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, and the page has since grown with details and hooks. 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. It needds to implements all layers of the OSI model since it interfaces with end-devices. The transmission medium may be wired or wireless. The bottleneck of end-to-end transmissions usually occurs at the access point. Access Networks must implement some kind of MAC.
Transport-/Core networks
The central part of a network. Typically consists of high speed copper or fiber cables. Main components are switches and routers. As opposed to Access Networks, core networks 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. Electromagnetic wireless transmission suffers from the difficulties of noise, limited usable frequencies, and the huge difference in power required transmitting opposed to recieve data. This makes doing both at the same time impossible.
(Multi-path) Propagation
A transmitted signal propagates via many different paths to the destination. The receiving antenna only measures the sum of these waves, making the received signal a combination of these varrious paths which have experienced 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. This also weakens the signal.
Refraction
Waves travel different speeds in different mediums or densities. At the intersection of two mediums, the wave "bends". This is called 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. Different wavelengths can be affected differently by refraction.
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 high risk, considering it has a wavelength at around the order of 10cm.
Shadowing
Large or dense object may block the path of the wave, causing "blind spots" within transmission range.
Attenuation
Signal strength attenuates (gradual reduction in the strength) with transmission distance. The strength/energy of a spherical signal wave gets divided over a larger and larger area as the spere grows. Attenuation is greater at higher frequencies.
Fading
Fading is the variation of the attenuation of a signal, due to various factors. A fading channel is a communication channel that experiences fading.
Frequency-selective fading
Different frequencies experience different amounts of fading, often due to constructive and destrucive interference of multiple paths. This can be combated by usings equalization or errorcoding.
Interference
Waves that come from the same source through different paths, or from different sources and with 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, often due to the many factors described above.
Signal to Noise Ratio (SNR)
The signal strength divided by the noise level. Higher is better. Often denoted in the logarithmicly scaled decibel units:
Range
A signal has a finite range that can be split into multiple sub-ranges, classified below:
Transmission range
Within this range, the error rate is low, and it is deemed possible to communicate.
Detection range
Within detection range, the error rate is so high that communication is impossible. But it is however still possible to detect there being a signal (received power is higher than usual background noise).
Interference range
Within the interference range it might not be possible to detect the signal. The signal adds to the background noise.
Channel capacity
The capacity of a channel is the maximal achievable data rate that can be reliably delivered over a channel.
Calculation
The capacity if often calculated from the modulation scheme, amount of noise, and the amount of bandwidth (frequency range) being used.
In the case of 64QAM modulation,
Noise free capacity:
The limitation on the data rate is in this case the signal bandwidth and the number of distinct pulses that can be detected by the receiver.
Noisy channel:
The achievable data rate depends on the bandwidth and the noise level.
Spectral efficiency:
AWGN Channel (Additive White Gaussian Noise):
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.
There are many parameters of the carrier wave we can modify to modulate our data.
ASK (Amplitude Shift Keying)
The amplitude of the carrier is controlled by the data we want to transmit.
It is simple to implement, requires low bandwidth, but is susceptible to distortion. Works best in optical transmission.
FSK (Frequency Shift Keying)
The frequency of the carrier is controlled by the data we want to transmit. It s 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)
The phase of the carrier is controlled by the data we want to transmit. More resistant to interference than ASK and PSK, but is harder to implement. The transmitters and receivers must be synchronized for PSK to work. Works best in wireless transmission.
QAM (Quadrature Amplitude Modulation)
A combination of both ASK and PSK: Modulation of both amplitude and phase. There exist many version of QAM, often denoted as nQAM, e.g. 16QAM, 64QAM, 256QAM. The n denotes the number of distinct combination of amplitude and phase, often charted in a circular constalation as seen below for 16QAM:
QPSK(4PSK) and 8PSK can be visualized the same way, but with a single aplitude level.
MSK (Minimum-shift keying)
MSK works a lot like PSK and QAM, but it shifts the phase and amplitude half a signal apart instead of at the same time. This is to minimize the amount of powerful amplitude fluctuations in the signal. This is the same as OQPSK, but it encodes bits as a half sinusoid instead of square pulses, to combat the problems of non-linear distortion.
