Patent Application
20100284425
The present invention relates to communications networks. More particularly, and not by way of limitation, the present invention is directed to a system and method utilizing variable frame lengths in a Time Division Multiplex (TDM) frame within a telecommunications network.
Digital transmission utilizing TDM technology is well-known in telecommunication systems. Such transmission has been deployed for many years. TDM technology employs so-called frames that typically have a duration of 125 microseconds. The beginning of each frame is normally signaled with a well-known bit pattern that assists receiving devices in recovering frame alignment, thereby facilitating recovery of the payload content of the frame. This pattern comprises part or all of a so-called TDM framing pattern or frame synchronization pattern.
Many telecommunications services, such as digital TDM telephony and digital leased line services, are based on the concept of a repetitive frame. However, the telecommunications network is rapidly migrating to a packet-based network in which there is no concept of a repetitive frame. However, such packet-based networks must continue to support legacy TDM services.
Some telecommunications protocols, such as those used in Gigabit-capable Passive Optical Networks (GPON), retain the concept of a precisely repeated framing pattern while fitting arbitrary payload fragments into the intervals between successive framing patterns. These arbitrary payload fragments are also typically called frames. In discussions of the present invention, frame types are distinguished either as TDM frames having a repetitive timing recovery frame type or as payload frames.
There are several problems with existing systems. The TDM framing synchronization pattern must recur at precisely repeated instants, which are referred to below as “reference instants”. Because payload frames may have arbitrary length, each potentially different from the other, there is a difficulty in fitting payload frames into an inter-TDM frame interval.
Currently there are two solutions utilized to overcome this problem. First, a lower-layer framing mechanism may be defined as part of the transmission protocol. In this existing system, a mechanism permits the fragmentation of payload frames such that the inter-TDM framing interval can be fully packed with these lower-layer frames and frame fragments, while a fragmented payload frame is reassembled from lower-layer frame fragments at the receiving end. This approach is taken in GPON, for example, whereby Ethernet payload frames are fragmented into so-called GEM (GPON encapsulation method) frames. A disadvantage of this approach is there is a significant hardware cost to reassemble payload frames.
In a second existing solution, the transmission logic may inspect each payload frame before transmitting it and transmit the frame only if there remains enough time in the inter-TDM framing interval X for the complete payload frame. X is the nominal time available for payload frames and does not vary in existing solutions. If there is not enough time, the frame is held until after the TDM framing pattern FP. It is typically not feasible to transmit some other (smaller) payload frame instead, so that the trailing end of the inter-TDM framing interval is left unused, which represents a loss in transmission capacity.
The present invention utilizes a TDM frame synchronization pattern which occurs at approximate intervals rather than at precisely periodic time intervals. The receiver of a transmission may reconstruct the precise reference time at which TDM framing pattern would have occurred through compensation with a dynamic offset indicator, which is transmitted in conjunction with the TDM framing pattern itself. Thus, it is possible to avoid the need to fragment payload frames, while retaining a reliable timing reference for TDM-domain applications.
In one aspect, the present invention is directed at a method of using variable payload frame lengths in a telecommunications system. During the interval between TDM framing patterns, a transmitting unit transmits a plurality of payload frames having arbitrary or variable frame length to a receiving unit, allowing the final payload frame to extend, if necessary, beyond the next reference instant. The transmitting unit then transmits the TDM frame synchronization pattern FP. The transmitting unit determines an offset value, which specifies the amount of delay imposed on the TDM frame synchronization pattern FP. The offset value is then sent to the receiving unit. The receiving unit receives the plurality of frames and the offset value and reconstructs the reference instant derived from the offset value. By reconstructing the precise reference instant, the receiving unit remains synchronized with the transmitting unit.
In another aspect, the present invention is directed at a system for using variable frame lengths in a telecommunications system. The system includes a transmitting unit that, during the interval between TDM framing patterns, transmits payload frames having arbitrary or variable frame length, allowing the final payload frame to extend, if necessary, beyond the next reference instant. The transmitting unit then transmits the TDM framing pattern FP. The transmitting unit determines an offset value, which specifies the amount of delay imposed on the TDM framing pattern FP. The system includes a receiving unit for receiving the plurality of frames and the offset value from the transmitting unit. The receiving unit also includes a reconstruction unit for reconstructing a precise reference instant derived from the offset value. By reconstructing the precise reference instant, the receiving unit remains synchronized with the transmitting unit.
In still another aspect, the present invention is directed at a node for using variable payload frame lengths in a telecommunications system. The node receives a plurality of frames having a variable or arbitrary frame length. The node also receives a TDM framing pattern whose position in time is delayed, possibly by zero, from the precise reference instant at which it would nominally have occurred. The node also receives an offset value providing an amount of deviation from the reference instant. In addition, the node reconstructs the precise reference instant derived from the offset value. By reconstructing the precise reference instant, the node remains synchronized with a transmitting unit transmitting the plurality of frames.
In the following section, the invention will be described with reference to exemplary embodiments illustrated in the figures, in which:
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
The present invention is a system and method utilizing variable frame synchronization transmission intervals in a framing pattern of a TDM system.
TDM framing pattern 212 may be transmitted at irregular or regular intervals. The receiving unit includes a reconstruction mechanism 120 for reconstructing the corresponding precise reference instants. This reconstruction mechanism permits complete pay-load frames to be transmitted at the end of an inter-TDM framing interval without fragmentation or lost transmission capacity.
The offset time from the reference instant may be conveyed as a field in, or in conjunction with, the TDM framing pattern itself.
In addition,
In one embodiment of the present invention, the framing may be carried in the transport definition of the framing pattern, such as the section overhead of Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH). The present invention is suitable for GPON and proposed 10G descendents. However, in another embodiment of the present invention, the present invention may be employed in a pure Ethernet transport system wherein all intelligence is carried in Ethernet frames. Variable frames may be used and received in the same manner as discussed above, but where the timing reference field is carded in a packet as ordinary traffic. This timing reference may specify the instant of occurrence of a well-known component of the packet, such as the boundary between the packet header and the packet body.
The present invention provides many advantages over existing systems and methods. The present invention avoids the need to fragment payload frames and reassemble the fragmented frames on the receiving end. Furthermore, the present invention avoids the requirement to determine whether a payload frame can or cannot be transmitted during the current interval as well as avoids losing transmission capacity in the event that a candidate payload frame is too large to be transmitted immediately.
As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide range of applications. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed above, but is instead defined by the following claims.
