CSE5808 QoS in Digital Communication Networks (1) 1 14.1 Introduction Ongoing demand for mobility in services has resulted in unprecedented growth of wireless

CSE5808 QoS in Digital Communication Networks (1) 1

14.1 Introduction• Ongoing demand for mobility in services has resulted in

unprecedented growth of wireless services and mobile satellite services.

• Any radio-frequency wireless channel introduces a wide range of possible impairments to QoS.

• Any user connected to a network with one or more wireless channels is exposed to QoS degradation during transmission.

• Loss of channels, reduced Eb/N0 and effects like fading and time varying latency can cause packet losses, resulting in retransmission and problems with transmission delay and jitter.

• Given the high likelihood that a large fraction of future systems will include a wireless link to the end user terminals, wireless link impairments must be considered in QoS modelling.


Wireless links can be divided into many categories:

• By frequency/wavelength:

1. Radio-frequency links in various bands

2. Optical coherent and incoherent links in various bands

• By link geometry:

1. Terrestrial links – static and carried by mobile platforms

2. Airborne links carried by manned or unmanned aircraft

3. Satellite links in Low Earth Orbit (LEO), Medium Earth Orbit (MEO) and Geostationary Earth Orbit (GEO)

• By platform mobility:

1. Land vehicles eg cars, trucks, busses

2. Maritime platforms eg ships

3. Aircraft operating at various altitudes

4. Satellites


• Each category is exposed to specific impairments to QoS which must be understood and modelled when designing the network.

• The consequences of improper or absent modelling will be QoS degradations which become difficult to predict and result in revenue losses to the users and providers

• Extensive research literature exists covering most of these problems in detail. Satellite link impairment modelling is most mature.

• Any model used is critically dependent on the geometry of the wireless link, and the propagation environment vs the wavelength used for the link.

• In the QoS context we are most interested in:

1. Link availability - dropouts and fading

2. Packet loss

3. Latency and delays

4. Jitter


14.2 Characteristics of Mobile and Satellite Links• All wireless link behaviour is governed by the physics of radio

propagation. A link must comprise a transmitter with an antenna (or optical aperture) and a receiver with an antenna (or optical aperture).

• The transmitter emits modulated radio-frequency power through its antenna, that power propagates through the environment, some fraction of that power impinges on the receiver antenna, is amplified and then demodulated.

• Antenna (and optical aperture) designs vary widely with operating wavelength and intended performance. The most important parameter of interest is gain G, which is inversely proportional to the antenna mainlobe width. The power at the receiver is then determined by the Friis formula:

• Pr = Pt .(Aet.Aer)/(r2 . lambda2) where Pr is power at the receiver, Pt is transmitter power, Aet.Aer are the effective antenna apertures, r is the distance and lambda is the wavelength at which the link is operating; gain is related to effective aperture by Gi = 4 pi Aei / lambda2


Friis Propagation Loss Model:

r3

Friis

Pathle

ngth L

oss [d

B]

Orbital Receiver

Friis Pathlength Loss [dB]

Ground Station

Terrestrial Receiver

Airborne Receiver

Pt

Pr

Pr

Pr

Aer

Aer

Aer

Propagation Losses

r1Friis Pathlength Loss [dB]

r2


• Distance determines the basic power budget for the link, assuming a given modulation, carrier wavelength, antenna and receiver performance. Even if this power budget is satisfied, the link QoS will still depend on other propagation losses incurred in transit.

• Distance also determines the latency for the link – c = 3.108

• Distance continuously changes with time, if the platforms carrying either or both the transmitter and receiver are in relative motion.

• With the exception of fixed terrestrial links and fixed ground station links to/from GEO satellites, we have to assume that distance and thus link losses and latency are variable in time.

• Because the Friis equation is an inverse square law relationship, ever increasing power or aperture sizes are required with increasing distance. In practical terms mobile links require a large power margin, or active control of transmitter power to achieve suitable Eb/N0.

• Variable latency in mobile and MEO/LEO satellite links means we have to assume worst case link distances to bound latencies when estimating QoS.


Latency in Satellite Links:

Ground StationGround Station

TroposphereCrosslinkUplink/Downlink

GEO Satellites35,786 km

LEO Satellites700-1200 km

MEO Satellites 7,000-10,000 km

Tropopause 11 kmAircraft


Pseudolites – Airborne Emulation of Satellites

• Several projects proposed or in development, including DARPA ABN, ‘Skysats’ and Angel Technologies HALOTM.

• Rather than place equipment on orbital platform outside the atmosphere, use High Altitude Long Endurance aircraft orbiting immediate area within atmosphere.

• Advantages – cheaper per bandwidth, reduced latency (varies with orbit size), easily redeployed, cheaper to maintain/sustain.

