1. Field of the Invention
This invention relates generally to optical fiber communications systems which use frequency division multiplexing, and more particularly, to maintaining the timing of data transmitted across such communications systems.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Upcoming widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3, OC-3, or equivalent connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of optical fibers and protocols such as SONET have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and transmits the resulting optical signal across the optical fiber to the receiver. The receiver recovers the original data from the received optical signal. Recent advances in transmitter and receiver technology have also resulted in improvements, such as increased bandwidth utilization, lower cost systems, and more reliable service.
Because of its large inherent bandwidth, an optical fiber is most efficiently used when multiple users share the fiber. Typically, a number of low-speed data streams (i.e., “low-speed channels”), for example transmitted by different users, are combined into a single high-speed channel for transport across the fiber. Conversely, when the high-speed channel reaches the destination for one of the low-speed channels contained within it, the low-speed channel is extracted from the rest of the high-speed channel. For certain applications, it may also be desirable or even required that the original timing of the low-speed channel be maintained when the low-speed channel is extracted from the corresponding high-speed channel. Alternately, there may be requirements on the maximum amount of timing jitter introduced and/or propagated by the overall transmission process. Tight jitter tolerances are beneficial since excessive timing jitter can significantly degrade the performance of the network. For example, if each data bit is expected during a certain timeslot, then the timeslot must be large enough to accommodate any jitter introduced by transmission over the network. Loose jitter tolerances will result in longer timeslots required for each bit which, in turn, will mean lower data transmission rates.
As one example, a network may be made up of a number of nodes connected to each other by optical fiber links. Data transmitted over the network may travel from node to node over several links before reaching its final destination. Jitter tolerances in such a network may be specified on a per-link basis so that end-to-end jitter requirements will be met independent of the number of links traversed. In other words, the specification of proper per-link jitter tolerances allows each link to be designed independently of the others while still guaranteeing that end-to-end jitter requirements will be met so long as each link meets the appropriate tolerances. The accumulation of jitter is maintained within tolerances as data travels across the network.
The GR-253 standard (e.g., see Bellcore, Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria, Generic Requirements GR-253-CORE, Issue 2, December 1995, with Revision 2, January 1999) is one example of a standard which specifies per-link jitter timing requirements. In particular, these standards specify requirements on jitter transfer, jitter generation, and jitter tolerance. The jitter transfer requirement for a link places a maximum on the jitter at the output of the link, assuming a certain amount of jitter at the input of the link and assuming no independent jitter sources within the link. In other words, it is a requirement on the proliferation of jitter as it propagates through the link. The jitter generation requirement places a maximum on the jitter at the output of the link, assuming no jitter at the input of the link. This is a requirement on the amount of jitter generated within the link. The jitter tolerance requirement specifies how much jitter at its input a link must be able to tolerate. In other words, each link must be able to accommodate a certain amount of jitter generated by the previous link.
Whether and with what difficulty these timing requirements can be achieved will depend in part on the specific technique used to combine the low-speed channels. Two widely used approaches to combining low-speed channels are wavelength division multiplexing (WDM) and time division multiplexing (TDM).
In WDM or its more recent counterpart dense wavelength division multiplexing (DWDM), each low-speed channel is placed on an optical carrier of a different wavelength and the different wavelength carriers are combined to form the high-speed channel. Crosstalk between the low-speed channels is a major concern in WDM and, as a result, the wavelengths for the optical carriers must be spaced far enough apart (typically 50 GHz or more) so that the different low-speed channels are resolvable. As a result, the number of different optical carriers is limited and if each carrier corresponds to a low-speed channel, as is typically the case, the total number of low-speed channels is also limited. Furthermore, if the bandwidth capacity of the fiber is to be used efficiently, each low-speed channel must have a relatively high data rate due to the low number of low-speed channels. The relative complexity of the components used in WDM systems further encourages the use of high data rates for each dedicated wavelength, and hence also for each low-speed channel. For example, some current WDM systems specify data rates of 2.5 Gbps and higher for each dedicated wavelength. This typically is a drawback since, for example, many data streams occur at a much lower bit rate, such as at 155 Megabits per second (Mbps) for OC-3, and will underutilize the higher data rate specified for each dedicated wavelength.
In TDM, each low-speed channel is compressed into a certain time slot and the time slots are then combined on a time basis to form the high-speed channel. For example, in a certain period of time, the high-speed channel may be capable of transmitting 10 bits while each low-speed channel may only be capable of transmitting 1 bit. In this case, the first bit of the high-speed channel may be allocated to low-speed channel 1, the second bit to low-speed channel 2, and so on, thus forming a high-speed channel containing 10 low-speed channels. The TDM approach is strongly time-based and requires precise synchronization of the low-speed channels between nodes in a network. As a result, TDM systems typically require complex timing, leading to increased overall cost. In addition, since the low-speed channels typically are combined on a bit-by-bit (or byte-by-byte) basis, TDM systems are heavily dependent on the bit rates of the individual low-speed channels and have difficulty handling low-speed channels of different bit rates or different protocols. As yet another disadvantage, a TDM channel typically consists of a header in addition to the actual data to be transmitted. When ten low-speed channels are combined into a single high-speed channel at a 10× higher data rate using SONET TDM protocols, the headers for the ten low-speed channels and the resulting high-speed channel typically must be manipulated in order to accomplish the conversion and also to undo the conversion (e.g., SONET pointer processing). This manipulation of the headers is not always straightforward and, in some cases, can even prevent the combination of certain types of channels.
Thus, there is a need for an inexpensive node which efficiently combines a number of low-speed channels into a high-speed channel and which can efficiently maintain the original timing of the low-speed channels and also meet timing jitter requirements for each channel, even when the low-speed channels are incorporated as part of a high-speed channel. The node preferably operates independent of bit rate, format, and protocol of the various channels and is capable of handling a large number of low data rate low-speed channels.