 The emergence of optical networks on the XS side is a relatively newer phenomenon. Relatively means it has been here for say last two decades. But at the core side optical networks have been there for quite some time. It is important to just look at the technology from the XS side perspective. How did it gain that much importance? And what are the variants of it? Specifically, we'd look at the optical networks in the context of NGNs. We'd explore the motivation and then we'd look at the standards which have emerged in the passive optical networks domain. Since the optical networks are a much higher data rate replacement of the traditional cable and DSL networks, it is the applications which determine if the optical networks are going to meet the due requirements. Although in the optical domain, the data rate that can be achieved is of the order of terabits per second or more. But at the end of the day, it is the end user using a certain application and the equipment that the user is using to determine the overall data rate requirement. The end user equipment that is typically used is normally a desktop or its variant with its own processing capability of a packet or a frame, the memory which is available in the form of a transmit buffer or a receive buffer and the availability of the interfaces through which this computer is going to put its data onto the optical network. Traditionally, we know that either it is the Ethernet or Wi-Fi. The optical network standards which have emerged as a consequence to the need for deploying fiber optic in the exercise are known as the passive optical networks. The most basic standard that emerged some time back is by the International Telecom Union, QDOT 834. This standard is primarily meant to interface the optical network with the ATM technology. Since the ATM technology is a switching technology at layer 2, the passive optical network serves as the physical layer for the ATM networks and this ATM integration with passive optical networks is known as the ATM passive optical network. It was proposed and later adopted by the ITUT, by the Full Service Access Network Working Group that was basically a consortium led by some companies from the telecommunication and optics world. The data rate that was poised for compatibility with the ATM was 622 megabits per second. It is the SDH sonnet hierarchy of STM level 4 at the downstream and the upstream it is STM 1 that is 155 megabits per second. Now the scope of integrating work from the exercise to the network side is between the central office and the user equipment. The next standard that proposes high data rates is the G.983. As the name suggests it is broadband passive optical network. Broadband passive optical network provides backward compatibility to the broadband technologies such as DSL and cable. Since the technology is all optical so it means that the data rates coming from the other broadband technologies need to be compatible. The data rates which are available in the downstream are STM 1, 155 megabits per second, STM 4, 622 megabits per second or even higher at the rate of 1.244 gigabits per second. The upstream however is only STM 1 and STM 4. The optical distribution that is how the overall distribution is done from the central office to the end users is also based on optical network and it is we have discussed it earlier on a device called a passive splitter. Typically in ITU standards the ratio is 1 ratio 32. It means one fiber strand actually splits into 32 outgoing fiber strands on the demultiplexer side. On the multiplexing side it is the 32 fiber optic strands that actually emerge into one optical strand. Then we have the ITU G.984 standard. This is actually known as the gigabit capable passive optical network or in other words GPON. Here the data rate is significantly higher for instance as compared to the last standard here it is double. That is instead of 1.2 gigabits per second it is 2.4 and in the downstream and in the upstream it is again 1.2 or 2.4 as you can see since there is a provision to have 2.4 gigabits per second in the upstream so there's a likelihood of implementing the symmetric service between the two endpoints. Here also the optical distribution network is passively deployed. An important consideration by the ITU and IEEE is it's quite interesting that when they move from one version or from one variant to the other IEEE and ITU look for significant gains in the data rates that they provide. A typical understanding in the ITU community is that a data rate increase should be twice or fourfold but in the IEEE community there's an understanding that the data rate can go as high as 10 times and we have seen that for instance when we moved from basic Ethernet to fast Ethernet to gigabit Ethernet. So it's an interesting analogy that we see here. Then in the IEEE arena we have 802.3 AH. Now we are well versed with 802.3 that is it's an Ethernet standard and 802.3 AH actually makes it an E-PON standard. E-PON actually means Ethernet Passive Optical Network. Here we are going to talk a lot about it but here for now we just have to understand that E-PON is basically providing access technology replacement using Ethernet. There is no more a LAN protocol here but Ethernet is a protocol that is used in the first mile or the last mile. So an entire suite of Ethernet protocols which include ARP, RARP are also used in conjunction with Ethernet. So Ethernet here is on the exercise. It works between the central office and the user equipment. Sometimes it is also known as Ethernet in the first mile or Ethernet in the last mile. It's again a jargon that is used by the consortium that promoted the deployment of E-PON across various exercise. Now let's look at the architecture for broadband passive optical network. Here we see certain entities. We've seen such a network deployment before. On the left hand side we have CO, the control office, central office. We have the OLT, the optical line termination unit. We have the ONTs on the user equipment side, optical network termination. The interface between the network and the service is known as service node interface and the interface between the user and the network is known as the user network interface. So we see on the service side we have SNI and on the user side we have UNI. Although there are no hard and fast rules because here you can see that a typical distance of 20 kilometers is encouraged. It's a physical distance but it can be extended logically up to 60 kilometers also. Here you can see again that like G-PON and like ATM-PON. Here also a typical splitter and aggregator is deployed in the ratio of 1 ratio 32 but it can be increased to 1 ratio 64 or in certain situations 1 ratio 128. So in B-PON what we conclude is that the data rate emphasis is to achieve broadband data rates of STM-1, STM-4 that is 155 megabits per second, 622 megabits per second. So in all using dedicated fiber to each user side or each customer premises and using respective ONTs to have a demultiplexed individual user traffic we can achieve such high data rates. Let's summarize the standardized bit rates which come from various PON standards. We've seen most of these. An interesting comparison here would be to look at the downstream and upstream bit rates which are placed across the B-PON, E-PON and G-PON. G-PON being the fastest goes up to 2.4 gigabits per second both for the upstream as well as for the downstream. An interesting observation can be made regarding the choice of wavelengths. Normally in optical communication low loss and low dispersion are the primary windows of transmission. So here we can see that in upstream 1.3 nanometers is used all across these different standards. In downstream there is a little bit of variation. 1550 nanometer is used all across, 1310 is used upstream all across but there is a slight modification depending upon the standard that we are talking about for instance in B-PON and G-PON we are seeing 90 nanometers but we don't see that in E-PON. An essential observation that you can right away make is that in E-PON it is mostly the ethernet traffic that is coming from the desktop users. So a higher data rate is not much of an expectation in E-PON which is coming from individual users but it can be catered for to be as high as possible and the transmission technology is also shown in the last row.