Abstract:
A high-speed access data network in which upstream and downstream traffic is logically or physically separated. The network can use the Ethernet media access control (MAC) layer protocol over distances that are much larger than conventionally possible with the Ethernet MAC layer protocol. In a configuration including a central office and multiple subscribers, the central office is removed from the collision domain, which can be made relatively small, without limiting the distance between the central office and the subscribers. The downstream data rate is not limited by the size of the collision domain and can thus be made almost arbitrarily large. Furthermore, by allowing a smaller collision domain, greater upstream data rates can be used. It is thus possible to use ubiquitous and inexpensive Ethernet LAN technology in highly cost-sensitive applications such as residential broadband access.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of data communications networks, in particular to broadband access systems. 
     BACKGROUND INFORMATION 
     The World-Wide-Web and the increasing demand for computer power and memory usage of software applications and data files puts tremendous pressure on networking infrastructure. While this pressure can be relieved at moderate cost in the premise Local Area Network (LAN) by migration to Fast Ethernet, LAN Switching or even Gigabit Ethernet technology, the residential local access loop presents a bottleneck. Many technologies such as ISDN, xDSL, HFC, FTTC, and FTTH have been proposed and tested in field trials. Unfortunately, most of these high-speed access technologies are too expensive for the typical residential consumer, and require expensive optical and electronic component technologies. Because of the telephone service providers&#39; insistence on carrying voice and data over the same wiring infrastructure, priority-based network protocols such as ATM are typically utilized, further increasing the cost of the system. 
     A potential approach, as identified and addressed by the present invention, is to attempt using Ethernet over a power-splitting fiber-to-the-home (FTTH) network. A block diagram of a generic power-splitting FTTH network is shown in FIG. 1 in which multiple optical network units (ONUs)  10  are coupled to a power splitter  15  which is coupled via fiber  18  to a central office (CO)  20 . Attempting to use Ethernet technology in such a configuration, however, is problematic. 
     In point-to-point links, Ethernet is typically limited to distances of less than 2-3 km due to either modal dispersion (over multi-mode fiber only) or power budget (over single-mode). The power budget limitation arises because no more than −4 to +2 dBm of optical power (depending on wavelength) may be emitted by an Ethernet LAN transceiver for eye safety reasons in an intra-building network. Such eye safety limitations, however, do not apply to access networks. 
     Although the constraints imposed by power budget and modal dispersion considerations can be overcome, the media access scheme of Ethernet imposes significant range limitations which impede scalability to access network applications. When sending data to the CO  20 , the ONUs  10  negotiate for media access using the Ethernet carrier-sense-multiple-access/collision detection (CSMA/CD) media access control (MAC) layer protocol. In order for this protocol to operate properly, however, the round-trip delay of each packet may not exceed the duration of the shortest packet. For conventional 10 Mbps Ethernet, this constraint typically imposes a range limitation of a few kilometers. Such a range is inadequate for an access network. As such, the use of Ethernet technology in access networks is not practicable. 
     SUMMARY OF THE INVENTION 
     The present invention provides an optical data communications network in which the Ethernet media access control (MAC) layer protocol can be used over distances that are much larger than conventionally supported by the Ethernet MAC layer protocol. In accordance with the present invention, upstream and downstream traffic is logically or physically separated on a power-splitting FTTH network. Such an arrangement makes possible the use of ubiquitous and inexpensive Ethernet LAN technology in highly cost sensitive residential broadband access applications. 
     By logically and/or physically separating the upstream and downstream traffic in accordance with the present invention, the long fiber run  18  between the CO  20  and the power splitter  15  is removed from the CSMA/CD collision domain. The separation of traffic into upstream and downstream directions is consistent with the flow of traffic in access networks which is predominantly between a central office and multiple stations, rather than from station to station. 
     By removing the long fiber run  18  between the CO  20  and the power splitter  15  from the collision domain, the collision domain is thus made relatively small, thereby allowing greater upstream data rates than would otherwise be possible with conventional Ethernet. Upstream data rates can be increased even further by placing the splitter  15  even closer to the ONUs  10  thereby allowing an even smaller collision domain. 
     Furthermore, by separating the upstream and downstream traffic and thus removing the downstream traffic from the CSMA/CD collision domain, the downstream data rate is not limited by the size of the collision domain and can thus be substantially greater than the upstream rate. As such, whereas 10 Mbps rates can be provided in the upstream direction, 100 Mbps or greater rates can be provided in the downstream direction. This comports nicely with the typical requirements of access networks. 