GMSK (Gaussian minimum-shift keying)
GMSK is similar to standard MSK, however the digital data stream is first shaped with a Gaussian filter before being encoded with MSK.
Performance
M-ary
Two or more bits (M bits) are transmitted simultaneously as a single symbol. This is done in e.g. QAM and 8PSK
Spectral efficiency
The amount of bits that are modulated per Hz
Error rate
The percentage of received erronous bits.
Coding rate
The Conding rate is used when describing the overhead and effectiveness of error correction coding.
Coding
The wireless channel is vulnerable to errors. Many channel encoding adds additional bits to improve the transmission reliability.
ARQ (Automatic Repeat reQuest)
ARQ is a error control method capable of detecting bit errors and retransmitting using detection codes. (ACK or timeout)
FEC/FEQ (Forward Error Correction)
FEC is an encoding capable of detecting and correcting a certain amount of bit errors using correction codes. The amount of bits it is capable of correcting is equal to the added overhead bits.
Performance enhancements
Spread spectrum
The signal is spread in the frequency domain over a wider bandwidth to combat frequency-selective fading. By using spread spectrum, interference will eventually despread and its influence will be reduced.
DSSS (Direct-sequence spread spectrum)
In DSSS, the original data signal is multiplied with a pseudo random noise spreading code. This spreading code has a higher chip rate (this the bitrate of the code), which results in a wideband time continiuous scrambled signal. DSSS is sometimes used in positioning systems
Orthogonal pseudo-random codes can be used to make the MAC called CDMA.
FHSS (Frequency-hopping spread spectrum)
The signal is spread over rapidly changing frequencies. Each available frequency band is divided into sub-frequencies. Signals rapidly change ("hop") among these in a pre-determined orderm known to both the transmitter and the receiver. Interference at a specific frequency will only affect the signal during that short interval. FHSS has the benefit of being easier to syncronize than DSSS.
Orthogonal pseudo-random codes can be used to make a MAC.
FHSS can be further dided into fast and slow hopping. In Fast Hopping, you hop to a different frequency multiple times per bit. In Slow Hopping, you transmit one or more whole bits before hopping to a different frequency.
OFDM (Orthogonal Frequency Division Multiplexing)
OFDM is an enhancement of FDM, where the sub-frequencies are able to be placed much closer to each other due to the sub-frequencies being orthogonal:
This avoids the need for any "guard space" between the sub-frequencies
You can divide a high data rate modulation stream into many subcarriers on parallel data streams. This makes the transmission of signals less likely to be ruined by frequency selective fading.
MIMO
The transmitter and/or receiver have multiple antennas. "Input" means the transmitter, "outut" means the receiver. There are multiple configurations possible:
SISO: Single Input Single Output
SIMO: Single Input Multiple Output
MISO: Multiple Input Single Output
MIMO: Multiple Input Multiple Output
Spatial Multiplexing Gain results in higher bit rates.
Spatial Diversity Gain results in smaller error rates.
Smart Antenna Gain (beamforming) results in less interference.
Exposed terminal problem
The exposed terminal problem occurs when a node is prevented from sending packets to other nodes because of a neighboring transmitter. Consider the case of S1 transmitting to R1 and S2 wanting to transmit to R2. S1 is in range to both S2 and R1.
Here, the node S2 concludes after performing a carrier sense, that it will interfere with the S1's transmission, even though this isn't the case since R1 is out of range of S2 and R2 is out of range of S1.
A RTS/CTS (Request to Send / Clear to Send) mechanism helps solve this problem if there is sufficient syncronization in place. If a terminal hears a RTS with no corresponding CTS, it can deduce it is a hidden terminal and transmit to other neighbors anyway.
Duplexing
Duplexing is having communication being able to travel both directions.
TDD (Time-division duplexing)
The channel is either used for either sending or for receiving at one point in time. The time spectrum is divided into receiving and transmitting slots.
FDD (Frequency-division duplexing)
Simultaneous transmission and reception, each at different frequencies. The spectrum is divided into sub-frequencies for sending and sub-frequencies for receiving.