• Disadvantages – same weather dependencies in propagation, smaller footprint.

Angel Technologies Image


Angel Technologies Image


Pseudolite - HALE Network Relay

• 50,000-60,000 ft operating altitude


Pseudolite - HALE Network Relay

OVERCAST

(c) 2001, Carlo Kopp

SATELLITE

HALE UAV WITH NETWORKING RELAY PAYLOAD


Mobile Ad Hoc Networks

• Ad Hoc Networks are cooperative self healing, self forming wireless networks in which every node acts as a router.

• There is no fixed topology in an ad hoc network. Topology is determined by the immediately available connections between nodes in the network.

• Packets hop from node to node until they reach their destination, with every participant carrying traffic from its neighbours, if required.

• In Ad Hoc Networks, route discovery is mostly dynamic. When a connection is needed, a route is discovered and used.

• QoS in Ad Hoc Networks presents unique problems since the topology of the network and thus optimal routes are continuously changing.

• Dropouts may arise due to loss of connectivity between nodes.

• Nodes which become a ‘cut vertex’ in the topology can become rapidly congested with traffic.


Mobile Ad Hoc Network Example

-> RESILIENT

EACH NODE ROUTES NETWORK

(c) 2001, Carlo Kopp

-> SELF ORGANISING

-> REDUNDANT-> SELF HEALING

TRAFFIC AS REQUIRED


Link Geometry Issues

• Fixed terrestrial link distances are bounded by the curvature of the earth and elevation of the antennas. In practical terms tens of kilometres are viable. The same is true for links between ships at sea.

• Mobile systems and mobile to base station links obey the same physics. Because a human portable or automotive antenna have an elevation below two metres, achieving even kilometres of distance can be problematic.

• Distances in airborne links – between aircraft and base stations – vary with the altitude of the aircraft. Jets cruise at about 11 km altitude, permitting a distance to a ground station of about 400 km, and between aircraft to about 600 km. High Altitude Long Endurance aircraft permit even greater distances.

• Satellite link distances depend on orbital altitude and the immediate relative geographical location of the ground station. MEO and LEO satellites are always in relative motion to the earth. The bounds on distances are set by the orbital parameters of the satellite.


14.3 Atmospheric Propagation Impairments• The earth’s atmosphere is an unfriendly environment for radio

propagation. A wide range of effects can impair transmission links, especially where the wavelength is less than several centimetres in size.

• Atmospheric losses arise mostly in the troposphere, which is laden with clouds, water vapour and dust particles.

• Gaseous propagation losses arise due to the interaction of the wave with water vapour molecules and oxygen molecules. These losses are especially severe at 22 GHz and 60 GHz, and increase with decreasing altitude and increasing humidity.

• Clouds are dense collections of microscopic water droplets which scatter microwave signals. Increasing cloud density [g/m3] and increasing frequency result in increasing losses.

• Rain droplets also scatter microwave signals, with losses increasing as a function of the rain rate [mm/hr] and frequency. Tropical monsoons can frequently make satellite links unusable.


• Melting layer losses occur at the 0o C isotherm in the troposphere. This is the transition between liquid water droplets and ice particles.

• Depolarisation can arise due to rain droplets and ice particles. The result of this is that energy in one polarisation is coupled to its orthogonal polarisation.

• Scintillation effects will arise in the troposphere. Pockets of air with slightly different temperatures will cause the beam to refract slightly as it passes through the boundary. The result is incremental defocussing and a small loss of power.

• Dust and aerosol clouds or layers can also cause propagation losses, the effect is strongest with shorter wavelengths.

• The design of any satellite or airborne link, or long haul terrestrial link, must consider propagation losses.

• Rain and cloud losses are usually dominant, and present difficulties in modelling since both will vary strongly over time.


40 kft20 kft

Low Altitude Adverse Weather

A (Ae) + Platform Altitude

Platform Altitude

Diffraction Region

Weather Penetration Distance

LOSLOS Range Limit

Hi

A (Ae)

Platform Altitude Range

Hg (Grazing Alt)

Weather Penetration Distance (Effective Path Length)

GEOMETRICAL MODEL FOR ATMOSPHERIC LINE OF SIGHT PROPAGATION - GRAZING (B) MODEL


Atmospheric Gaseous Absorption Losses


Gaseous Losses – Upper Millimetric Bands


Gaseous Losses vs Altitude


Water Vapour Density vs Altitude


Cloud Vapour Losses – Staelin/CCIR


Precipitation Loss Models


Optical Band Transmissivity


• To model the power budget for any mobile, airborne or satellite link, we must know the distance between the stations, and the atmospheric propagation conditions for the calculation. Without the latter the calculation will be unrealistically optimistic.

