The present invention pertains to T1 transmission systems and more particularly to unrestricted 32 kb/s (32,000 bits per second) and 64 kb/s (64,000 bits per second) transmission channels which perform the robbed signaling function.
T1 transmission line facilities operate at 1.544M bits per second. The T1 line transmission facilities utilize an alternating mark inversion (AMI) signaling scheme. This AMI signal contains no more than 15 consecutive zero bit positions in a particular T1 bit stream. This requirement of not more than 15 zero bits in a string derives from the operation of repeaters in T1 transmission line systems. Commonly used repeaters may not operate within specification if more than 15 consecutive zeroes are transmitted. Repeater synchronization cannot be guaranteed for bit streams with more than 15 consecutive zeroes.
One technique used for T1 line 0 bits suppression is bipolar with 8 zeroes substitution (B8ZS). The B8ZS technique utilizes two special coding patterns containing bipolar violations which are substituted for strings of 8 zeroes in the input signal stream. Upon detecting bipolar violations with either of the two special patterns, the receiver places 8-bits of zeroes in the corresponding positions in the output bit stream.
One technical disadvantage of the B8ZS technique is that it violates the AMI signaling standard on T1 lines. Since bipolar violations normally indicate transmission errors, the deliberate introduction of these transmission errors by B8ZS signaling reduces the effectiveness of determining real errors which are detected by the AMI signaling scheme. Another disadvantage of the B8ZS technique is that it detects strings of 8 zeroes and substitutes the special violation patterns more frequently than the specified T1 line operation requires. In addition to these disadvantages, the B8ZS patterns will not propagate through standard multiplexer derived DS1 facilities or protected T1 facilities.
Severe economic disadvantages obtain for the introduction of the B8ZS technique into the North American transmission network. This introduction would require a global replacement of existing multiplexers, automatic protection switches, electronic cross-connect device (DACS), digital switch interface hardware and any other item in the network with violation monitor and removal (VMR). Such an introduction of the B8ZS technique presents an unreasonably large capital investment requirement to change network hardware.
Another more commonly used technique for meeting the T1 line zero suppression requirement is to place restrictions on the sources of bit sequences allowed in payload channels. For example, this restriction is applied at CODECS to transform a 0 bit to a 1 bit whenever an all zero 8-bit channel appears as a PCM code word. This same technique of forcing 1 bits into all zero 8-bit channels is used to further assure that no all zero 8-bit channels and therefore not more than 15 consecutive zero bits are transmitted in the T1 line format. This is a sufficient, but not a necessary condition for achieving the requirement.
The disadvantage of the technique of T1 line zero suppression by restricting sources of bit sequences is that all zero 8-bit strings are not permitted to be transmitted on any channel. This prevents the use of T1 lines for transmission of clear channel data (which require sending all possible 8-bit strings including an all zero 8-bit string) and standard CCITT 32K bit per second ADPCM. This technique also requires special treatment of all payload 8-bit strings by hardware such as CODECS and digital trunk units to force 1 bits into all zero 8-bit octets.
Robbed signaling typically occurs by using the least significant bit of each channel during every sixth frame to provide signaling functions such as, on-hook, off-hook, etc. Thus, the processing at a digital switching system for any channel may not begin until six frames of data have been received by the receiving digital switching office.
It is an object of the present invention to provide signaling data during each frame in order to facilitate uniform processing of the signaling data by the digital switching system.