Digital loop carrier system having CPU to channel unit protocol

Digital loop carrier system having CPU to channel unit protocol. A method of communicating with the protocol is used in a telecommunication system having at least a central processing unit (CPU) connected to a plurality of digital interface units (DIU) via a communication module (CM). Each of the digital interface units is connected to a plurality of channel units (CU). The method has the steps of: sending from the CPU to the COM a command message having at least a first DIU address of a selected DIU of the plurality of DIU's, a channel address of a selected CU of the plurality of CU's, a second DIU address, the second DIU address being substantially identical to the first DIU address, and a first information portion; sending substantially the command message from the COM to the plurality of DIU's, the selected DIU receiving the command message; and in the selected DIU removing the first DIU address from the command message to form a modified message, and sending the modified message from the selected DIU to the plurality of CU's, the selected CU receiving the modified message. The selected module provides a response message to the CPU. The protocol provides a comprehensive response format which identifies the responding module, type of response, and if in error, who detected the error and type of error detected.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus and method for use in 
channelizing telephone transmission equipment, and in particular, for 
providing a protocol for communication between a central processing unit 
and channel units. 
The primary purpose of a PCM digital multiplexer system is the conversion 
of voice frequency signals to digital signals using standard sampling and 
encoding techniques and the multiplexing of 23 additional digitized 
samples into a T1 serial transmission format. Within the format of the T1 
serial bit stream are 24 eight bit locations commonly referred to as time 
slots into which the encoded data can be placed. The normal approach to 
assigning these time slots to the individual channels has been on a fixed 
physical slot to time slot basis. In other words, each physical slot is 
identified to a particular time slot. 
With the evolution of PCM carrier systems and a number of different 
timeslot assignment sequences, there is a requirement to provide the 
ability to re-assign physical slots to different timeslots within the T1 
format in order to be backward compatible with the older systems. 
In addition, there is a requirement to provide special conditioning of the 
analog portion of the circuit due to those transmission parameters which 
are caused by cable losses such as gain and equalization. Normally these 
parameters are adjusted by switches or potentiometers located on each 
channel unit. In order to provide a means to adjust these parameters and 
assign timeslots from a common database, a central processing unit must be 
able to communicate this information to each channel unit. The 
communication protocol devised for this particular application is the 
subject of this invention. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method and 
apparatus having an improved protocol for communicating with channel units 
which can utilize one of a plurality of timeslots in a pulse code 
modulation serial transmission format, such as a T1 line. 
The method of communicating is used in a telecommunication system having at 
least a central processing unit (CPU) connected to a plurality of digital 
interface units (DIU) via a communication module (CM). Each of the digital 
interface units is connected to a plurality of channel units. The method 
in general has the steps of: 
sending from the CPU to the CM a command message having at least a first 
DIU address of a selected DIU of said plurality of DIU's, a channel 
address of a selected CU of said plurality of CU's, a second DIU address, 
the second DIU address substantially identical to the first DIU address, 
and an information portion; 
sending substantially the command message from the CM to the plurality of 
DIU's, the selected DIU receiving the command message; and 
in the selected DIU removing the first DIU address from the command message 
to form a modified message, and sending the modified message from the 
selected DIU to the plurality of CU's, the selected CU receiving the 
modified message. 
The protocol of the method further includes the response of the selected CU 
to the CPU.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention has general applicability but is most advantageously 
utilized in a digital loop carrier system of the type shown in FIG. 1. 
As shown in FIG. 1, a central office location 10 is connected to a remote 
location 12 by digital transmission line system 14 and a bypass pair line 
16. The transmission line system 14 is to be understood as having digital 
multipliers and digital lines between the central office terminal 18 and 
the remote terminal 34. The bypass pair line 16 is utilized for the pair 
gain test system. The central office 10 has a central office terminal 18 
having channel banks. The channel banks are composed of a plurality of 
channel units in a blue channel system 22 and a white channel system 24. 
Each of the channel systems 22 and 24 contain at least one spare channel 
26 and 28 respectively. The references to "white" and "blue" channel 
systems are terminology adapted by Rockwell International for its 
equipment. 
