guided medium. Typically using electromagnetic waves.
##(Multi-path) Propagation
A transmitted signal propagates via many different paths to the destination. The received signal is a combination of these versions and experience random variations in the amplitude and phase (resulting in constructive or destructive effect at the receiver). Causes diversity gain.
Issues related to multipath propagation can be solved by MIMO, spread spectrum, frequency hopping, OFDM, etc.
##Reflection
A smooth surface reflects the signal. Weakens the signal.
##Refraction
A wave that travels into a dense medium will bend. Waves in the air bend towards the earth since the atmosphere has higher density closer to the ground.
##Difraction
A wave is bent around a large object.
##Scattering
A signal hits an obstacle around the same size as the wavelength and is scattered into several weaker outgoing signals.
GSM is at risk considering it has a wavelength at the order of 10cm.
##Shadowing
Large objects might block a signal.
##Attenuation
Signal strength attenuates (gradual reduction in the strength) with transmission distance. Attenuation is greater at higher frequencies.
##Fading
Fading is the variation of the attenuation of a signal.
###Frequency-selective fading
##Interference
Waves that come from the same source or have similar frequencies might superimpose at the reciever. The result could be either **constructive interference** or **destructive interference**.
###Intersymbol interference (ISI)
Symbols can interfere with each other causing noise.
##Noise
A signal might suffer modifications during transmission or capture.
###Signal to Noise Ratio (SNR)
$SNR_{dB} = 10log_{10}SNR$
$SNR = 10^{SNR_{dB}/10}$
##Range
A signal has a finite range that can be split into multiple sub-ranges.
###Transmission range
Within this range, the error rate is low, and it is possible to communicate.
###Detection range
Within detection range the error rate is so high that we cannot communicate, but it is still possible to detect the signal (received power is higher than background noise).
###Interference range
Within the interference range it might not be possible to detect the signal, but it adds to the background noise.
##Channel capacity
Capacity the maximal achievable rate that can be delivered reliably over a channel.
###Calculation
$C$: Channel capacity
$B$: Channel bandwidth
####Noise free
The limitation on data rate is the signal bandwidth.
$C = 2B log_2M$
####Noisy
The achievable data rate depends on the bandwidth and the noise level.
$C = B log_2(1+SNR)\, bps$
####Spectral efficiency
$C = log_2(1+SNR)\, bps/Hz$
####AWGN Channel
$C = B log_2(1+\frac{P}{N_0B})\, bps$
$P$: received signal power
$N_0$: Noise spectral density
##Modulation
There are two types of modulation: **Digital Modulation**: translation of digital signals into (baseband) analog signals.
**Analog Modulation**: shifting baseband analog signals into passband signals.
###ASK
Amplitude Shift Keying. Information data controls the amplitude of the carrier. Simple to implement, require low bandwidth, but is susceptible to distortion. Works best in optical transmission.
###FSK
Frequency Shift Keying. Information data controls the frequency of the carrier. Less susceptible to errors than ASK, but requires more bandwidth. Vulnerable to sudden changes in phase. Works best in wireless transmission.
###PSK
Phase Shift Keying. Information data controls the phase of the carrier. More resistant to interference than ASK and PSK, but is harder to implement. Transmitters and receivers must be synchronized. Works best in wireless transmission.
###QAM
Combination of ASK and PSK. Modulation of both amplitude and phase.
###MSK
####GMSK
##Performance
###M-ary
###Spectral efficiency
Spectral efficiency: how many bits are modulated per Hz
$log_2 M bits/Hz$
###Error rate
###Coding rate
$R_c = \frac{No.\, of\, information\, bits}{No.\, of\, coded\, bits}$
##Coding
The wireless channel is vulnerable to errors. Channel encoding adds additional bits to improve transmission reliability.
###ARQ
Automatic Repeat reQuest is capable of detecting bit errors and retransmitting using detection codes.
###FEQ
Forward Error Correction is capable of detecting and correcting bit errors using correction codes.