Multiplexing and MAC (Multiple Access Control)
Multiplexing is the process of combining multiple data streams/channels into a single medium.
MAC is any method that allows multiple clients/users to share a common medium with minimal interference. MAC is often achived by multiplexing the datastreams of each client together.
MAC schemes can be divided into contention-free or contention-based MAC schemes. Contention-based MAC have each client "compete" for medium access, while contention-free schemes divides the medium among the users ahead of time.
Contention-free MAC/Multiplexing methods
SDM/SDMA (Space-division Multiplexing/Multiple Access)
SDM divides space into sectors and assigns each sector to a communication channel. This is often used in optical communication or with MIMO solutions, like beam-forming and cellular networks.
TDM/TDMA (Time-division Multiplexing/Multiple Access)
TDM divides the spectrum into timeslots, assigning certain slots to different channels
FDM/FDMA (Frequency-division Multiplexing/Multiple Access)
FDM divides the spectrum into smaller sub-frequency bands that is assigned to different channes.
OFDM/OFDMA(Orthogonal frequency-division Multiplexing/Multiple Access)
OFDM is the same as FDM, but it is able to squeeze the sub-frequencies much closer due to the sub.frequencies being orthogonal to each other. this avoids having as much "guard space" between the sub-frequencies. You can read more about it in the OFMD section listed under "performance enhancements".
CDM/CDMA (Code-division Multiplexing/Multiple Access)
CDMA allows multiple users to communicate at the same time on the entire spectrum by having them combine their data to transmitt witha known unique orthogonal code before transmitting. To decode the signal, you correlate the received signal with the code corresponding to the user to want to receive from.
Contention-based MAC
ALOHA
ALOHA was the first public demonstration of a distributed random access MAC. Pure ALOHA was simple:
- If you have data to send, send the data
- If, while you are transmitting data, you receive any data from another station, there has been a message collision. All transmitting stations will need to try resending "later".
Note that the first step implies that Pure ALOHA does not check whether the channel is busy before transmitting. Pure ALOHA cannot use 100% of the capacity of the communications channel. How long a station waits until it transmits, and the likelihood a collision occurs are closely interrelated, and both affect how efficiently the channel can be used. The quality of the backoff scheme chosen significantly influences the efficiency of the protocol.
Slotted ALOHA
Slotted ALOHA introduced discrete time slots. A station can only start a transmission at the beginning of a timeslot, and thus collisions are reduced, resulting in twice the throughput of pure ALOHA.
CSMA (Carrier Sensing Multiple Access)
If a user wants to transmit, it first senses the carrier. If the carrier is idle, transmit with a certain probability otherwise, wait for some time and try again. If there was a collisison, then retransmit the data.
CSMA/CD (CSMA with Collision Detection)
If a collision is detected, stop and wait before trying again. This is not possible in the wireless medium, because receiving data is impossible while transmitting. This means only the receiver can detect whether there was any collision or not.
CSMA/CD was often used in wired networks, but has fallen out of favor once switches started replacing hubs, meaning traffic became routed instead of broadcasted.
CSMA/CA (CSMA with Collision Avoidance)
If the medium is busy, then the transmitter waits until it the medium idle before it itself starts transmitting.
Often used in the wireless medium, since Collision Decetion is not possible at the transmitters side
MACA (Multiple Access with Collision Avoidance.) (RTS/CTS)
No carrier sensing. A user must send a RTS (Request To Send) and receive a CTS (Clear To Send) before being allowed to transmit. When other users hear a RTS or CTS, then they remain silen to allow the message to be transmitted undisturbed. These frames are usually sent in a discrete timeslot framework.
RTS and CTS have slight misleading names. RTS can be better described as a "back off" request, while the CTS is an ACK of the RTS.
MACA solves the hidden teminal problem, and partially solves the exposed terminal problem. You can deduce you're an exposed terminal if you hear a RTS with no matching CTS.
MACAW
MACA for WLANs (wireless LAN). It requires nodes sending acknowledgements after each successful frame transmission, as well as the additional function of Carrier Sense (Don't send a RTS unless the medium has been idle for some time).