The central office 10 also has a central office switch 30 which is 
connected by leads 32 to the central office terminal 18. Central office 
switches which are known in the prior art may be utilized for the central 
office switch 30 shown in FIG. 1. The central office terminal 18 provides 
derived subscriber channels over outgoing digital lines 14. A 
corresponding remote terminal 34 of the system separates the signals on 
the digital line 14 into a plurality of voice frequency analog signals on 
local drop wires 33 for delivery to a plurality of subscriber stations 35. 
As shown in FIG. 1, each of the channel banks are divided into a white 
channel system 40 and a blue channel system 38 with reference to the 
remote terminal 34. The central office terminal 18 has a similar structure 
of channel banks. Typically, each of these channel systems 38 and 40 will 
contain 48 channel units. 
As shown in FIGS. 1 and 2, each of the blue and white channel unit groups 
22, 24 and 38, 40 in the central office terminal 18 and the remote 
terminal 34 have 96 channel units 100 divided into four digroups 102 of 24 
channel units. A digital interface unit (DIU) 104 multiplexes the 24 
channel units 100 onto the line interface unit (LIU) 106. The multiplexing 
function is symbolically represented by the switch 108. The LIU 106 is 
connected to a predetermined T1 line 110. The DIU 104 provides formatting, 
main clock source, multiplexing, etc. and the LIU 106 provides signal 
shaping, power, voltage levels, etc. A bus 112 connects a central 
processing unit (CPU) 114 to each DIU 104 via a communication module 103. 
The architecture of the digital loop carrier system of the present 
invention requires a unique communication protocol in order to effectively 
communicate with all system modules on a timely basis and which meets the 
constraints of certain time dependent processes found in a digital loop 
carrier system. 
The structure of the system showing the communication module, the 8 DIU 
modules, and the 24 channel units connected to each DIU is shown in FIG. 3 
along with the data links which utilized this protocol. A description of 
the protocol for each data link and the action of each element in 
processing the data link format follows. 
DEFINITIONS OF ACRONYMS USED 
SOM - Start of message byte, all 0's. 
MAD - Message # for message identification and address of DIU. 
CAD - Channel Unit (CU) address. 
COM - Command to be interpreted by addressed unit. 
RSP - Response to be interpreted by CPU. 
DTL - Data length byte indicating number of data bytes. 
DTA - Data bytes. 
ECH - Error check performed on message by exclusive or 'ing consecutive 
bytes on a bit by bit basis. 
DESCRIPTION OF PROTOCOL 
The following command protocol is defined for the CPU 114 to CM module 103 
data link 112 in FIG. 3. 
Byte 
1. Start of message (SOM) (Al zero's) 
2. Message #/Address #1 (MAD) (DIU,FSU,COM,EEM,Etc.) 
3. Channel address (CAD) (all zero's if not applicable) 
4. Message #/Address #2 (MAD) (same as byte 2) 
5. Command (COM) 
6. Date length N (DTL) 
6+N Data byte(s) (DTA) 
7+N Error check (ECH) (2 to 6+N EX/or Byte wide)* 
The following response protocol is expected by the CPU 114 from all 
modules: 
Byte 
1. Channel address (CAD) (all zero's if not applicable) 
2. Message/address (MAD) 
3. Response (RSP) (See byte description) 
4. Data length N (DTL) 
4+N Data byte(s) (DTA) 
5+N Error check (ECH) 
The following command protocol is defined for messages from the CM module 
103 to the DIU 104: data link 120 in FIG. 3. 
Byte 
1. Message #/Address #1 (MAD) (DIU,FSU,COM,NUM) 
2. Channel address (CAD) (all zero's if not applicable) 
3. Message #/Address #2 (MAD) (same as byte 1) 
4. Command (COM) 
5. Data length N (DTL) 
5+N Data byte(s) (DTA) 
6+N Error check (ECH) (1 to 5+N) 
The following protocol is defined for the DIU 104 to the channel units 100: 
data link 122 in FIG. 3. 