##Performance enhancement
###Spread spectrum
By using spread spectrum, interference will eventually despread and its influence will be reduced. Resistant to frequency-selective fading.
####DSSS
####FHSS
Orthogonal pseudo-random codes determine frequency changes.
Transmitter and/or receiver have multiple antennas.
Spatial Multiplexing Gain results in higher bit rates.
Spatial Diversity Gain results in smaller error rates.
Smart Antenna Gain (beamforming) results in less interference.
**SISO**: Single Input Single Output
**SIMO**: Single Input Multiple Output
**MISO**: Multiple Input Single Output
**MIMO**: Multiple Input Multiple Output
##Hidden terminal problem
The hidden terminal problem occurs when node B is visible from node A, but not from node C who wants to communicate with node A.
##Exposed terminal problem
#Duplexing
Duplexing allows communication in both directions.
##TDD
The channel is either used for sending or for receiving. The entire spectrum is divided into timeslots.
##FDD
Simultaneous transmission and reception. The spectrum is divided into frequencies for sending and frequencies for receiving.
#Multiplexing and MAC
Multiplexing lets multiple users can share a medium with a minimum of interference.
##SDM/SDMA
Space Division Multiplexing divide space into sectors and assigns each sector to a communication channel.
##TDM/TDMA
Time Division Multiplexing assigns the whole spectrum to each communication channel at different time slots
##FDM/FDMA
Frequency Division Multiplexing divides the spectrum into smaller frequency bands that is assigned.
###OFDM/OFDMA
Orthogonal Frequency Division Multiplexing.
##CDM/CDMA
Code Division Multiplexing lets each communication channel use the entire spectrum at the same time but with a unique orthogonal code.
##Contention-based MAC
###ALOHA
Distributed random access MAC.
####Slotted ALOHA
Twice the throughput of pure ALOHA.
###CSMA
Carrier Sensing Multiple Access.
If a user wants to send it senses the carrier.
If the carrier is idle, transmit with a certain probability
otherwise, wait for some time and try again
####CSMA/CD
Carrier Sensing Multiple Access with Collision Detection
If a user wants to send it senses the carrier.
If the carrier is idle, transmit with a certain probability
otherwise, wait for some time and try again
if a collision is detected, stop and wait before trying again
####CSMA/CA
Carrier Sensing Multiple Access with Collision Avoidance.
CSMA/CD is not applicable in wireless since we might not be able to detect a collision.
###MACA
Multiple Access with Collision Avoidance.
No carrier sensing. A user must send a RTS (Request To Send) and receive a CTS (Clear To Send) before transmitting.
Solves the hidden teminal problem, partially solves the exposed terminal problem.
####MACAW
MACA for WLANs.
#Cellular networks
##Cells
Coverage area of a base station.
###Cell breathing
When there are many users in a cell that iterferes with each other, the cells effective range decreases. Theese changes in cell size is refered to as cell breathing. In GSM, where each user has an assigned timeslot, this is not an issue because the cell "doesnt breathe", but in for example UMTS the cell size is related to the number of users which complicates cell planning.
##Sector
Part of a cell covered by a directional antenna.
##Frequency reuse
Frequencies can be reused in many cells if the cells are far enough apart.
###Frequency reuse factor
$\frac{D}{r} = \sqrt{3N}$
$N$: frequency reuse factor, $D$: Reuse distance, $r$: cell radius
##Handover
The act of moving the connection from one basestation to another.
###Hard handover
Radio link is interrupted during the switch.
###Soft handover
Data flow is not interrupted. A connection to both basestations might be active for a while.
###Seamless handover
Data flow is not interrupted. Radio link to the old base station is released as soon as the new radio link is up.
##GSM (2G)
Groupe Spécial Mobile. Circuit-switched voice communications. Using FDMA/TDMA with FDD. Modulation: GMSK.