Cellular networks
To get better coverage and capacity over a area, you can divide the area it into smaller chunks, which in access networking is called "cells". This can be regarded as a form of SDM/SDMA
A cellular network structure/topology is usually theoretically modeled using hexagonal cells. This however doesn't always align with reality, due to area-dependent fading and obstacles.
Some basic terminology:
- Base Station(BS): Stationary nodes/towers broadcasting the signal
- Mobile Station(MS): The user being served by the Base Station. May also be called a Subscriber Station(SS), Mobile Terminal(MT) or Mobile Node(MN)
- Downlink(DL) vs Uplink(UL):
Cells
A single cell, is the area covered by a base station. A cell can be further divided, using directional antennas, into "sectors".
Cell breathing
When there are many users in a single cell that iterferes with each other, the effective range of the cell may decrease.
Changes in the cell sizes are often refered to as "cell breathing". In GSM, where each user has assigned timeslots in TDMA, 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.
Sectors
A sector is partial area of a cell that is served/covered by a directional antenna in a base station.
Frequency reuse
Frequency reuse is a term used when using a MAC which can't handle multiple base stations too close to each other broadcasting on the same frequency. But a frequency can be reused in multiple cells if they are far enough apart.
Cells are bunched together into clusters where each cell in the cluster uses a unique frequency. (seen as colors on the figure below) This cluster pattern may then be repeated accross the cellular network. The "Frequency reuse factur" describes the number of cells in this cluster.
UMTS/CDMA is unaffected by frequency reuse, and may have a reuse factor of 1.
Frequency reuse factor
The Frequency reuse factor describes the number of cells in the clusters. This equation is used to calculate this factor.
Example:
The distance between cell centers with the same frequency band is required to be more than 6km. What is the cell radius for a cluster size of 12?
Typical values of N: 1,3,4,7,9,12,13,16,19,...
Handover
The act of changing the serving base station with an another base station. These can be further categorized as "Hard" and "Soft" handovers, each with their own benefits and weaknesses.
(Handover is called "handoff" in the US)
The Mobile Station smoothly changes the serving Base Station due to:
- Movement the a new cell
- Radio channel variation
- Load balancing
- Minimization of mobile power consumption and global interference
- Change of access technologies (Switching from GSM to UMTS)
The handover may be initiated both by the mobile or by the network, or it may be a cooperative effort where the mobile provides decision making data to the base stations.
There are multiple types of handover:
- Intra-cell handover: Switch to the same Base Station, but different channel. This happens often due to interference.
- Inter-cell handover: Transition between different base stations.
- Inter-system handover: Between 3G and LTE, etc
Hard handover
In a hard handover, the radio link to the existing Base Station is released before the new link to new BS is established. It can cause interruption of the data flow, but this means the mobile station only needs to support using 1 radio channel at a time. A hard handover may also be implemented as a single event in the network, which is simpler to implement.
Soft handover
I Soft handover however, the data flow is not interrupted as opposed to hard handovers. During the handover, all data is transmitted over 2 or more radio links simultaneously, resulting in a improved Quality of Service (QoS). Soft Handover is however more complex to implement, since it requires coordinationg of multiple base stations, and can not be reguarded as a single event, but instead a series of states.
Seamless handover
In performing a seamless handover, the data flow is not interrupted, like in soft handover. The radio link to the old base station is however released as soon as the new radio link is up. This still requires the mobile station to be able to maintain 2 simultainious radio links for a short period, but it is simpler to implement on the network side.
GSM (2G)
GSM stands for "Groupe Spécial Mobile". GSM is a Circuit-switched network intended for voice communications. It divides the frequency bandwidth into 125 channels (124 effective) with FDMA, and further divides these channels using TDMA with FDD. It uses FEC for error correction. The use of TDMA ensure GSM have no cell breathing.
Modulation: GMSK (Gaussian Minimum Shift Keying)
GPRS (2.5G)
Stands for "General Packet Radio Service". GPRS is an extention of GSM that allows one user to occupy any number of timeslots (ie. higher datarate). GRPS also introduces packet switched data transfer.
