Tag Archives: passive optical network

A PON Based on Code-division Multiplexing Access

Because of the Internet and broadband networks were introduced, the emerging applications – such as teleconferencing, video on demand, and high quality audio transmission has strict high flux optical access network quality of service (QoS) capabilities. However, the infrastructure of current access networks suffers from limited bandwidth, high network-management costs, poor flexibility, and low security, which prevent networks from delivering integrated services to their users. Due to sophisticated optical components and electronic circuits, optical fiber links has become used in access networks. Passive optical network (PON) and different multiplexing technology is put forward under this background, including the wavelength division multiplexing (WDM), 1 time division multiplex (TDM), 2 and optical code division multiplexing (OCDM).

PONs have been standardized for FTTH solutions and are deployed by network-service providers worldwide. Even though PONs based on TDM (TDM-PON) effectively use fiber bandwidths, they have limitations regarding transmission speed, burst synchronization, security, dynamic bandwidth allocation, and ranging accuracy. Wavelength division multiplexing (WDM) technology has also been proposed for PONs. When used in conjunction with PONs (WDM-PON), this emerging technology becomes more favorable as the required bandwidth increases, but it failed to attract attention from industry due to the high cost of optical component. Other schemes for optical access networks are currently under study worldwide.

Optical code division multiplexing access (OCDMA) systems have attracted attention in recent years, because the number of advantages, including its asynchronous access ability and flexibility of user distribution, support for variable bit rate, traffic and security “bursts” unauthorized users. OCDMA is an very attractive multi-access technique for access systems like local-area networks and the first mile, but no detailed network -design schemes have been developed to date. A PON based on code-division multiplexing access (CDMA) has been proposed using pseudo-random and Walsh codes for user identification. However, signature processing for multiple access is done in the electrical domain using an application – specific integrated circuit (ASIC), and not in the optical field as we are pursuing.

We have developed the OCDMA-PON, a network structure of PON in conjunction with OCDMA, i.e., a different multiple-access technology from TDM and WDM. OCDMA technology achieves signature processing light rather than the electrical domain using an optical encoder/decoder.

Optical-line terminator (OLT) of the optical-code-division multiplexing access/passive optical network (OCDMA-PON) system. Rx, Tx: Receiver, transmitter. OOC: Optical orthogonal code. ODN: Optical distribution network. ONU: Optical network unit. ONT: Optical network termination. n: Counter.

For the forward channels, the source is encoded at the OLT and the downstream signal is transmitted at a wavelength of 1550nm. Every user is a assigned a unique optical orthogonal code and identified by a correlation operation at the optical decoder based on fiber Bragg gratings (FBGs). To reduce multi-user interference (MUI), the nonflattened source spectrum can be compensated by a flatness +compensator before entering the FBG encoder. Another benefit is a flattened source is that it relaxes the accuracy requirements regarding the achievable precision with which the phase can be controlled and maintained stably when the number of users increases. For the backward channel from the optical network unit (ONU) optical network terminal (ONT) to the OLT, the upstream is transmitted at a wavelength of 1310nm. After upstream traffic passes the optical transceiver module, which it is sent to multiple decoders, each of which recovers the information for each user.

Upstream of a stable wavelength is usually required with a stabilized laser source at the ONU’s transmitter. The downstream signal from the OLT to ONU/ONT passes through a circulator to the detector, where the user’s information is separated through optical correlation with his or her unique OCC using a balanced receiver. The downstream control signal is also obtained and the control unit is passed to the network. For the upstream, the signal from the ONU to the OLT is encoded by the OCC for user identification by the optical encoder. This is then transmitted through optical fiber link from tifert. Our scheme has several advantages. For example, any user may add or drop into the network at random and the network is running asynchronously.

Our OCDMA-PON system is composed of an OLT and an ONU. Every ONU is identified by its own cde address. The signal is modulated with both frame information and an address-code sequence. The former is used to complete the data load switch, which help to identify different users.

Figure 1 compares the bit-error rate (BER) with the ONU-transmitter input power for two cases, one affected by interference (for example, MUI or from different noise contributions) and a second based on a back to back configuration. In this case, we assume that the amount ONUs/ ONTs is 30 and the bit rate of downstream flow is 1.25gb/s. We found that the sytem that includes interference exhibits much worse performance than that using the back to the back configuration, with approximately 6dB penalty at a BER of 10−9. For the OCDMA-PON, we confirm that MUI is the dominant degradation source (see Figure 3) and must be included in the network design.

Figure 1. Performance of the OCDMA-PON for back-to-back configuration and with interference. N-active: Number of active users. R-downstream: Downstream-traffic bit rate.

Figure 2 compares the BER performance as the number of active users increases for the CDMA-PON and OCDMA-PON systems. We assumed that the bit rate of downstream traffic is 1.25Gb/s for a fiber-link length of 10km. The OCMDA-PON scheme exhibits a similar performance to the CDMA-PON when the number of users is large. For example, for 30 users and a BER of 10−9, OCDMA-PONs can only increase the number of users compared to that supported by CDMA-PONs by 10%. However, the advantage of OCDMA-PONs over CDMA-PONs becomes obvious when the number of users supported by the OCDMA-PON is small (see Figure 4). Note that the signature is processed in the electrical domain by ASIC with Walsh code for user identification in CDMA-PONs, while the optical domain is used by the FBG encoder/decoder based on source-spectrum flattening and a balanced detector with the prime codes for user identification in OCDMA-PONs. Taking into account the higher bit rate and higher bandwidth offered by OCDMA-PONs based on optical processing of user signatures, we still prefer an OCDMA-PON scheme with intensity modulation. To further improve the performance of the OCDMA-PON, we need to solve the MUI-imposed degradation problems and improve the optical encoder/decoder.

