1 dte – data terminal equipment dce – data circuit terminating equipment modem standards ...
TRANSCRIPT
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DTE – Data Terminal EquipmentDTE – Data Terminal Equipment DCE – Data Circuit terminating EquipmentDCE – Data Circuit terminating Equipment
Modem StandardsModem Standards V.32 bisV.32 bis V.34 and V.34bisV.34 and V.34bis V.90 V.90
ADSLADSL Advantages & DisadvantagesAdvantages & Disadvantages SpecificationsSpecifications TechnologyTechnology LimitationsLimitations SplitterSplitter Splitting the signals – CAP & DMTSplitting the signals – CAP & DMT
Other Types of ADSLOther Types of ADSL
CHAPTER 6 - CHAPTER 6 - MODEMS and ADSLMODEMS and ADSL
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DTE – Data Terminal EquipmentDTE – Data Terminal Equipment
Ultimate source or destination, e.g. PC
Typically do not have the means to transmit appropriate electrical waveforms directly onto the transmission links into the network
For data transmitted over the PST, the telephone line from the telephone network terminates at a DCE (modem) which in turn connects to the DTE.
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DCE – Data Circuit–Terminating EquipmentDCE – Data Circuit–Terminating Equipment
A DCE provides the interface for the DTE to the network
DCE may sit within the physical DTE box e.g. a modem card in a PC.
DCE transmits and receives data in the form of appropriate electrical signaling waveforms accepted by the network link
It exchanges data and control information with the DTE it is connected to.
Standards are defined for communications between DTE and DCE.
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MODEM StandardsMODEM Standards
Modems operations for the Telephone line interfacing is standardized by the International Telecommunications Union – part of ITU.
Modem standards are set as ITU-T “V-series” Specifications
When 2 modems attempt to establish a connection a process called “handshaking” takes place.
Three most important things negotiated are the speed, compression technique and error control.
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MODEM – SpeedMODEM – Speed
Speed is the data transmission rate between DCE to DCE.
Examples of speeds used today are:- 14400 bps (V.32 bis) 28800 bps (V.34) 33600 bps (V34 bis -with software enhancement) 57000 bps (V.90)
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MODEM – Speed … continueMODEM – Speed … continue
Speed depends on line quality for e.g. two V.34 modems (V.34 bis) are trying to communicates
i. A 33600 bps connection is negotiated. If this is not possible the next higher rate will be tried.
ii.If line qualities deteriorate then the modem will “fall back” to a lower speed or “fall forward” to a higher vice versa.
iii.If V.34 modem is connected to a V.32 modem the maximum speed is only 14400 bps regardless of line qualities.
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MODEM – MODEM – Compression
Compression the process of reducing the repetitive bits pattern and redundant bits within a data set.
Eg. a text message contains the string XXXXXX, a compression technique code the string into 2 bytes i.e. :i.one byte to identify the repetitive character andii.one byte to specify the number of times it is repeated.
Compressed file can be sent at a higher speed than uncompressed file.
The two primary standards used for modems are V.42 and the Micro Networking Protocol (MNP) level 5.
V.42 generate 4 to 1 data compression ratio depending on the type of file being transmitted.
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MNP5 compress data by a factor of 2 to 1.
To benefit from compression the PC must be able to transmit with its connected modem at a rate that is equal to the speed achieved by the compression ratio.
This speed is called the port speed (serial port speed) or DTE-to-DCE rate i.e :i)V.34 modem should maintain a port speed of 115 200 bps ii)V.90 should maintain a serial port speed of 230 400 bps to benefit from a 4 to 1 compression ratio.
MODEM - MODEM - Compression …… continue
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Error Control refers to error detection and correction.
The standards used are V.42 and MNP levels 1 through 4.
V.42 uses cyclic redundancy check (CRC) for error detection and automatic repeat request (ARQ). ARQ prevents the modem from accepting any more data until the defective frame has been retransmitted successfully.
Default error control is V.42 but models can negotiate to use MNP 1-4.
MODEM – MODEM – Error Control
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V.32bis standard modem operates at a maximum rate of 14.4 kbps
2400 symbols per sec. signalling rate (baud rate).
Based on a QAM constellation of size 128, with 6 data bits transmitted with each symbol (6x2400=14400).
128-point QAM constellation, essentially a 12x12 square constellation with 4 points removed at each corner and rotated by 45 deg.
The extra bit redundancy available (128=27) is used for error correction.
The bandwidth efficiency 14400/3000 = 4.8 bits/Hz(Note: 3000 refers to Shannon Law that bandwidth limit of a voice : typical telephone line is 300Hz -3300 Hz or 3000 Hz )
Operate in full duplex mode using the two-wire (single wire-pair) telephone line by employing hybrid transformers and echo cancellers.
