PON: Easing the last mile?

The synergy of PON and XDSL seems to be a panacea for the last-mile broadband access problem. Here, is a description of PON techniques for audio, video and data transmission

The recent developments in fiber optic communication have enhanced the transmission bitrate from megabit/sec range to terabit/sec range in the core network. This is equivalent to six-order (10 to the power of 6) increase in transmission capacity over a single fiber, during the last decade. However, there is this last-mile problem of extending the seamless broadband connectivity to a remote subscriber.

Traditionally, copper pairs have been used to provide connectivity between the local switching exchange and subscriber premises. Every subscriber is connected via a dedicated pair to the serving exchange in a star configuration. This copper-pair was sufficient to provide 3.4 KHz speech telephony over several kilometers.

Recently, the transmission capacity of copper-pair has been enhanced by several orders due to the deployment of XDSL technologies. However, XDSL-based services are limited by bandwidth or physical constraints such as distance from the serving switching exchange. Further, the fault liability of copper-based access network is still too high.

A logical solution is to extend a dedicated fiber from the serving exchange to a subscriber. However, the cost of extending a dedicated fiber to a remote subscriber is prohibitively very high.

A promising alternative is to use a mix of fiber optic and DSL technologies in the access network. Here, fiber optic connectivity is extended from the serving exchange to remote area using shared fiber-optic network. Thus, a three-tier network is evolved to extend the seamless broadband connectivity to a remote subscriber.

Using the three-tier approach, the access network is divided into feeder network, distribution network and the copper-based XDSL network. The feeder network uses SDH technologies in ring topology to extend the broadband capability to tens of kilometers from the serving exchange.

A distribution network extends the fiber reach from one to two kilometers from the subscriber premises. PON (Passive optical network) is a promising technology in a distribution network. Here, like cable television network, the fiber network is shared among hundreds of subscribers using bus/tree topology. This significantly reduces the cost. Further, no active electronics is used in PON technology.

Finally, copper-based XDSL technology is used to extend the broadband connectivity within one to two kilometers range. Thus, the combination of PON and DSL technologies extend the broadband connectivity to masses in a cost-effective manner.

Passive optical network (PON): (see Figure-2) It is a shared tree network; the root (headend) is located on the feeder networks. The transmission path from the head-end to the farthest node is transparent. This implies that the optical network does not contain any active electronics or power source.

Each intermediate node uses optical splitter that divides the optical signal into multiple paths. The signal splitting process also attenuates the signal level. This necessitates optical transmission at higher power level at the headend.

PON can be used for multi-service delivery from the headend to a curb or to the home directly. The PON can be used both for broadcasting entertainment programs and interactive communication. The two-way interactive communication can be realized using single fiber or two fibers.

When the fiber is extended to the curb then such a system is known as 'Fiber-To-the Curb' (FTTC). From curb, the copper-based XDSL technology extends broadband connectivity to multiple subscribers. The interface unit between PON and XDSL network is known as 'Optical Network Unit' (ONU). It is necessary to make power supply arrangements at the curb. The interface unit located at the subscriber end is known as 'Network Termination' (NT). The NT links copper-based XDSL network and the home network.

Alternatively, fiber can be extended to the home directly. Such a system is known as 'Fiber-To-The-Home' (FTTH). However, the deployment cost of such a network is high. The interface unit located at the subscriber end is known as 'Optical Network Termination' (ONT). This unit acts as interface between home network and PON.

Broadcast using single fiber PON: It is similar to cable television distribution. From the headend, using tree network, the optical signal is distributed to remote nodes. At a remote node, optical network unit converts optical signal into electrical signal for distribution to customers. Using copper-based network, it is transmitted to subscribers.

Power loss in PON: In addition to splitter loss, there is signal attenuation due to fiber splicing and fiber loss.

For a splitter with two outputs, the input power is equally divided at the two output ports. In other words, power on each output port is 3db less than the input power. Further to the splitter loss, there is excess loss in the range of 0.2 to 0.4 db at splitter. Thus, typical power loss for a two-port splitter is 3.4 db.

