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雅思阅读
2016-08-29 10:24
来源：新东方整理
作者：上海新东方
Revision: Communications and Networked Systems
Network Topologies
Star
All the network devices connected to a central computer which is often used as the file server.
Advantages of a Star
If one cable fails the other stations are not affected
Consistent performance even under heavy use
Reliable market proven system
No problems with collisions of data since each station has its own cable to the server/hub/switch
Easy to add new stations, provided hub/switch can manage, without disrupting the network
Security can be implemented in the hub/switch
Disadvantages of a Star
May be costly to install because of the cable lengths required
Cannot cable more than 100 m from the central system without additional equipment
In large networks a central hub/switch is required
The speed of the network is dependent on the quality of the switch/hub
If the central hub/switch fails, the whole network goes down
Ring
Each of the devices on the network is connected in a ring or a loop. (Each machine has a connection to the one previous and the one after it in the loop).
Advantages of a Ring
No dependence on a central computer or file server and each node controls communication to and from itself
Transmission around the network is one way only
High data transmission rates are possible
Easy access
Easy detection of problems
Disadvantages of a Ring
If one node breaks down transmission is disrupted
If used over large distances a repeater is required because of degradation of signal
If used over large distances the amount of cable required is great
Bus
Each of the devices is connected directly to a main communications line, called a bus.
Advantages of a Bus
Easy and inexpensive to install as it requires the least amount of cable of the
network types
Easy to add more stations without disrupting the network but there is a limit to the number of stations
Disadvantages of a Bus
The whole network goes down if the main cable fails at any point
Cable failure is difficult to isolate without the correct equipment
Network performance degrades under a heavy load
Information can be transmitted in either direction and occasionally there is data collision.
Network Components
Routers
A communications device (can be hardware or software) which receives data and forwards it to the correct location via an appropriate route.
Can be a modem as it allows an internal network to connect to the internet.
Can be used to connect two LANs together even if they use different protocols
Repeaters
A device used to link two cable segments. With a loss of signal over distance, a repeater amplifies the signal before passing it on. Allows you to extend the distance of a cable run and the overall length of the network.
Bridges
A connection between two (same protocol) local area networks. Creates a logical network which can appear to users as a single network although it may physically be several different networks. Bridges use a different OSI layer for addressing to a router.
Switches
A switch is sometimes known as a multi-port bridge. It receives data and forwards it only to the correct port. Switches have dropped in price and now form the network backbone for large installations. Some devices can manage the flow of data in the network.
Hubs
These are sometimes referred to as multi-port repeaters. That is they receive data and broadcast it to all the output ports.
A hub cannot screen network traffic unlike a switch.
Patch Panels
These extend socket points into central cabinets to enable links to be easily made from Workstations onto switches, etc.
Cabling
Cat5 cabling has a maximum run of 96 metres before a repeater is needed. It should not be used outside as the copper in the wires may degrade
Fibre optic can run for much greater distances, even miles, but this depends very much on the type of fibre optic cable used – details on advantages of fibre optics…
Wireless links can now be up to 2km, but walls/buildings significantly reduce this range
Filters
Examine requests for pages and contents of pages and e-mail and can block communications based on a set of rules.
Hardware OR Software based which allows for flexibility (e.g. blocking key search words or sites with certain content, e.g. images – firewalls are common forms of filters)
Protocols
A set of rules used to ensure proper transfer of data between devices. The protocol determines:
The format of the data packets, such as header content and the type of error checking to be used
The data compression method (if any)
how the receiving device indicates that it has received
how the sending device indicates it has finished sending
Gateways
A combination of hardware and software that links two dissimilar computer systems. It converts data passing through into the format required by the other system. It allows two different network types to be connected securely.
For example, a gateway on an e-mail system allows different e-mail systems to exchange data.
Networks used for Communication Services
Analogue & Digital Signals
Methods (analogue and digital) used to organize local and wide-area networks:
Internal networks are digital
These can be connected to external analogue networks such as POTS (Plain Old Telephone System)
That are connected to digital internal networks at the other end
Involves the use of:
……
ISDN and Telephone
Facilities available on modern (tone) telephone systems and ISDN (digital telephone lines) include:
……
Digital Service – this means that there are less errors and as a result less frequent requests for retransmission of data resulting in higher bandwidth
Higher bandwidth and faster transmission of data
Use of ring patterns to denote a set of callers
Caller Line Identification
Answer service, missed call (1471) etc.