MODEM – MODEM – V.32bis Standard:
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A "hybrid" acts like a "traffic circle" used to separate the two directions of transmission on a single wire pair link,
Electrical imperfections cause a portion of the transmission in the forward direction to be reflected in the reverse direction called echoes.
Echo cancellers reduce to insignificance the strength of any echo signal.
Automatic equalizers, synchronization circuits, data scramblers, data compression, and error control used to maintain reliable data and speed
MODEM – MODEM – V.32bis Standard:
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MODEM – MODEM – V.32bis Standard:
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128-Point QAM Constellation Used in a V.32bis Modem
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MODEM – MODEM – V.34 and V.34bis Standard
V.34 specifies a full-duplex data rate of 28.8 kbps V.34bis includes specifications for a data rate of 33.6 kbps.
Constellation size of 768, a symbol rate of 3200 baud and 9 data bits per symbol gives a 28.8 kbps data rate.
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Most recent modems for data transmission of 56 Kbps modems.
K56flexTM and x2TM, predate the development of V.90
56K implementations takes advantage of the digital PSTN network and the direct digital connections
MODEM – MODEM – V.90 Standard
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Without analog-to-digital conversions in the downstream side, the signal-to-noise ratio is improved resulting in higher speed transmissions.
The higher speed only possible on the downstream link
End-user to ISP transmission is still limited to the current maximum provided by V.34bis of 33.6 kbps.
Typical traffic from the end-user to the ISP is mostly lower bandwidth data
Downstream (download) needs the most bandwidth for graphics, video and sound.
For faster download speed both ends of the connection must be compatible i.e. both ends must be x2TM, or K56flexTM or V.90.
MODEM – MODEM – V.90 Standard
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MODEM – MODEM – V.90 Standard
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Asymmetric Digital Subscriber Line (ADSL)Asymmetric Digital Subscriber Line (ADSL)
ADSL is a broadband connection technology which utilises existing copper wired telephone networks.
ADSL uses a wide range of frequencies over an existing telephone line to deliver much higher speeds than your standard 56k modem (between 10 and 140 times faster).
It is also possible to use your phone while online
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Asymmetric Digital Subscriber Line (ADSL) converts existing twisted-pair telephone lines into high speed data communications
Upstream data rate of ADSL is 16 to 640 kbps.Downstream data rate of ADSL is 1.5 to 6 Mbps.
ADSL transform the existing network to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to everyone's home this century.
New broadband cabling will take decades to reach all prospective subscribers. ADSL will make multimedia and broadcasting markets viable, and profitable, for telephone companies and application suppliers alike.
Asymmetric Digital Subscriber Line (ADSL)
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Asymmetric Digital Subscriber Line (ADSL)
ADSL uses two pieces of equipment: one on the customer end and one at the provider end:
Transceiver - At the customer's location, there is a DSL transceiver, which may also provide other services.
DSL access multiplexer (DSLAM) - The DSL service provider has a DSLAM to receive customer connections.
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ADSL – Advantages and Disadvantages
Advantages of DSL:
You can leave your Internet connection open and still use the phone line for voice calls. The speed is much higher than a regular modem (1.5 Mbps vs. 56 kbps) DSL doesn't necessarily require new wiring; it can use the phone line you already have. The company that offers DSL will usually provide the modem as part of the installation.
Disadvantages:
A DSL connection works better when you are closer to the provider's central office. The connection is faster for receiving data than it is for sending data over the Internet. The service is not available everywhere.
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An ADSL creates three information channels: A high speed downstream channel. A medium speed duplex channel. A Plain Old Telephone Service (POTS) channel.
Asymmetric Digital Subscriber Line (ADSL) - Specifications
Downstream data rates depend on length of the copper line, its wire gauge, presence of bridged taps, and cross-coupled interference.
Line attenuation increases with line length and frequency, and decreases as wire diameter increases.
ADSL connection requires ADSL modem at the local end and at the telco switching office.
ADSL does not requires a second line or a new termination unit
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Asymmetric Digital Subscriber Line (ADSL) – Technology
ADSL uses advanced digital signal processing and creative algorithms to pump info through telephone lines
Signals at one megahertz attenuate very much over long telephone lines
ADSL modems divide the available bandwidth of a telephone line in either ways -- Frequency Division Multiplexing (FDM) or Echo Cancellation.
FDM assigns one band for upstream data and another band for downstream data.
The downstream path is further divided by TDM into one or more high speed channels and one or more low speed channels.
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ADSL modems can tolerate impulses of arbitrary magnitude whose effect on the data stream lasts no longer than 500 µsec.