EDGE (2.75G)
Stands for "Enhanced Data Rates for GSM". An extention of GPRS. It uses 8PSK modulation which has 3 bps/Hz spectral efficiency. It uses hybrid ARQ to error handling.
UMTS (3G)
Stands for "Universal Mobile Telecommunications System" It uses DSSS over two 5MHz FDD channels. On these channel it uses W-CDMA (Wideband CDMA) with QPSK (4PSK) modulation. UMTS has a circuit switched and packet switched side for voice and data transmissions respectivly. The use of W-CDMA (autocorrelation with orthogonal codes) allows UTMS a to have frequency reuse factor of 1, making soft handovers possible with a single antenna on the mobile station.
HSPA (3.5G)
Evolution of W-CDMA, uses up to 64QAM modulation, with adaptive coding and error correction.
LTE (4G)
Stands for "Long-Term Evolution". Has no circuit switched component, it is purely packet switched. You may use Voice over IP (VoIP), but LTE is designed to work alongside 2G/3g, which may handle the voice transmissions instead. This results in a more flat and simple backend/core network. LTE uses OFDM/OFDMA on the downlink and SC-FMDA (Single-Carrier FDMA) on the uplink with TDMA and MIMO (beamforming on downlink), with QPSK/QAM modulation. Channel-dependent scheduling is used to give users good channels.
LTE-A (Advanced)
Harder, better, faster, stronger
WLAN (IEEE 802.11)
WLAN stands for "Wireless Local Area Network" WLAN can be found in different configurations:
- Can be infrastructure based, with certian access points (AP) which is connected to the internet.
- Can also be ad-hoc based, where stations have peer-to-peer (P2P) connections with each other.
- Can also be a mesh, where multiple P2P connections propagating the signal of distant APs.
The IEEE 802.11 MAC is CSMA-based with with MACA(RTS/CTS) with IFS and backoff. IEEE 802.11 has procedures when scanning for networks (both passive and active) and Association/Reassociation.
IFS
Stands for "Initial Interframe Space"
- SIFS: Short IFS - used for highest priority frames such as ACKs
- PIFS: Point Coordination Function (PCF) IFS - used for medium-priority time-critical frames
- DIFS: Distributed Coordination Function (DCF) IFS - used for asynchronous data frames
- Station waits for a DIFS amount of time before sending any RTS frames
- The receiver acknowledges the RTS with a CTS frame after waiting for SIFS amount of time.
- The sending station sends data immediately and waits for an ACK
- Other stations store medium reservation based on the heard RTS and/or CTS
PCF and DCF access
- Time critical services use PCF(point coordination function)
- The access point sends a beacon frame to all stations. Then uses a polling frame to allow a particular station to gain contention-free access in a reserved time period.
- This is not really used in practice
WPAN (Bluetooth and IEEE 802.15.4 aka ZigBee)
WPAN stands for "Wireless Personal Area Network" Networks that connect devices within a short range. Used as replacement of cables and ad-hoc networking.
Bluetooth
Bluetooth as a small and low-power radio, designed for low cost. It uses FHSS with 79 hopping channels, dividing each channel with TDMA. It uses Gaussian FSK modulation. Both FEC and ARQ are used for error correction depending on the packet.
Bluetooth MAC is controlled by a Master that polls slaves explicitly or implicitly. Multiple piconets coexists with one master each. The piconets (up to 10) may form a scatternet, which has efficient bandwidth utilization.
Piconet
Basic unit of Bluetooth networking. It may consist of one master and up to 7 slaves.
IEEE 802.15.4 / ZigBee
- Desired in IoT
- DSSS CSMA/CA with backoff
- NO support for voice
- Supports both star and mesh topologies
Optical fibre
Optical fibres are well suited for high bandwidth data transfer because they a lot of usable bandwidth (50THz in theory), low attenuation, it is light and thin and easy to install, cheap to produce(it's made of sand!). It is immune to electromagnetic interference, but is difficult to splice and the connectors are complex.
Modulation
The fiber is a waveguide guiding the light along the cable. The light is usually generated by either a laser or a LED. LEDs are cheaper and easier to control, but has low power, range and is susceptible to dispersion. Lasers on the other hand have high power, but are difficult (read: more expensive) to control.