Optical amplifier: To compensate for splitter loss, splicing loss and fiber attenuation, an optical amplifier is used at the headed (root). An optical amplifier accepts weak optical signal as input and without electrical conversion, amplifies the signal within optical domain. The typical transmitted power is in the range of 25-30 db.

Receiver sensitivity: The sensitivity of distant optical receiver is dependent on the transmission bitrate. A typical value at 155 Mb/s transmission is 40 db.

Typical power budget available for distribution network planning is in the range of 65 to 70 db. This may be enough for 16 branch-outs using two port splitters. Thus, the distant 'Optical Network Unit' (ONU) can be located within the 20km range of headend.

Bi-drectional transmission using dual fibers
Here, for upstream and downstream transmission dedicated tree networks exist. The downstream transmission deploys 'Time Division Multiplexing' (TDM) and the upstream transmission uses 'Time-Division Multiple Access' (TDMA).

In downstream direction, the headend broadcasts the time-multiplexed signal to all ONUs. Here, each ONU in the distribution network is allotted a pre-defined time-slot. Thus, the headend puts pertinent information in assigned time-slot and the same is accessed by OLT. However, this simple scheme lacks privacy because every ONU has access to data pertaining to other ONUs. To address this security problem, it is necessary to use signal encryption techniques.

In upstream direction, to avoid collision of optical signals from different ONUs, TDMA technique is used. Here, each ONU restricts its transmission to predefined time-slot only. Thus, it is easier to identify the data of an individual ONU at the headend.

In general, the distance between various ONUs and headend is different. Thus, it is necessary to ensure strict bit synchronization, stable burst transmission and wide dynamic range signal recovery, etc.

Bi-directional transmission using single fiber
In such schemes, a single fiber is used both for upstream and downstream transmission. Both 'time compression multiplexing' (TCM) and 'wave division multiplexing' (WDM) techniques are used extensively.

Time compression multiplexing: Here, a single fiber is used alternately manner for downstream and upstream transmission. The wavelength for each direction of transmission is the same. This method is also known as optical ping-pong transmission. It is an optical burst-mode, time division multiplexing technique.

Here, information for each direction is first time compressed and is stored in a buffer. The compression by a factor of 2 enables the information of 'T' sec duration to be represented by 0.5T sec. This time-compressed information at the destination end is decompressed to recover the original signal.

The time compression-multiplexing scheme can be symmetrical or asymmetrical. In the symmetrical scheme, the time-slots for upstream and downstream direction have equal time duration. In asymmetrical scheme, upstream and downstream time-slots are unequal. In some cases the time-slots are programmable as well.

WDM: WDM is similar to frequency division multiplexing of electrical domain. WDM transforms a fiber into hundreds of virtual fibers/wavelengths. These virtual fibers have non-interfering nature. All sorts of data patterns/formats including analog and video traffic can be transported simultaneously without any interference.

WDM combines the different optical carrier's output on a single fiber. The light signals combine in a manner similar to frequency division multiplexing.

There are two incoming fibers, fiber-1 and fiber-2. These fibers carry coherent optical signals at wavelength 'l1' and 'l2' respectively. These signals appear as input to a prism or diffraction grating.

These two optical signals are combined in prism and the composite optical signal containing optical signals at wavelength 'l1' and 'l2' respectively travel on a shared fiber. Here, the prism is combining (multiplexing) two optical wavelengths and acts as an optical combiner.

At the receiving end, the combined optical signal is applied to another prism. Since the prism has different refractive indices for different wavelengths, the two beams emerge. Thus, prism-2 has split/demultiplexed the composite optical signal into two separate 3 and 4 fibers respectively.

Thus, here one fiber is used to carry two distinct optical signals simultaneously and results in transmission-capacity doubling.

For two-way duplex communications another shared fiber carries the composite signal in reverse direction.

In practice, the optical signals at wavelengths ' l1' and 'l2' can be two optical signals at 1300 nm and 1500 nm respectively.

