TTM4105: Access and Transport Networks
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.
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.
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 transmission is the transmission of data between points that are not connected by a guided medium. Typically using electromagnetic waves.
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.
A smooth surface reflects the signal. Weakens the signal.
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.
A wave is bent around a large object.
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.
Large objects might block a signal.
Signal strength attenuates (gradual reduction in the strength) with transmission distance. Attenuation is greater at higher frequencies.
Fading is the variation of the attenuation of a signal.
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.
A signal might suffer modifications during transmission or capture.
Signal to Noise Ratio (SNR)
A signal has a finite range that can be split into multiple sub-ranges.
Within this range, the error rate is low, and it is possible to communicate.
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).
Within the interference range it might not be possible to detect the signal, but it adds to the background noise.
Capacity the maximal achievable rate that can be delivered reliably over a channel.
The limitation on data rate is the signal bandwidth.
The achievable data rate depends on the bandwidth and the noise level.
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.
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.
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.
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.
Combination of ASK and PSK. Modulation of both amplitude and phase.
Spectral efficiency: how many bits are modulated per Hz
The wireless channel is vulnerable to errors. Channel encoding adds additional bits to improve transmission reliability.
Automatic Repeat reQuest is capable of detecting bit errors and retransmitting using detection codes.
Forward Error Correction is capable of detecting and correcting bit errors using correction codes.
By using spread spectrum, interference will eventually despread and its influence will be reduced. Resistant to frequency-selective fading.
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
Exposed terminal problem
Duplexing allows communication in both directions.
The channel is either used for sending or for receiving. The entire spectrum is divided into timeslots.
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.
Space Division Multiplexing divide space into sectors and assigns each sector to a communication channel.
Time Division Multiplexing assigns the whole spectrum to each communication channel at different time slots
Frequency Division Multiplexing divides the spectrum into smaller frequency bands that is assigned.
Orthogonal Frequency Division Multiplexing.
Code Division Multiplexing lets each communication channel use the entire spectrum at the same time but with a unique orthogonal code.
Distributed random access MAC.
Twice the throughput of pure ALOHA.
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
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
Carrier Sensing Multiple Access with Collision Avoidance. CSMA/CD is not applicable in wireless since we might not be able to detect a collision.
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.
MACA for WLANs.
Coverage area of a base station.
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.
Part of a cell covered by a directional antenna.
Frequencies can be reused in many cells if the cells are far enough apart.
Frequency reuse factor
The act of moving the connection from one basestation to another.
Radio link is interrupted during the switch.
Data flow is not interrupted. A connection to both basestations might be active for a while.
Data flow is not interrupted. Radio link to the old base station is released as soon as the new radio link is up.
Groupe Spécial Mobile. Circuit-switched voice communications. Using FDMA/TDMA with FDD. Modulation: GMSK.
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.
Enhanced Data Rates for GSM. Extention of GPRS.
W-CDMA. Voice is circuit switched. Data is packet switched.
OFDM/OFDMA, TDMA, FDMA, MIMO. Pure packet switched. Channel-dependent scheduling is used to give users good channels.
CSMA-based, with IFS, with RTS/CTS, with backoff
Polling (TDMA), FHSS
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.
Pulse spreads in long fibre lines. Results in intersymbol interference, effectively limiting the maximum transmission rate. Dispersion depends on fibre type.
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)
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
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.
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.
Coarse Wave Division Multiplexing. The most cost effective solution.
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.
Send the same signal on two alternative paths. If one breaks we have a backup!
Have a backup path available for each path, but it may be used for other low priority traffic when the primary path is working.
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)
One fiber runs from each home to the service provider. No external power is needed, but uses a lot of fiber cable.
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.
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)
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.