Optical fibres uses OOK modulation (on-off-keying)
The receiver uses photodiodes.
Light propagation thorugh fibre
The fibre cable is made of glass which acts as a waveguide, due to is total internal reflection. Lighpulses are reflected in the core of the fibre when hitting the cladding. This is due to the core having a higher refractive index than the cladding (the index is 1468 at 1550nm).
This allows light to be reflected if the angle of entry is not too steep. Due to this, fibre optic cable can not be bent beyond certain tolerances before losing its light propagating properties. The critical angle for total reflection may be calculated using Snells law.
Attenuation
Attenuation: a gradual reduction in the strength.
Signal strength attenuates with transmission distance. The attenuation is low in materials such as fibre, but it is not non-existant. If you replaced the oceans with fibreglass, you'd be able to see the ocean floor from the surface.
Attenuation in fibre may be due to Rayleigh-scattering, Absorbtion or radiation loss.
The fibre has two slices of usable bandwidth where the attenuation is low.
Attenuation must be accounted for over larger distances. This is done with DWDM components like the optical amplifier EDFA (Erbium Doped Fiber Amplifier).
Dispersion
Pulses are gradually spreading/becoming wider when propagating through the fibre. Too much spreading results in intersymbol interference (the bits bleed into eachother), effectively limiting the maximum transmission rate. Dispersion depends on the fibre type. There are multiple types of dispersion due to different factors. You may use dispersion compensation employing fibre or electronic compensation to compensate for this.
Chromatic dispersion
Different wavelengths of light travel at different speeds through the fibre. This is due to the reflection being a function of the wavelenght, causing the different colors to travel different distances. Chromatic dispersion is a sum of material and waveguide dispersion. You may use Dispersion Compensator (DCM / DCU) to align the colors again.
Zero dispersion
Zero total dispersion is achieved when the effects of the multiple factors of dispersion add up to 0. This can be achived at set frequencies. This is usually 1300nm in standard fibre, or ~1500nm in dispersion shifted fibre.
Dispersion Compensating Fibre (DCF)
Dispersion Compensating Fibres counteracts the effects of dispersion. It has negative dispersion compared to standard transmission fibre. It has a Much higher dispersion/km. This means shorter streches of DCF fibre than transmission fibre are required for achieving zero dispersion.
Regeneration
The types of regeneration is often refered to as RRR, or the 3 R's:
- 1R regeneration (amplification)
- usually an optical amplifier
- Amplifies the signal without conversion to electrical
- Typically transparent for signal (shape, format and modulation)
- 2R Reamplification & Reshaping
- Reshapes the flanks of the pulse as well as the floor and roof of the pulse, removes noise.
- Usually electronic
- Optical solutions still subject to research
- 3R Reamplification & Reshaping & Retiming:
- Synchronisation to original bit-timing. (regeneration of clock)
- Usually involves electro-optic conversion
- Optical techniques in the research lab.
Erbium Doped Fiber Amplifier (EDFA)
A DWDM component which amplifies the optical signals passing through. These are places along the span of longer distances to boost the signal.
These can not be used on Coarse WDM signals. EDFA may introduce some noise which is taken care of using filters.
Optical switches
Optical switches are switches that enables optical signals to be selectively switched from one circuit to another.
Opaque Network - Fixed Patch panel
Fixed patch panel between WDM systems with transponders. Here a patchboard is used to patch the different channels
Opaque Network - Opaque Switch
Electrical switch fabric between WDM systems with transponders.
Opaque Network - Transparent Switch
Transparent switch between WDM systems with transponders, complemented by a OEO switch for drop traffic.
Transparent Network - Transparent Switch
Transparent switch on a transparent network. The signal stays optical until it exits the network.
Optical couplers
One or more fibers in, one or more fibres out. It divides the optical signal on several fibres. Signal power is divided on the output-fibres. Splitting ratio may vary. (50/50 means 50% on each wire) Attenuation from input to output depends on splitting ratio: 50/50 splitter results in 3 dB attenuation (halving the power). Couplers may be deployed as both splitters and combiners.