     In an exemplary embodiment of the present invention, an overlay passive FTTH network for data communications to residential customers is provided which can be used, for example, for internet access, working-from-home, pay-per-view, Web-TV, etc. Voice encapsulated in Ethernet packets may also be carried over such a network. Other packet formats such as IP may also be used on the network layer. 
     While embodiments of the present invention described herein may be referred to as FTTH networks, naturally, the present invention is in no way limited to residential applications and can also be used, for example, in a wide variety of commercial, industrial and institutional applications, among others. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a power-splitting fiber-to-the-home network. 
     FIGS. 2A and 2B,  2 C illustrate the logical and/or physical separation of downstream and upstream networks in a data network in accordance with the present invention. 
     FIGS. 3A,  3 B and  3 C show three embodiments of an upstream portion of a network in accordance with the present invention. 
     FIG. 4 shows a data network in accordance with the present invention in which upstream and downstream data traffic is separated using coarse wavelength division multiplexing (WDM). 
     FIG. 5 is a block diagram of an exemplary embodiment of an optical network unit which uses coarse WDM, in accordance with the present invention. 
     FIG. 6 shows the RF spectral densities of Ethernet and Gigabit Ethernet signals. 
     FIG. 7 is a block diagram of an exemplary embodiment of an optical network unit which uses subcarrier multiplexing, in accordance with the present invention. 
     FIG. 8 shows a data network in accordance with the present invention in which upstream and downstream data traffic is separated using a ping-pong scheme. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a power-splitting FTTH optical data communications network, such as the network of FIG. 1, which uses the Ethernet MAC layer protocol. In accordance with the present invention, the network is logically and/or physically divided into upstream and downstream portions. As a result, the fiber run  18  from the CO  20  to the power splitter  15  is not part of the CSMA/CD collision domain and can thus be much longer than is possible with conventional Ethernet applications. 
     As such, by separating the upstream and downstream traffic, the long fiber run from the CO  20  to the splitter  15  can be made almost arbitrarily long, limited only by the power budget. The power splitter  15  can be located close enough to the customer premises so that the distance between the ONUs  10  and the power splitter  15  does not exceed the upstream collision domain. Since upstream traffic is relatively modest in a typical access network, conventional Ethernet data rates are sufficient in the upstream direction. For a data rate of 10 Mbps, the shortest packet is 512 bits or 5.12 μsec long, resulting in a collision diameter of roughly 4 km, which is long enough for most installations. 
     Several embodiments of networks, in accordance with the present invention, will now be described in which the upstream and downstream data traffic are logically and/or physically separated. 
     The logical separation of the upstream and downstream networks can be implemented by providing separate upstream and downstream fibers and power splitters. In other words, the logical separation of the upstream and downstream networks can be achieved by physically separating the networks. 
     FIGS. 2A and 2B illustrate an exemplary embodiment of an FTTH network in which the downstream and upstream data traffic is physically separated. In this embodiment, in the downstream direction (FIG.  2 A), the CO  20  is coupled via a downstream fiber  17  to a downstream power splitter  14  which is in turn coupled to a plurality of ONUs  10 . In the upstream direction (FIG.  2 B), the ONUs  10  are coupled to an upstream power splitter  16  which is coupled via an upstream fiber  19  to the CO  20 . 
     In the downstream direction (FIG.  2 A), the CO  20  is the only station transmitting, and no contention for media access occurs. As such, the downstream data rate can be substantially greater than the upstream data rate. Data for the different ONUs  10  is statistically multiplexed onto the fiber and is characterized by a different address (e.g., MAC address) for each ONU. In accordance with the MAC layer protocol, the ONU receivers filter out the appropriate packets. 
     In the upstream direction (FIG.  2 B), the ONUs  10  negotiate for access to the shared upstream fiber  19  between the splitter  16  and the CO  20 . A portion of the light transmitted to the power splitter  15  is reflected back to all ONUs  10  for carrier sensing and collision detection by using, for example, a partial reflector  25  in the fiber run  19  to the CO  20 . However, the fiber run  19  to the CO  20  is not part of this collision domain, because it does not need to obtain media access. The upstream fiber  19  thus “listens” to the traffic between the ONUs  10 , without actively participating. Therefore, the relevant collision domain  12  contains the ONUs  10  and the power splitter  16 , but not the CO  20 . 
     In the embodiment of FIG. 2B, a 3 dB splitter (not shown) at each ONU  10  can be used to separate the transmitted and received power on the upstream portion of the network. 
     In the embodiment of FIGS. 2A and 2B, each ONU  10  includes one transmitter (upstream) and two receivers. A first receiver of each ONU is used for carrier sensing and collision detection on the upstream traffic, and a second receiver is used to detect the downstream traffic from the CO  20 . 