Figure 2. CDMA-PON versus OCDMA-PON comparison as the number of ONUs/ONTs increases. L: Length.

Our OCDMA-PON system is different from classical PONs in that OCDMA is used for multiple-user access instead of TDM or WDM, and the OCDMA-PON scheme combines the advantages of both the PON and OCDMA technologies, including flexible network assembly, fair bandwidth division, differentiated services or QoS in teh physical layer, Asynchronous access, support for variable bit rate and burst, and the security of an unauthorized user traffic. Based on comprehensive comparisons between the OCDMA-PON and the previously studied CDMA-PON, we find that the former scheme exhibits a better BER performance than the latter for small numbers of users.

In summary, OCDMA-PON offers several advantages over CDMA-PON, including higher bit rate, higher bandwidth, and better security against unauthorized users. We continue to work towards realizing an experimental demonstration of the OCDMA-PON system.

How FTTH Broadband Works?

Stop and think how your Internet usage has evolved during the last few years. If you’re like most people, you will do, and looking forward to more online interaction, such as increasing rich media and upload and download images and video.

More large files are moving across the cyberspace network these days, and experts expect that trend will only increase. In January 2008, the study by the Discovery Institute estimates new technologies will drive Internet traffic up by 50 times its current rate within the next 10 years.

The pressure for better connectivity is one of the main reasons providers and users to view its fibre to the home broadband connections as a potential solution.

FTTH broadband connections, refer to optical fibre cable connection for individual residences. Such optical based system can provide large amounts of digital information, telephone, video, data, and so on, more efficiently than traditional copper coaxial cable for about the same price. FTTH premises depend on both active and passive optical networks to function.

FTTH network cables connection is a reality of more than 1 million consumers in the United States, and more than 6 million Japanese and 10 million global to enjoy its benefits, broadband property according to the magazine. Many people think that the FTTH technology standard to predict network connection can solve traffic congestion.

More than 10 million homes worldwide already have fibre to the home broadband connections because the technology holds many advantages over current technologies.

What are the advantages to FTTH broadband connections?

A key advantage to FTTH – also called FTTP, for “fibre to the premises” broadband – is that it provides for far faster connection speeds and carrying capacity than twisted pair conductors, DSL or coaxial cable. Experts at the FTTH Council say fibre-to-the-home connections are the only technology with enough bandwidth to handle projected consumer demands during the next decade reliably and cost effectively. The technology is already, affordable, as businesses around the world are demonstrating by getting into the business as they speculate on consumer demand.

Fibre has a virtually unlimited bandwidth coupled with a long reach, making it “future safe,” or a standard medium that will be in place for a long time to come.

However, greatly improving the bandwidth cost and current technology. According to the FTTH Council, cable companies spent about $84 billion to line family ten years ago, but it costs less in today’s dollars line those houses with FTTH technology.

FTTH will be able to handle even the future Internet use some experts see the future. Technologies such as 3D holographic high definition television and games will one day become daily necessities of families all over the world. FTTH will be able to handle estimated 30-gigabyte-per-second needs of such equipment.

Active and Passive Optical Networks

There are two important types of systems that make FTTH broadband connections possible. These are active optical networks and passive optical networks. Each offers ways to separate data and route it to the proper place, and each has advantages and disadvantages as compared to the other.

An active optical system uses electrically powered switching equipment, such as a router or a switch aggregator, to manage signal distribution and direct signals to specific customers. This switch opens and closes in various ways to direct the incoming and outgoing signals to the proper place. In such a system, a customer may have a dedicated fibre running to his or her house.

A passive optical network, on the other hand, does not include electrically powered switching equipment and instead uses optical splitters to separate and collect optical signals as they move through the network. A passive optical network shares fibre optic strands for portions of the network. Powered equipment is required only at the source and receiving ends of the signal.

Active and Passive Optical Networks of the advantages and disadvantages

Passive optical networks, or PONs, have some distinct advantages. They’re efficient, in that each fibre optic strand can serve up to 32 users. PONs have a low building cost relative to active optical networks along with lower maintenance costs. Because there are few moving or electrical parts, there’s simply less that can go wrong in a PON.

Passive optical networks also have some disadvantages. They have less range than an active optical network, meaning subscribers must be geographically closer to the central source of the data. PONs also make it difficult to isolate a failure when they occur. Also, because the bandwidth in a PON is not dedicated to individual subscribers, data transmission speed may slow down during peak usage times in an effect known as latency. Latency quickly degrades services such as audio and video, which need a smooth rate to maintain quality.
Active optical networks offer certain advantages, as well. Their reliance on Ethernet technology makes interoperability among vendors easy. Subscribers can select hardware that delivers an appropriate data transmission rate and scale up as their needs increase without having to restructure the network.

Active optical networks, however, also have their disadvantages. They require at least one switch aggregator for every 48 subscribers. Because it requires power, an active optical network inherently is less reliable than a passive optical network.