Asymmetric Digital Subscriber Line (ADSL) – Technology
The upstream path is also TDM into corresponding low speed channels. Echo Cancellation assigns the upstream band to over-lap the downstream, and separates the two by means of local echo cancellation.
Echo cancellation uses bandwidth more efficiently, but at the expense of complexity and cost.
ADSL modem organizes the aggregate data stream into blocks, and attaches an error correction code to each block for receiver to corrects errors.
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Asymmetric Digital Subscriber Line (ADSL) - Limitations
Local loop restriction i.e. exact data rate of ADSL connection need an accurate measurement.
The quality of the copper affects the maximum connection speed.
Interoperability problem between different manufacturers.
Service availability as deployment of ADSL service is not widespread yet
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How ADSL connect You to The WEB
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What is a splitter?Everybody uses splitters on their telephone lines. If you want to connect up more than one telephone into the same socket, the chances are you're using a splitter. When you're connected via ADSL, you need a special type of splitter which contains a micro-filter.
Why do we need a micro-filter?There are two reasons why they're required.
The first is to stop the ADSL signal interfering with the voice part of your phone line - this sounds a bit like 'fast clicks' when it happens.
The second reason is to ensure that the ADSL line is terminated correctly. An unmatched termination can cause reflections back down the line. This is a bad thing and can cause errors on your ADSL connection.
Micro Filter - Splitter
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Micro Filter - Splitter
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Splitting the signals - CAP
CAP operates by dividing the signals on the telephone line into three distinct bands:
• Voice conversations - 0 to 4 kHz • The upstream channel (from the user back to the server) - 25 and 160 kHz. • The downstream channel (from the server to the user) - 240 and above (max1.5 MHz)
With the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.
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Splitting the signals - DMT
DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 kHz wide.
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Upstream: 25 channels (6-30), one channel used for control, so 24 channelsx 4000 Hz per channel x 15 bits/baud yields 1.44 Mbps. Theoretically.Downstream: 225 channels, one channel used for control, so 224 channelsx 4000 Hz per channel x 15 bits/baud yields 13.4 Mbps. Likewise.
Splitting the signals - DMT
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Splitting the signals - DMT
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CAP vs DMT
DMT constantly shifts signals between different channels, searching for the best channels for transmission and reception.
Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel
In addition, some of the lower channels (those starting at about 8 KHz), are used as bidirectional channels, for upstream and downstream information.
Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, makes DMT more complex to implement than CAP, but gives it more flexibility on lines of differing quality.
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When your telephone line arrives at the local exchange, the ADSL and voice components of the signal are split apart in the same way as your splitter works at home or in the office.
The voice wiring is connected to Telco's PSTN network and the ADSL wiring is connected to a device known as a DSLAM.
The Local Exchange
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The picture shows a series of Fujitsu
patch panels used to split wiring pairs
into their respective POTS and ADSL
components.
All telephone lines are connected to
panels on the right side and
ADSL lines are fed into patch panels
on the left side ready to connect to
the DSLAM.
The Local Exchange – Patching The DSLAM
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DSLAM is the heart of DSL, Digital Subscriber Line Access Multiplexer
A device that connects many digital subscriber lines to a network by multiplexing the DSL traffic onto one or more network trunk lines.
A DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. Flexible and able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation -- both CAP and DMT
DSLAM may provide additional functions including routing or dynamic IP address assignment for the customers.
The DSLAM
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ADSL advantages over cable modems:
Cable-modem users generally share a network loop that runs through a neighbourhood, adding users means lowering performance in many instances.
ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won't see a performance decrease as new users are added -- until the total number of users begins to saturate the single, high-speed connection to the Internet.
The DSLAM
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PVC1 = Home500 USB users connected at 50:1PVC2 = Business500 Ethernet users connected at 20:1PVC3 = Business1Mb Ethernet users connected at 20:1PVC4 = Business2Mb Ethernet users connected at 20:1
The DSLAM
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The way in which most ISPs are connected to the gateway is normally either via Fast Ethernet (FE - 100Mbps) or Gigabit Ethernet (GigE - 1000Mbps).
Obviously if your ISP has a 155Mbps gateway installed then Fast Ethernet is not up to the job and so they'll have Gigabit Ethernet connections.
Transit to ISP
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Other types of ADSL
Very high bit-rate DSL (VDSL) - This is a fast connection, but works only over a short distance.
Symmetric DSL (SDSL) - This connection, used mainly by small businesses, doesn't allow you to use the phone at the same time, but the speed of receiving and sending data is the same.
Rate-adaptive DSL (RADSL) - This is a variation of ADSL, but the modem can adjust the speed of the connection depending on the length and quality of the line.