TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to a telecommunications environment and more particularly to a multi-rate transmission system.

BACKGROUND OF THE INVENTION

Many modern transmission products, such as SONET fiber optic transmission systems, are required to have variants in order to operate at different clock rates. Development of these products becomes unnecessarily complicated and time consuming because one set of components is required for one clock rate and another set of components is required for another clock rate. Upgrading the transmission system or changing the clock rate requires disposal and replacement of a large number of components, significantly increasing the cost and design cycle time of the system. Further, changes in system clock rates cannot be performed remotely, requiring the system to suffer down time in order to affect the changes. Therefore, it is desirable to have a transmission system that can perform in response to changes in clock rates without adversely affecting performance and development.

SUMMARY OF THE INVENTION From the foregoing, it may be appreciated that a need has arisen for a transmission system that provides multi- rate operation. In accordance with the present invention,

a multi-rate transmission system is provided that substantially eliminates or reduces disadvantages and problems associated with conventional fixed rate transmission systems. According to an embodiment of the present invention, there is provided a multi-rate transmission system that includes a receiver capable of receiving a serial data stream. The serial data stream has a first clock rate. A clock recovery unit generates a first clock signal w th the first clock rate in response to the serial data stream. A serial to data converter converts the serial data stream into a parallel format in response to the first clock signal. A frame recovery unit places the parallel format into a frame format. The frame recovery unit is capable of operating at any of a plurality of clock rates to include the first clock rate. With the frame recovery unit operating at the first clock rate, the serial data stream is made up of a plurality of consecutive unique bits. With the frame recovery unit operating at a second clock rate that is an integer value fraction of the first clock rate, the serial data stream is made up of unique bits wherein each unique bit occupies a number of consecutive bit positions equal to the integer value in order to provide multi-rate operation. The present invention provides various technical advantages over conventional fixed rate transmission systems. For example, the multi-rate transmission system can operate with any of a plurality of clock rates. Another technical advantage is that unique bits can be repeated to adjust for differences m clock rates between system components. Yet another technical advantage is the ability to remotely change the operating clock rate without swapping out system components. Other technical advantages

are readily apparent to one skilled in the art from the following figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: FIGURE 1 illustrates a simplified block diagram of a multi-rate transmission system;

FIGURE 2 illustrates a bit sequence of a receive side of the multi-rate transmission system at a first clock rate; FIGURE 3 illustrates a bit sequence of the receive side of the multi-rate transmission system at a second clock rate; and

FIGURE 4 illustrates a bit sequence of a transmit side of the multi-rate transmission system at the second clock rate.

DETAILED DESCRIPTION OF THE INVENTION

FIGURE 1 is a simplified block diagram of a multi-rate transmission system 10. Multi-rate transmission system 10 includes a receive section 12 and a transmit section 14. Receive section 12 includes a receiver 16, a clock recovery unit 18, and a serial to parallel converter 20 operating at a first clock rate M. Receiver section 12 also has a frame recovery unit 22 operating at any one of a plurality of clock rates, including first clock rate M and a second clock rate M/n. Transmitter section 14 includes a transmitter 24, an oscillator 26, and a parallel to serial converter 28 operating at first clock rate M. Transmitter 14 also has a frame formatter 30 operating at any one of a

plurality of clock rates, including first clock rate M and second clock rate M/n.

In operation, a serial data stream is provided to receiver 16 of receiver section 12 from a transmission medium 32. Transmission medium 32 may be any conventional carrier of telecommunications information to include a fiber optic communication path. Clock recovery unit 18 generates a receive clock signal having first clock rate M m response to the serial data stream received by receiver 16. Serial to parallel converter 20 converts the serial data stream into a parallel format in response to the receive clock signal generated by clock recovery unit 18. Frame recovery unit 22 identifies the frame information transferred by the serial data stream for further processing by telecommunication components in communication with receiver section 12.

FIGURE 2 shows a bit sequence of the serial data stream for frame recovery unit 22 operating at first clock rate M. Each bit position of the serial data stream has a unique bit associated therewith at each clock pulse of the frame information transported from transmission medium 32. Frame recovery unit 22 selects every bit position of the received data in order to properly interpret the data for appropriate processing. FIGURE 3 shows a bit sequence of the serial data stream for frame recovery unit 22 operating at second clock rate M/n, where n is equal to 4. Each unique bit of the frame information occupies a number of bit positions corresponding to first clock rate M associated with an integer value ratio between first clock rate M and second clock rate M/n. For the example shown, each unique bit of the frame information occupies four bit positions of first clock rate M. Frame recovery unit 22 receives data at first clock rate M with each unique bit spanning four cloc ¹ -

pulses of first clock rate M. Frame recovery unit 22 continues to operate at first clock rate M, but selects every fourth bit position of the received data in order to correctly interpret the data. Frame recovery unit 22 arbitrarily selects the every fourth bit position sequence. Though frame recovery unit 22 does not know where the boundaries are for each bit position, arbitrary selection will result m recovery of a valid bit sequence.

For operation of transmit section 14, frame formatter 30 receives unframed data from processing components in communication with transmit section 14 and provides the appropriate overhead and data manipulation according to the clock rate desired for proper transmission of frame information. Frame formatter 30 generates a parallel format from the frame information that is converted into a serial data stream by parallel to serial converter 28. Oscillator 26 generates a transmit clock signal having first clock rate M to drive parallel to serial converter 28. Transmitter 24 places the serial data stream onto transmission medium 32 at first clock rate M.

For operation at first clock rate M, frame formatter 30 processes the frame information such that a unique bit s present at each bit position of the serial data stream for each clock pulse of first clock rate M as shown in FIGURE 2.

FIGURE 4 shows a bit sequence of the serial data stream for frame formatter 30 operating at second clock rate M/n, where n is equal to 4. Each unique bit of the frame information is repeated a number of times associated with the integer value ratio between first clock rate M and second clock rate M/n. For the example shown, four repeats of each unique bit of the frame information are performed by frame formatter 30. This allows for frame formatter 30 to provide data on transmission medium 32 at the slower

second clock rate M/n through the components of transmit section 14 operate at the faster first clock rate M.

Frame information can be transported at a maximum clock rate or at any integer value fraction of the maximum clock rate in the telecommunications environment. The clock rate for multi-rate transmission system 10 can be changed through software modifications without replacing all the components of multi-rate transmission system 10.

In summary, a multi-rate transmission system can operate at a plurality of rates by having the majority of components operating at a fixed first clock rate and a single multi-rate component on both the receive and transmit side operating at any of a plurality of clock rates. Unique bits occupy each bit position of a serial data stream when all components operate at the first clock rate. Unique bits are repeated into a number of bit positions in the serial data stream when the single multi- rate component of both the receive and transmit side operates at a second clock rate which is an integer fraction of the fixed first clock rate.

Thus, it is apparent that there has been provided in accordance with the present invention, a multi-rate transmission system that satisfies the advantages set forth above. Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations readily ascertamable to one skilled m the art can be made herein without departing from the spirit and scope of the present invention as defined by the following claims.