• To calculate a power budget for such a link, we need the following:

1. Transmitter power and aperture specifications.

2. Receiver sensitivity (at what C/N-Eb/N0 is BER OK) and aperture specifications.

3. Distance – or range of distances applicable.

4. Carrier wave frequency / wavelength.

5. Altitude of the propagation path.

6. Cloud density [g/m3]

7. Rain rate [mm/hr]

• Numerous texts have example satellite link power budget calculations (eg Ippolitto LJ, Propagation Effects Handbook for Satellite Systems Design, NASA 1082 (04), 1989.)


14.4 Fading Effects vs QoS• Fading is a common problem observed with ground mobile and

airborne links, the latter at low altitudes (eg helicopters, light aircraft).

• Fading arises as a result of the receiver seeing multiple copies of the incoming signal, one arriving via direct path from the transmitter, and the other via indirect paths, bouncing off terrain, buildings and other objects in the vicinity of the receiver (‘ghosting’ on analogue TV is essentially the same problem).

• At the receiver, the multiple copies of the signal arrive with small relative time differences. They add or subtract changing the total signal power seen by the receiver.

• If either station in the link is moving, the total signal power at the receiver will change over time – hence the term fading.

• When a signal experiences fading, the C/N ratio varies over time. For many modulation types, this can produce strong changes in Eb/N0 and thus strong changes in BER.

• Some modulations, such as COFDM or FM, are more resistant to fading than others, such as QAM or ASK techniques.


Fading Behaviour vs Multipath

Mobile Station

Mobile Station B

BuildingsElevated Terrain

Pathlengths DifferLatencies Differ

Mobile Station A

Automobile with


Fading models (Steele 1992)


• In general, fading is an unavoidable reality where the link modulation type is known to be susceptible.

• Some receiver designs are more resistant to fading than others, but in worst case situations dropouts may still occur.

• From a QoS perspective fading manifests itself in time variant packet losses, or complete link dropouts.

• Fading can also result in increased jitter as the receiver switches between carrier signals.

• When assessing packet losses and jitter performance, knowledge of the modulation type and its performance in fading environments is essential.

• Extensive research exists on fading behaviour in mobile channels.

• The most commonly used models for fading are Ricean and Raleigh channel models.


14.5 Ensuring QoS when using Satellite Links• In many remote areas satellite links are the only available channel

providing access to the global network infrastructure. Many such areas are in the tropics (Africa, South East / South Asia) where dense cloud and heavy rain are seasonal occurrences. How can be guarantee link availability and Eb/N0 in such environments?

• Spatial Diversity – install multiple satellite systems at locations separated by many kilometres. At any time choose the system which is providing the best Eb/N0. This technique relies on cloud and rain never being uniform in density at any time.

• Wavelength Diversity – use two satellite systems each operating in a different frequency band, preferably very widely apart. At any time choose the system which is providing the best Eb/N0. This technique relies on propagation losses varying with wavelength.

• In practice Spatial Diversity is the preferred method. It is necessary to gather detailed statistical data on rainfall and cloud patterns in the area of interest to establish the best distance for ground stations.


14.6 Ensuring QoS when using Airborne Links• Airborne links are expected to be available in the post 2010 period

• While they will operate at much higher bit rates than satellite links, they will share the same difficulties seen with satellite systems, especially where shallow antenna elevation angles are involved.

• Airborne links will exhibit cyclic variations in latency as the platform carrying the networking equipment flies in a circular orbit – the size of the orbit will bound the latency for a fixed ground station.

• Packet loss behaviour will be limited primarily by carrier wavelength weather dependencies in Eb/N0, and fading behaviour resulting from local terrain and obstructions.

• Spatial diversity may not be viable given the geometry of the orbit.

• In general, link quality is maximised by using steep antenna elevation angles, ie placing the ground station as near to the orbit as possible.


14.7 Ensuring QoS in Ad Hoc Networks• Ad Hoc Networks presents QoS difficulties due to propagation and

fading problems in wireless links, but also due to the continuously changing topology.

• These problems are most pronounced in networks formed using human portable or automobile carried nodes, where fading contributes to topology changes, and antenna elevation is inherently small.

• Guaranteed connectivity is nearly impossible in low density networks, where there are no redundant routes.

• Even where redundant routes exist, topology changes present difficulties for latency and jitter guarantees.

• At this time no protocol is defined for Ad Hoc Network QoS – services provided on a ‘best effort’ basis.

• An Ad Hoc Network QoS protocol will have to be highly dynamic to permit rapid switching between routes and reallocation of resources.

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CSE5808 QoS in Digital Communication Networks (1) 1 14.1 Introduction Ongoing demand for mobility in services has resulted in unprecedented growth of wireless