Arrayed Waveguide Grating (AWM)
Here a splitter and coupler divides light on N waveguides of different length. On each of the N outputs, constructive interference is achieved for a specific wavelength and destructive interference for the other wavelengths
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
Optical multiplexers are used to divide the fiber into multiple channels. Wavelength-Division Multiplexing(WDM) technology combines mutliple optical carrier signals onto a single optical fiber by using different wavelength as carrier. WDM systems uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split the signals apart. There are multiple methods and techniques:
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
CWDM stands for "Coarse Wave Division Multiplexing". The most cost effective solution, capable of providing 8 channels with 8 wavelengths. CWDM can not be amplified at in limited to around 160km distance.
DWDM
CWDM stands for "Dense Wave Division Multiplexing". Much more expensive than CWDM, but capable of providing 40, 80 or even 160 channels due to narrower wavelength spans. DWDM mutliplexing systems are capable of having longer transmitting ranges with less interference.
Optical Transport Network (OTN)
OTN is a standard for optical networks. It is used for framing of different protocols for transport over the physical optical layer (IP/Ethernet over fiber). It has management functionality, monitoring functionality and FEC (forward error correction).
Protection schemes
Optical transport networks need protection agains cut/hit cables and link.
Dedicating resources
1+1 protection
Continuous signal on two alternative paths, we simply choose the best. If one breaks we have a backup! It provides hitless protection switching (switching without loss)
1:N protection
Several parties share a single common protection path. Enables the path to be employed by low priority traffic when not in use. Implies information loss because of switching happens after hit discovery. Data in the fibre is lost. Not hitless.
Carrier(-Grade) Ethernet
"Carrier-Grade Ethernet" is a loosely defined marketing term. In the context of this means the extensions of Ethernet used by service providers. Ethernet for MANs (Metro Area Networks). In carrier ethernet, you simply wrap incomming ethernet/ip packets in an another ethernet header for the ethernet level on the transport ethernet. This means you can use routingprotocols already made for ethernet, you get VLAN, scalability, reliability, QoS and service management.
MAC-in-MAC
The MAC header is added at the edge of the service provider, chosen from a dedicated set of MAC addresses. The header contains customer ID, VLAN ID, service ID and more. This provides total separation of the customer and service provider networks.
Fiber to the X/Fiber to the Premise (FttP/FttX)
Used to descrive the fiber setup in a neighberhood/for a customer.
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. This is expensive, and could simply be multiplexed together instead, which leads to:
Curb-switched
One fiber runs to a switch near the homes (the curb switch). One fiber runs from each home to the switch. Uses less fiber cable, but the switch itself 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/attenuation than a powered switch.
Fiber to the Building (FttB)
Fiber to the Curb (FttC)
GMPLS
Generalized Multi-Protocol Label Switching. Common control plane for optical and packet networks. Extended MPLS, but now supports the optical layer. GMPLS differs from MPLS because it doesn't add labels to the packets. Implicit tags are defined by physical properties. We want this behaviour because we want high bandwith in core networks. Switching on physical properties allows us to avoid header lookups.
Switching can be defined by eg.: Fibre in a bundle Wavelength in a waveband Wavelength in a fibre Or even a timeslot on a waveband in a fibre
GMPLS also implements a lot of other services. For example routing protocols like OSPF (Open Shortest Path First) and Link Management Protocols.
GMPLS is so amazing that it supports protection and restoration aswell.
GMPLS controlling across network layers
Burst, packet and hybrid switching
Goal: Want to utilize the resources better than OCS (Optical Circuit Switching), but still have the same behaviour (ish) Also want packet switching/routing as in OPS.
Realised by sending Control Packets on designated channels. We have separated the routing information (OH) from the user data/payload. Controled Packets are sent ahead of the payload with an offset sufficient for each node to convert the Control Packet to electrical, analyse the path and configure the node. When the payload arrives the node, there is no need for OEO convertion and lookups, we can simply forward the payload. Maybe it's unclear, but by doing this, the payload experience a circuit switched path without having the whole path reserved at the same time as OCS does.
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.