communication systems an overview paul norman communication and information systems modelling

Communication Systems

An Overview

Paul NormanCommunication and Information

Systems Modelling

of 27

Communication Systems Overview - Topics

• Speaker Background

• Introduction to BAE SYSTEMS Communication and Information Systems Modelling Group (4 Minutes)

• What is a Communications System ?

• Circuit Switching

• Packet Switching

• Packet Switching Networks - Routing

• Packet Switching Networks - Congestion Control

• Packet Switching Networks - Quality Of Service

• Management

• Washup

of 27

Speaker Background

• Software Background, involvement with communication systems throughout career.

• Past Application Areas:

– Commercial Security and Fire Systems

• Development of communications for distributed security system– Telecommunications

• Design & development of telecommunications equipment and certification against national standards

• Design & development of test systems for telecommunications testing– Traffic Management Systems

• Development of traffic management systems and motorway sign control systems for the UK Highways Agency.

• BAE SYSTEMS - CIS Modelling

– Development of software systems for the UK Army Communications Test and Reference Centre.

– Leading Air Traffic Management Modelling Projects

of 27

Off Line Modelling

• The Ptarmigan Modelling Facility (PMF) provides high fidelity circuit switch and packet switch network modelling over a terrain database

• Models of communication systems which closely represent the real system - “from bit to battlefield”

of 27

On-line Real Time Modelling

• Synthetic battlefield scenarios directly interacting through a model and real command information systems

• Real Time modelling of communications systems such that the model operates on an event by event basis in real time

of 27

Synthetic Environments

• Allows all elements of the command structure within a given scenario to be represented

• A synthetic environment for the integration of military systems

PJHQ

MCC/JFHQ

LCC

ACC

Watchkeeper

Tracer

Communications Model

of 27

Testing Infrastructure

• Testing Infrastructure provides integrated environments to support the development, integration, and testing of complex communications systems.

• The Land Systems Reference Centre is the British Army Communications Test and Reference Centre.

• Modern Warships such as the Type 45 Destroyer currently under development have complex communication systems.

• Currently working with the Type 45 Destroyer team to provide a testing infrastructure to facilitate the complex system and software integration issues.

of 27

Air Traffic Management

The CIS Modelling group has also had a lead role in a number of studies in the ATM domain including FARADEX, PAMPAS and PATIO

• SIEM - SSR/IFF Environment Model

• models the interference effects of various civil and military identification systems

• models a wide variety of SSR/IFF scenarios and equipment, including Mode S, TCAS and CWS

• used by CAA for national frequency clearance within UK airspace and to support EUROCONTROL and international studies

of 27

What is a Communications System ?

• A Communications System is any system that allows one party to communicate with a different party

• Can be as simple as two cans connected by string, or the waving of flags using semaphore.

• Examples of communication systems:

– PSTN (Publics Switched Telephone Network)– Cellular (Mobile Systems)– Internet

of 27

Circuit Switched Networks

• Circuit Switched Systems - Originally developed for voice traffic

• Circuit has to be established prior to communications

• Dedicated communications path between caller and receiver

– Inefficient for data transfer

– Fixed data rate, constant transmission delay

– Virtually no delay at nodes

• Circuit needs to be removed once communications complete

• Once all circuits between two nodes are utilised, further call requests are blocked

E xchangeE xchange

S ubscribe r Loop

In te rc ityT runk

C onnec tingTrunk

of 27

Packet Switched Networks

• Packet Switched Systems

• Data is broken down into a number of “Packets” for transmission

• Greater line efficiency as node to node links are dynamically shared

• Data is buffered at nodes - queues

• Different data rates can be handled across the network

• When traffic is heavy, packet switched networks still accept packets but delivery delay is increased

• On a circuit switched network, when traffic becomes heavy some calls are blocked

• Priorities can be used

User Data

Control Inform ation (packet header)

Packet

of 27


• Significant relationship between packet size and transmission time

• Smaller packet size results in faster transmission across link

• Eventually, control information overhead for packets causes increased transmission time

Tim

e

A B C D

Packet flowbetw een nodes

A B C D


A B C D


1-packet message 2-packet message

Data1

Data1

Data1

Data1

Data1

Data2

Data1

Data1

Data2

Data2

Data2

Data1

Data1

Data2

Data2

Data3

Data3

Data3

Data4

Data4

Data4

Data5

Data5

Data5

of 27


• Two approaches to packet switching techniques– Virtual circuits

» Preplanned route is established prior to data transmission

» Similar to circuit switching technique, slower due to queues at nodes

» Better for data transfer over extended periods of time as routing decisions made once

Com puterCom puter

Com puter

Virtual Circuit #1

Virtual Circuit #2

of 27


• Two approaches to packet switching techniques– Datagrams

» Each packet treated independently

» Routing decisions made by each node independently meaning that packets may arrive out of sequence

» No virtual circuit set up or close down delays

» Node congestion or failure has less effect than with virtual circuits

Com puterCom puter

Com puter1

1

2

2

3

3

of 27

Packet Switched Networks - Routing

• Routing Algorithm Considerations

– Correctness > Data transfer without error– Simplicity > The simpler the route the less to go wrong

– Robustness > Ability to cope with localised failures or overloads

– Stability > Avoid fast changing load conditions

– Fairness > Not favouring local routing to the detriment of long distance routing

