PON
FEATURE
Homes of future will be stacked with technology , but what will the next stage in Internet delivery be ?
a twisted-pair telephone line and 56Kbps is very close to channel capacity .
ADSL overcomes the voice channel ’ s Shannon limit simply by not using the standard voice channel ; rather it uses another channel .
In the telephone system , each user is connected to the CO via a twisted-pair , which is used only for a fraction of the time when we call someone and only with the lower 4KHz of channel bandwidth .
What about the bandwidth above 4KHz ? That is clearly unused . ADSL uses that untouched bandwidth and leaves the lower 4KHz free for the standard voice channel . You can use the phone for voice calls and exchange data at the same time .
How wide and how noisy is the ADSL channel ? That is not standardised , which is why each ADSL modem measures line noise at start-up and then establishes the optimal bit rate depending on the user channel .
Each user connects to the CO at a speed that depends on the channel itself . You can read this line rate in the control panel of your ADSL modem at home .
ADSL is really a wonderful idea . It makes better use of the wires that are already buried in the ground , requires no change in last mile technology and old phones are still compatible with the new technology . You just need a filter at home ( it is called a splitter ) that separates the telephone ’ s voice bandwidth from the ADSL bandwidth . It is , in short , elegant and very cheap .
The CO has a similar filter for each line . This filter connects the voice channel to the Muldex and the line ’ s upper bandwidth to a new , data-only machine called a DSLAM ( Digital Subscriber Line Access Multiplexer ). Telecom operators need only set up a DSLAM close to each Muldex in each CO to provide ADSL service to customers .
The DSLAM is a datacom-only machine with an analogue front end . It collects all ADSL data from a wide subscriber set . All this data is usually sent to an FPGA ( Field-Programmable Gate Array ) is where it is processed and aggregated on an Ethernet link .
A high-speed Ethernet link is generally connected to the Internet or transported via SDH or OTN . The ADSL standards have been constantly evolving and the backside connections of a DSLAM , where it connects to the Internet , are available in many flavours depending on network configuration : Ethernet , XAUI , SDH and OTN just to mention a few .
These are the ideal conditions for using an FPGA , because it ’ s possible to build a completely programmable backside connection and to track evolving ADSL standards using programmable devices . As you can verify in the Xilinx Vivado Design Environment ’ s IP Catalogue , you can set up nearly any datacom standard in a Xilinx FPGA at the touch of a button .
The ADSL architecture looks so ingenious , especially in places where it naturally upgrades a telephone network , that it is difficult to understand why everybody wants more , but ADSL has limits . That ’ s why the market has advanced toward PON ( Passive Optical Networking ) technology .
Again , ADSL ’ s limit is dictated by the Shannon Theorem . It is hard to take
ADSL beyond 15Mbps using twisted pairs . This is not a limit of the ADSL technology itself ; rather the average distance from the subscriber to the CO sets the limit . If we want to go faster , we must change the ‘ last mile ’ while also minimising the cost of changing the last mile . We could of course deliver an SDH ( carrying Ethernet ) to each customer to meet these needs , but this would be too expensive . PON has been the best answer to this problem because it provides an optimal balance of upgrade costs , delivered performance and minimal last-mile rework .
Let ’ s see how PON works in detail .
Service providers bring a single fibre to the ‘ curb ’ within a radius of hundreds of meters to customers . There is not one fibre per user . One fibre replaces tens of twisted pairs . A passive optical splitter delivers a fibre to each user and users access only their own portion of the multicast data from the CO , restricted by an encryption mechanism .
In the upstream direction a fibre from each user goes into a passive splitter and is multiplexed into a single fibre to the CO . The machine in the CO receiving the data from this fibre is called an OLT ( Optical Line Termination ). This architecture is radically different from ADSL . PON ’ s advantage is that the boxes on the streets are now optical , and still passive . There are no active components in the box . This is a key advantage of PON technology : it minimises maintenance costs for providers .
The disadvantage is that providers must replace existing twisted pairs with a ( limited ) number of fibres . To mitigate the migration cost ( at the expense of performance ), PON is being set up in many countries as a hybrid technology . Users connect to the box on the street with ADSL , but it ’ s optical from the street to the OLT .
ADSL works much faster with this hybrid approach because the DSLAM is only a few hundred meters from the user , not back at the CO . The disadvantage is that the hybrid box in the street is now active because it hosts a small DSLAM .
PON represents a balanced cost and performance trade-off . It is not a technically optimal solution , as our old 56Kbps modem was , but it does scale for the future .
There is another key technology aspect to OLTs : their front end . In the upstream direction , all users are connected to the same receiver via
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