###GPRS (2.5G)
General Packet Radio Service. Extention of GSM that allows one user to occupy any nomber of timeslots (ie. higher datarate). Introduces packet switched data transfer.
###EDGE (2.75G)
Enhanced Data Rates for GSM. Extention of GPRS.
##UMTS (3G)
W-CDMA. Voice is circuit switched. Data is packet switched.
###HSPA (3.5G)
##LTE (4G)
OFDM/OFDMA, TDMA, FDMA, MIMO. Pure packet switched. Channel-dependent scheduling is used to give users good channels.
###LTE-A
#WLAN (IEEE802.11)
CSMA-based, with IFS, with RTS/CTS, with backoff
#WPAN (Bluetooth)
Polling (TDMA), FHSS
##Piconet
#Optical fibre
##Attenuation
Signal strength attenuates (gradual reduction in the strength) with transmission distance. Attenuation is low in fibre, but not non-existant. Must be accounted for over large distances.
##Dispersion
Pulse spreads in long fibre lines. Results in intersymbol interference, effectively limiting the maximum transmission rate. Dispersion depends on fibre type.
###Chromatic dispersion
Different wavelengths travel at different speeds through the fibre. Sum of material and waveguide dispersion.
##Dispersion Compensating Fibre (DCF)
Dispersion Compensating Fibre counteracts the effects of dispersion.
##Erbium Doped Fiber Amplifier (EDFA)
Optical amplifier.
#Optical switches
Light is reflected by the cladding into the core.
##Opaque Network - Fixed Patch panel
##Opaque Network - Opaque Switch
##Opaque Network - Transparent Switch
##Transparent Network - Transparent Switch
###Optical couplers
One or more fibers in, one or more fibres out.
###Optical add/drop multiplexer (OADM)
Filter out a wavelength or a set of wavelengths and/or add a wavelength or a set of wavelengths. Reconfigurable versions exist (ROADMs).
###Arrayed waveguide grating (AWG)
Commonly used muxer/demuxer for WDM.
#Optical multplexing
##Passive Optical Network with TMDA (TDMA-PON)
Passive and cheap, but has low scalability.
##Passive Optical Network WDM (WDM-PON)
Common method of optical multiplexing: Wavelength Division Multiplexing. Coupling several wavelengths
together into a single fibre.
###CWDM
Coarse Wave Division Multiplexing. The most cost effective solution.
###DWDM
Dense Wave Division Multiplexing. Much more expensive than CWDM.
#Optical Transport Network (OTN)
OTN is a standard for optical networks. Frames different protocols for transport over fibre. Has management functionality, monotoring functionality and FEC.
#Protection schemes
##Dedicating resources
###1+1 protection
Send the same signal on two alternative paths. If one breaks we have a backup!
###1:1 protection
Have a backup path available for each path, but it may be used for other low priority traffic when the primary path is working.
##Shared protection (M:N protection)
Have M backup paths for every N paths. If unused a backup path may be used by other low priority traffic. Problematic if many paths break and need to share few backup paths.
#Carrier-Grade Ethernet
Extension of Ethernet for service providers. Ethernet for MANs (Metro Area Networks).
#Fiber to the X/Fiber to the Premise (FttP/FttX)
##Fiber to the Home (FttH)
###Point-to-point
One fiber runs from each home to the service provider. No external power is needed, but uses a lot of fiber cable.
###Curb-switched
One fiber runs to a switch near the homes. One fiber runs from each home to the switch. Uses less fiber cable, but the switch needs power.
###Passive
One fiber runs to a passive splitter near the homes. One fiber runs from each home to the splitter. Does not need power, but the splitter causes higher power loss than a powered switch.
##Fiber to the Building (FttB)
##Fiber to the Curb (FttC)
#GMPLS
Generalized Multi-Protocol Label Switching. Extended MPLS.
#Optical Packet Switching (OPS)
Switching packets at the optical layer. Pure OPS is SciFi as of today. Current solution is optical payload switching, but electronic header processing.
#Optical Burst Switching (OBS)