– Optimality> Optimising the routing for best performance

– Efficiency > Penalty of overheads less than benefit accrued

of 27


• Routing Algorithms

– Fixed Routing• Data transfer between two points always fixed, typically using a

least cost routing• No reaction to traffic loading conditions

– Flooding• Packet sent to every connecting node except node received from• Final destination node will receive multiple copies which are

discarded after the first is received• Highly robust• Traffic load directly proportional to the network connectiviity

– Random• Packet sent on to random node, could have probability formula

applied rating links based on data rate, spreading loading

of 27


• Status information from other nodes on the network affects the routing decision at any given time

• Key influences on routing decisions:– Failure: A node or trunk failure means it can no

longer be used by the network– Congestion: When one portion of the network is heavily

congested, route around rather than through

• Some form of adaptive routing is used by virtually all packet switched networks

• Routing Algorithm

– Adaptive Routing

of 27


– Requires network status information to be shared between nodes• Trade off between quality of information and it’s lifetime and the

overhead placed on the networke.g. Better information and more frequent updates equates to a higher network loading and lower network performance visible to the user

– Early adaptive routing algorithms used for ARPANET (Fore runner of the Internet) highlighted a problem of oscillation.

• Adaptive Routing

Computer

Computer

A

B

of 27


– Oscillation caused by all nodes seeking best route for their traffic– Oscillation is undesirable because:

• A significant proportion of capacity is unused when it is needed most: under heavy traffic load

• The large swings in traffic cause more frequent routing updates being sent, increasing the load on the network further

– Later algorithms make routing decisions based on giving the average route a good path instead of attempting to give all routes the best path• Link utilisation is calculated for link rather than delay time previously• Utilisation then averaged with last utilsation to smooth out routing

oscillations

• Adaptive Routing

of 27

Packet Switched Networks - Congestion Control

• The objective of congestion control is to maintain the number of packets within the network below the level at which performance falls off dramatically

• Refresher:

– In essence a packet switching network is a network of queues– At each node there are queues of packets for each outgoing channel– If the rate at which packets arrive and queue up exceeds the rate at

which packets can be transmitted, the queue size grows without bound and the delay experienced goes to infinity

– Even if arrival rate is less than packet transmission rate, queue length will grow dramatically as the arrival rate approaches the transmission rate

of 27


• If packets arrive faster than routing decisions can be made or transmitted from the output buffers then saturation will eventually be reached.

• Once saturation is reached two strategies can be employed:

– Discard any incoming packet for which there is no buffer space

– Node exercises flow control on the nodes to which it is connected so that traffic flow remains manageable• However if one node restrains

another node from sending packets, then that node’s queues will fill causing the congestion to spread

O utput Buffer

Input Buffer

Packet Switching Node

of 27


• Uncontrolled

– Graph shows throughput against offered load, as nodes discard packets, the source node then retransmits the discarded packets in addition to the new packets, the congestion gets worse.

– Even successfully transmitted packets may be retransmitted because of the time taken to acknowledge them.

– System reaches GRIDLOCK

of 27


• Controlled

– Adaptive routing algorithms reduce congestion across the network enabling higher offered load before congestion inevitably occurs

– Congested node sends congestion information to source nodes to enable them to reduce flows to the congested node

• Even with congestion control, as the offered load approaches the capacity of the network the network will start to become congested, eventually reaching GRIDLOCK

of 27

Packet Switched Networks - Quality Of Service

• Recent communication technologies such as Asynchronous Transfer Mode (ATM) and IPv6 have a concept of a Quality of Service (QOS). This defined the importance or criticality or time sensitivity of the data being transmitted.

– As congestion increases, packets with a higher QOS are routed in preference to packets with a lower QOS.

– In military systems the QOS refers to the military significance of the data.

– In commercial systems the QOS could be defined as the timesheet transmission overnight to head office would have a low QOS, whereas the incoming sales data being transmitted to the warehouse for packing would have a higher QOS.

• Users whose data is classified with a low QOS would be the first to receive Denial Of Service and data transmission delays

of 27

Management

• Modern communications networks and especially Military systems have introduced complex network management systems.

– Initially help with the optimum deployment of the network– Monitor the health and load utilisation of all links in the network in real

time– Make strategic routing decisions for the network, whereas nodes only

make local decisions– May be distributed (especially true in military systems) in a master-

multiple slaves combination such that all have the information to make the decisions, but only the master does

– Define policies for Quality of Service and link and total system capacity

of 27

Washup

• Messages will always get from source to destination, only the time taken is variable depending on the load on the network.

– The Internet news sites (CNN, BBC etc) effectively gridlocked for a time during the 11th September terrorist attacks. The data would get through, just the time taken for it to arrive meant that its value was degraded to the user

• Adaptive routing and congestion control messages help to prevent the forming of bottleneck. But only until such time as the requested load approaches the capacity of the system

– If requested load increases further then gridlock will occur

• Quality of service inducing denial of service is one way of controlling load requests until congestion eases

• Escalating knock effects from an initial problem, such as a node failure involve a combination of the technques described here in a managed system.

Paul Norman

BAE SYSTEMS Avionics

PO Box 5 New Filton House

Filton Bristol

BS34 7QW

Tel: +44 (0)117 918 8254

Fax: +44 (0)117 918 8149

Email: [email protected]

Contact Details

communication systems an overview paul norman communication and information systems modelling

Documents

systems data

battlefield slide

blocked slide

real time slide

networks packet

complex communication

bae systems communication

networks routing packet