     FIGS. 3A,  3 B and  3 C show three embodiments, in accordance with the present invention, of an upstream portion of a network using Ethernet CSMA/CD to negotiate access to the common fiber  19  to the CO  20  via the power splitter  16 . 
     In the embodiment of FIG. 3A, the splitter  16  comprises a (N+l)×N power splitter, so that each ONU  10  has an input as well as an output fiber to the power splitter  16 , resulting in two fiber runs between the splitter and each ONU  10 . 
     The embodiment of FIG. 3B uses a 2×N power splitter  15 , with a high reflector  35  attached to the second port of the splitter to reflect power to all of the ONUs  10 . 
     In the embodiment of FIG. 3C, a partially reflecting fiber grating  45  is inserted in the fiber run to the CO  20  to reflect some portion of the upstream power back to the ONUs  10  for carrier sensing and collision detection. 
     In the embodiments of FIGS. 3B and 3C, a 3 dB splitter (not shown) at each ONU  10  can be used to separate the transmitted and received power on the upstream portion of the network. 
     In a further embodiment of a network in accordance with the present invention, the upstream and downstream paths share the same physical fiber plant  18  but are logically separated using coarse wavelength division multiplexing (CWDM). FIG. 4 illustrates such an embodiment. This embodiment has the advantage of avoiding the cost of the extra fiber and power splitter of the two-fiber embodiment, described above. 
     In the embodiment of FIG. 4, wavelengths of substantially 1.5 μm can be used for downstream traffic, while wavelengths of substantially 1.5 μm can be used for upstream traffic. In an alternative embodiment, 1.5 μm can be used for upstream traffic, and 1.3 μm can be used for downstream traffic. In the embodiment of FIG. 4, a dichroic fiber grating  55  which is partially reflecting at 1.3 μm and transparent at 1.5 μm is located on the fiber  18  between the splitter  15  and the CO  20 , proximate to the power splitter  15 . The grating  55  serves to reflect a portion of the upstream light back to the ONUs  10  for CSMA/CD purposes. 
     In the CWDM embodiment of FIG. 4, a CWDM splitter (not shown) at each ONU  10  can be used to separate the upstream and downstream data, whereas a 3 dB splitter (not shown) can be used to separate the transmitted and received upstream data. 
     The power-splitting network architecture of the present invention can be implemented using commercially available Ethernet components. A function performed by each ONU  10  is packet filtering, so that only those downstream packets corresponding to the ONU&#39;s address (e.g., MAC address) are available at the subscriber interface of the ONU. In the upstream direction, only packets destined for the CO  20  should leave the ONU  10  to travel onto the network. 
     Most likely, the subscriber would like to have a standard (e.g., 10 Base-T) interface to connect PC&#39;s or other data networking equipment to the network. To perform the appropriate packet filtering in both directions, the functionality of a (OSI Layer  2 ) bridge is provided at the ONU  10 . 
     Because the downstream traffic is independent from the upstream traffic in the system of the present invention, it is possible to freely choose the downstream data rate. For example, the downstream traffic could be 100 Mbps Fast Ethernet, or even Gigabit Ethernet. In this case, different services such as internet access, pay-per-view video, and even voice telephony can be statistically multiplexed onto the large available downstream bandwidth. Each of these services would have a separate MAC address for each ONU  10 , and a multiport bridge can separate the services at the ONU. 
     FIG. 5 is a block diagram of an exemplary embodiment of an ONU  10  for multiple services. The ONU  10  of FIG. 5 includes a 1.5 μm receiver  100 , a 1.3 μm transceiver  110  and a multiport bridge  120 . The receiver  100  can be used to receive downstream 100 Mbps fast Ethernet traffic whereas the transceiver  110  handles two-way, 10 Mbps Ethernet traffic. The two streams are separated using CWDM or by physical separation, as described above in connection with FIGS.  2  and  3 A- 3 C. The multiport bridge  120  filters packets with different Ethernet MAC addresses corresponding to different service and directs them to corresponding ports. In the exemplary embodiment of FIG. 5, the multiport bridge  120  provides a bidirectional 10 Base-T subscriber interface, such as for internet access, a one-way 100 Base-T subscriber interface, such as for downstream video, and a third interface, such as for voice. The multiport bridge  120  is preferably implemented as an ASIC which can be incorporated into the ONU  10  at low cost. 
     It should be noted that in accordance with the present invention, the routing of downstream traffic need not be limited to using MAC addresses. For example, traffic may be routed in accordance with IP addresses instead. In this case, an IP router can be used instead of a multiport bridge. 