Limitations for handling data traffic
Bandwidth is not enough for video on demand really need 6 ISDN lines for acceptable quality
Only supports 30 simultaneous voice calls (insufficient for major businesses)
A high bandwidth line does not mean high speed internet access, the web server may be under heavy load and only provide a slow link for data download
NOTE: - Satellite links are cheaper today and are being used more and more by businesses due to their higher bandwidth being able to cope with the growth of data traffic
Broadband
……
Cable Networks
Cable television implies digital television and also includes satellite with services such as:
……
Tree and Branch
A method of distribution in which an area is served by a main cable and branches from this cable serve a group of subscribers.
The disadvantage of this method is that each subscriber cable must have sufficient bandwidth to carry all available channels, also less suitable for interactive services as return signals are more difficult to handle.
Switched Star
A method of distribution in which the main cable serves a series of junctions and lines connect the individual subscribers to these junctions
Advantage is that subscriber lines do not need to be high bandwidth because each subscriber takes only the signal required. It also allows users to send signals back up the line.
……
Benefits and limitations of one are reversible for the other.
Distributing cable networks
The topologies shown use signal boosters to transmit signal in the local area.
Bandwidth has grown since the first cable services in the 1960s, so now almost 1000 channels can be transmitted (most recently with benefits of fiber optic cable, which often reduced boosters from 30/40 to about 6, then to just 1 or 2 in recent years!)
Less boosters hugely improved signal quality and reliability, which were originally very variable
Satellites are used to beam down the signal into the local area (useful for remote locations, e.g. in America)
A single fiber optic cable cab supply 500 homes – the market targeting possibilities of this are huge!
These local areas were ideals for organizing fast Internet access through cable modems
Signal Scrambling/Encryption
Pay per view uses encryption when distributing the service
The first system to "scramble" a channel on a cable system was demonstrated in 1971. In the first scrambling system, one of the signals used to synchronize the television picture was removed when the signal was transmitted, then reinserted by a small device at the customer's home. Later scrambling systems inserted a signal slightly offset from the channel's frequency to interfere with the picture, then filtered the interfering signal out of the mix at the customer's television. In both cases, the scrambled channel could generally be seen as a jagged, jumbled set of video images.
In a digital system, the signal isn't scrambled, but encrypted. The encrypted signal must be decoded with the proper key. Without the key, the decoder can't turn the stream of bits into anything usable by the television's tuner. When a "non-signal" is received, the cable system substitutes an advertisement or the familiar blue screen.
Mobile Telephone Networks
How a cellular radio network operates:
……
Digital Vs Analogue
The most common (digital – 2G) mobile networks can carry many more signals/frequencies (phone calls) than the previous analogue networks (1G). Approx 3 times more calls/signals
The latest (3G) phones can provide much greater bandwidth allowing streaming of video, etc.
Privacy for phone calls is much better for digital than analogue as the number of possible “keys” to break the encryption is much greater than the signal variances possible to scramble an analogue signal
Satellite Communications
Description of satellite communications:
Spacecraft, using solar power, launched for the purpose of providing broadband communication channels between ground stations by boosting data traffic signals
Each satellite has a footprint on the ground
Satellites can be geostationary so a fixed aerial on the ground is used but higher powered signals are required to cover the distance to the satellite than for Low Orbit satellites
Series of repeaters/relay stations can form a communications link on the ground
Advantages
Continuously available for the transmission of data
Three geostationary satellites can cover the globe
Can transmit and receive from any point on the globe
High bandwidth means it can carry thousands of phone calls and video
Cost is distance independent (same cost to send a few miles as a few thousand) and no longer notably expensive
Disadvantages
Geo-stationary orbit can only hold a limited number of satellites above the equator, hence the use of low Earth orbit satellites
Distance between satellites and the earth has consequences:
……
How cellular technology works with satellite systems
The idea of cells connecting devices is being used by non-geostationary (low Earth orbit) satellites
Satellites that do not remain over a fixed position on the Earth’s surface
As each satellite passes a point on the earth it connects to the cell. When the satellite passes out of range the connection with the next satellite is automatically made and the call continued.
These cells have a much bigger footprint (size) than land based ones
Geo-stationary satellite cells work in the same way as land based antenna networks
You must know the difference between cellular land based and satellite phones
Examples
Weather Satellite Systems
Sun monitoring
Navigation (GPS)
Communication Systems – e.g. mobile phones, video phones
Electronic Funds Transfer (SWIFT)
Mapping the Earth using a wide range of sensors for military or peaceful purposes
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