Byte 
1. Start of message (SOM) (inserted by DIU, all zero's) 
2. Channel Address (CAD) 
3. Message #/Address (MAD) 
5. Data length N (DTL) ** 
5+N Data byte(s) (DTA) ** 
6+N Error check (ECH) (2 to 5+N) 
7+N End of message (EOM) (inserted by DIU, all 1's) 
DIU 104 must check for all zero's in data length and data bytes and insert 
an additional zero byte when detected for messages sent to channel unit 
100. Data length does not include repeated bytes. 
MODULE PROCESSING OF PROTOCOL 
For outgoing messages from the CPU 114, the CM 103 performs the following 
action (must buffer entire message before error response): 
(1) Read First Address (MAD) 
(2) Compare with Second Address (MAD) 
(3) Perform error check 
If there is not a good address comparison, or if there is a bad error 
check, or invalid command, then send CAD, MAD, set appropriate bits in 
error response RSP, and regenerate ECH back to CPU 114. If there is no CM 
Module Address, then send MAD, CAD, MAD, COM, DTL, DTA and regenerated ECH 
to DIU 104. If there is a CM Module Address, then execute command and send 
CAD, MAD, good RSP, DTL, DTA, and regenerated ECH back to CPU 114. 
For DIU Action: 
(1) Read Address (MAD) 
(2) Check (CAD) for zero's (all zero's indicates DIU message) 
(3) Compare with Second Address (MAD) 
(4) Perform error check 
If there is not a good address comparison, or if there is a bad error 
check, or invalid command, then send CAD, MAD, set appropriate bits in 
error response RSP, and regenerated ECH back to CPU 114. If there is a 
channel Address, then check DTL and DTA bytes for zero's and repeat zero 
bytes, strip first MAD, add SOM, and send SOM, CAD, MAD, COM, DTL, DTA, 
regenerated ECH, and added EOM to CU 114. If there is a DIU Address, then 
execute command and send CAD, MAD, good RSP, DTL, DTA, and regenerated ECH 
back to CPU 
DIU must repeat any zero bytes to prevent other channel units from 
interpreting all zero's as a start of message. Channel unit should detect 
consecutive zero bytes and discard one of them. 
For Channel Unit Action: 
(1) Recognize Start of Message 
(2) Recognize Address 
(3) Perform Error Check 
If there is a bad error check or bad command, then send CAD, MAD, set 
appropriate bit in error response RSP, and regenerated ECH back to CPU 
114. 
If there is no error, then execute command and send CAD, MAD, good RSP, 
DTL, DTA, and regenerated ECH back to CPU 114. 
If two consecutive zero bytes occur in DTL or DTA, one zero byte shall be 
saved and the other discarded. If only one zero byte occurs (not 
consecutive) the receiving process should be terminated and no response 
sent back to CPU 114. 
For an incoming response from CU 100 to CPU 114, the Channel Unit Action is 
the same as described above. The DIU 104 Action is to pass entire channel 
unit response on to CM module 103 with no error check or regeneration of 
error check. The CM Module Action is to Perform Error Check and if error 
check is bad, then write over entire response byte with appropriate bits. 
If there is no error, then entire message is passed on to CPU 114. 
The major features of this communication protocol are as follows: 
Multiple Primary/Secondary Node Structure to Minimize Backplane Routing; 
A synchronous Serial Format to Minimize Pin-Outs; 
Command and Response Message Format; 
Variable Message Length; 
Message Tracking Capability; 
Message Response Verification (Who, Type or Response, Error Location); and 
Check sum Error Check at Each Node. 
RESPONSE BYTE CODING 
A good response or acknowledge of the command message is indicated by a 
zero in bit 8 of the returned response byte. A bad response to a command 
message is indicated by a one in bit 8 of the returned response byte. Bits 
1 & 2 of the error response byte indicate where in the outgoing message 
the error was detected while bits 3 & 4 indicate where in the incoming 
message the error was detected Bits 5, 6 & 7 indicate the type of error 
detected. Outgoing error responses result in an immediate error response 
at the point of detection as there is no reason to send the command to the 
addressed module. Incoming error responses will write over the returned 
response byte regardless of either a good or bad received response. 
FIG. 4 represents a flow chart showing the various paths a message can take 
depending upon its error check, its address, or its interpretation of the 
command or other error responses 
The invention is not limited to the particular details of the apparatus 
depicted and other modifications and applications are contemplated. 
Certain other changes may be made in the above described apparatus without 
departing from the true spirit and scope of the invention herein involved. 
It is intended, therefore, that the subject matter in the above depiction 
shall be interpreted as illustrative and not in a limiting sense.