     As discussed above, the downstream and upstream traffic are logically separated, in accordance with the present invention, thereby allowing the downstream traffic to have a substantially higher bit-rate than the upstream traffic. As described above, the upstream and downstream traffic can be separated by providing separate fiber and power splitters for each direction or by CWDM. Two additional arrangements for logically separating the upstream and downstream traffic will now be described, in accordance with the present invention. 
     In one such embodiment, the upstream and downstream traffic are separated spectrally. For instance, the upstream data bit-rate can be at the conventional Ethernet rate (i.e., 10 Mbps), whereas the downstream bit-rate can be at the Gigabit Ethernet rate (i.e., 1.25 Gbps). FIG. 6 illustrates the RF spectral densities of Ethernet and Gigabit Ethernet, as used in the upstream and downstream directions, respectively, of the present embodiment. As shown in FIG. 6, 10 Mbps Manchester-encoded Ethernet has a power spectrum that has a first zero at approximately 15 Mhz. The power spectrum of  8 B/ 10 B-encoded 1.25 Gbps Ethernet has a first zero at approximately 940 Mhz and a peak at 470 Mhz, with no appreciable spectral content below 50 Mhz. 
     A block diagram of an embodiment of an ONU  10  for use in a network in accordance with the present invention which uses spectral separation is shown in FIG.  7 . The configuration of such a network is similar to that of FIG. 1, with a common distribution fiber  18  and a common power splitter  15 . In this embodiment, each ONU  10  includes one optoelectronic transceiver  200 , an RF splitter  210  and MAC and physical layer circuitry  220  and  230  for 10 Mbs Ethernet and Gigabit Ethernet, respectively. The splitter  210  routes signals below a threshold frequency to the 10 Mbs circuitry  220  and signals above the threshold frequency to the Gigabit circuitry  230 . For the present embodiment which uses 10 Mbs Ethernet and Gigabit Ethernet, an appropriate threshold frequency for the splitter is in the range of 15-50 Mhz (e.g., 30 Mhz). 
     Unlike the ONU  10  of FIG. 5, which includes two receivers, one for carrier sensing and collision detection in the upstream direction and a second for receiving the downstream traffic, the ONU of FIG. 7 includes one receiver which is used for carrier sensing and collision detection as well as for detecting downstream traffic. 
     As a variant of the spectral separation embodiment described, true subcarrier multiplexing (SCM) can also be used. In such an embodiment, each of a plurality of baseband data streams is modulated with a different carrier signal. 
     In a further embodiment of a network in accordance with the present invention, the upstream and downstream traffic is logically separated using a ping-pong scheme. A block diagram of such an embodiment is shown in FIG.  8 . 
     In the ping-pong scheme of the present invention, the CO  20  acts as a network master. Unlike conventional Ethernet operation in which the CO  20  transmits only when it detects that the shared fiber  18  is free of any other carrier, the CO  20  transmits an Ethernet jamming signal, such as would be used when a collision is detected, immediately before the CO transmits a data packet. The duration of the jamming signal is selected to correspond to the length of the longest possible packets (as specified by the Ethernet standard) transmitted by each of the ONUs  10 . Transmitting the jamming signal ensures that the ONUs  10  have stopped transmitting and that all ONU packets have left the network before the CO packet arrives at any of the ONUs  10 . In the ping-pong scheme of FIG. 8, in which the CO  20  transmits a jamming signal, no separation (such as by CWDM, separate fiber or spectral separation) is required as the CO  20  acts as the master controller. The CO  20  uses moderation in accessing the shared fiber  18 , i.e., it leaves enough time between transmissions so that the ONUs  10  have a chance to transmit their own packets back to the CO  20 . 
     In the embodiment of FIG. 8, the CO  20  is coupled to the fiber  18  via a coupler  21 , preferably a 3 dB coupler. An isolator  23  can be inserted between the transmit output of the CO  20  and the coupler  22 . The isolator  23  prevents feedback of the upstream light into the downstream transmitter. The ONUs  10  are coupled to an N×N passive splitter  15 . The splitter  15  includes an input for downstream data and an output for upstream data which are coupled to a second 3 dB coupler  22 . The couplers  21  and  22  are coupled via the common fiber  18  which carries both upstream and downstream traffic. 
     The couplers  21  and  22  and the isolator  23  can be eliminated if the feeder fiber  18  is replaced with separate upstream and downstream fibers. 
     It should be noted that all of the embodiments of FIGS. 3A-3C can be adapted for use in a pong-pong scheme similar to that of FIG. 7, in accordance with the present invention.