In general, a typical WDM system multiplexes and de-multiplexes four optical channels or less. Thus widely spaced laser devices can be used. Further, such a WDM system also has lower cost.

Duplex WDM (see Figure-4): Normally, with WDM technology, two fibers are needed to carry composite optical signals in trans and receive directions respectively. In 'duplex WDM', the single fiber carries and receives signals simultaneously.

In the duplex WDM, directional couplers are deployed at trans and receive ends. The directional coupler is a three-port device like a hybrid transformer, used in a copper-based access network. A brief description of 'duplex WDM' is given below:

In practice, the minimum separation between trans and receive direction wavelengths should be 50 nanometers.

Standardization in distribution network
In June 1995, a consortium of major telecommunication operators collaborated to define specifications for a multi-service communication system. Known as the 'Full Service Access Network' (FSAN), it defines a basic set of communication requirements for a flexible broadband access network.

In the year 1998, the FSAN developed specifications were adopted by the 'International Telecommunication Union' (ITU) as standard G.983. This standard defines the broadband optical access network using 'ATM-based Passive Optical Network' (APON). It also defines the use of WDM technology for two-way interactive communication and video broadcast using single fiber. APON can be used to extend fiber to curb or alternatively to home. The basics of optical transmission between ONUs / ONTs and OLT remains same.

APON A typical home APON system comprises an 'optical line transmission unit' (OLT), an 'optical network termination unit' (ONT), passive optical network and a network management system. The OLT is located at the headend (service node) and an ONT resides at the subscriber premises. Some features are:

Advantages of APON

Current Status

Here, a node on feeder network is known as 'service-point-of presence' (SPOP).

At SPOP, OLTs corresponding to multiple PONs can be located. The Ethernet traffic from Lotus can be connected to a LAN switch and can be switched to the desired OLT.

In the near future, Terrawave plans to offer higher speeds including 1.2 Gbps and 2.4 Gbps. To extend the reach of PON network beyond 20km, Terrawave proposes to use repeaters.

Terrawave offers protection switching within 50msec. The dynamic bandwidth allocation feature will offer idle bandwidth to any customer on the fly.

The ONT contains ' DSL Access Multiplexer ' (DSLAM). It multiplexes/ demultiplexes voice, Ethernet and video traffic in to ATM traffic.

Technical glossary
Churning: It encrypts user data between OLT to a respective ONT/ONU. The encryption is achieved by data scrambling. It offers low level of protection for data confidentiality.

Grant: It is the permission granted by the OLT to an ONT/ONU for the use of specified time-slot for upstream cell transmission.

Optical Line Termination unit (OLT): It is the network side interface to PON. It is located at the headend.

Optical Network Unit (ONU): It is a remote unit. It interfaces PON and copper network. It is located at the curb.

Optical Network Termination unit (ONT): It is the remote unit connected at the user side of PON. It resides at building / home.

Ranging: It measures the logical distance between the OLT and an ONU/ONT. This enables collisionfree cell transmission between various ONTs / ONUs and OLT.

Distribution Network: It is passive optical network between network service node i.e. OLT and ONTs/ONUs located at building / home or curb.

ATM: The term stands for 'Asynchronous Transfer Mode'. It allows the integration of bursty data and continuous stream. It maps incoming asynchronous traffic into 53 byte cells. In a cell, there are 5 overhead bytes and 48 payload bytes. ATM provides cell sequence integrity. It offers excellent quality-of-service for the end-to-end services. ATM is standardized by ITU.

Narrowband services: nx64 Kbit/s transmission. Here, n' may take integer values from 1 to 31.

Broadband services: The boundary between narrowband and broadband transmission is 2 Mbit/s. Thus, transmission of 2Mbit/s and beyond constitutes broadband transmission.

XDSL: A generic term for copper-based digital subscriber line. Here, 'X' denodes the flavour of DSL technology. Some of often used DSL technologies are :

SDH : Synchronous Digital Hierarchy.

References

A. K. Vanwasi is GM (R&D), ITI LTD. Naini, Allahabad. He can be reached at Vanwasi_nni@itiltd.co.in.

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