Apparatus and method for transferring operation, administration and management cells across and ATM data frame user network interface

A method and switch appratus for transporting an ATM layer OAM cell across a FUNI are disclosed. The method broadly includes generating a DXI (FUNI) frame from the ATM layer OAM cell by using certain bits of the five byte ATM overhead as bits in the two byte DXI (FUNI) header, using the forty-eight byte ATM OAM cell payload as the payload for the DXI (FUNI) frame, and providing an indication in a predetermined bit field of the two byte DXI header that the forty-eight byte payload is an ATM OAM cell. Preferably, the predetermined bit field includes the previously reserved second lsb of Octet 1 of the DXI frame header, and the previously reserved third lsb of Octet 2 of the DXI frame header, which are set to values of "0" and "1" to indicate an ATM OAM cell, and to "0" and "0" to identify "regular" data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of telecommunications. 
More particularly, the present invention relates to operation, 
administration, and management (OAM) cells used across the User Network 
Interface (UNI) of an Asynchronous Transfer Mode (ATM) network. 
2. State of the Art 
Perhaps the most awaited, and now fastest growing technology in the 
telecommunication field in the 1990's is known as Asynchronous Transfer 
Mode (ATM) technology. ATM is providing a mechanism for removing 
performance limitations of local area networks (LANs) and wide area 
networks (WANs) and providing data transfers with at a speed of on the 
order of terabits/second. The variable length packets of LAN and WAN data 
are being replaced with ATM cells which are relatively short, fixed length 
packets. Because ATM cells can carry voice, video and data across a single 
backbone network, the ATM technology provides a unitary mechanism for high 
speed end-to-end telecommunications traffic. 
In order for ATM technology to develop, it must be functional in its own 
right as well as compatible with existing technology. To be compatible, on 
the one hand, the ATM cells must be capable of receiving and accommodating 
voice, video, and LAN and WAN type data; while on the other hand, ATM 
cells must be capable of adapting to high speed technology such as the 
synchronous optical network (SONET). In order to meet these and other 
requirements, a technical group called the ATM Forum which is comprised of 
numerous corporate representatives has been proposing ATM "standards" 
which are being provided to ANSI and the ITU-T for their consideration and 
adoption. Details of those standards may be found in proceedings of the 
ATM Forum. Of particular interest among the ATM Forum documents is a 
document which is incorporated by reference in its entirety herein and 
which is dated Mar. 15, 1995 and entitled "af-saa-0030 000 Frame 
User-to-Network Interface (FUNI) Specifications", listing G. Shenoda and 
D. McDysan as editors. The Mar. 15, 1995 document incorporates therein the 
present invention which was described by one of the present inventors on 
Sept. 27, 1994 to the ATM Forum Technical Committee SAA/DX-UNI Sub-working 
Group. Of additional interest for the present invention are a Draft 
(11/94) ITU-T Recommendation I.610 entitled "B-ISDN Operation and 
Maintenance Principles and Functions", pp. 1-47; a Committee T1, Working 
Group T1S1.5 report dated Jul. 11-15, 1994 entitled "dpANS B-ISDN 
Operations and Maintenance principles and Functions", M. Methiwalla, 
editor, pp i-ix; an undated Draft ANSI standards document entitled 
"Broadband ISDN Operations and Maintenance Principles and Functions", 
Secretariat: Exchange Carriers Standards Association, (pp. 1-19) , all of 
which are hereby incorporated by reference in their entireties herein. 
Additional background material on ATM is readily available, such as a 
booklet entitled "Asynchronous Transfer Mode: Bandwidth for the Future", 
published by Telco Systems, Inc., Norwood, Mass., and a pamphlet entitled 
"Tutorial-ATM in private Networking", published by Hughes LAN Systems, 
Mountainview, Calif. 
As is set forth in Section 2 of Draft Recommendation 1.610, operation, 
administration, and management (OAM) cells provide performance monitoring, 
defect and failure detection, system protection, defect information, and 
fault localization functions. performance monitoring is a function which 
processes user information to produce maintenance information specific to 
the user information. The maintenance information is added to the user 
information at the source of a connection/link and extracted at the sink 
of a connection/link. Analysis of the maintenance event information at the 
connection sink allows analysis of the transport integrity. Defect and 
failure detection is accomplished by continuous or periodic checking and 
results in the production of various alarms. In the system protection 
function, the effect of a defect on the transport of user information is 
minimized by blocking or changeover to other entities, and the failed 
entity is excluded from operation in order to protect the system. Defect 
information is given to other management entities. As a result, alarm 
indications are given to other management planes. Response to status 
report requests will also be given. Fault localization involves a 
determination by internal or external test systems of a failed entity if 
defect information is insufficient. 
Section 3 of the 1.610Recommendation states that OAM functions in the 
network are performed at five OAM hierarchical levels associated with the 
ATM and physical Layers of the B-ISDN protocol reference model. The five 
hierarchical levels include: the virtual channel level which extends 
between network elements performing virtual channel connection termination 
functions, the virtual path level which extends between network elements 
performing virtual path connection termination functions, the transmission 
path level which extends between network elements assembling/disassembling 
the payload of a transmission system and associating it with its OAM 
functions, the digital section level which extends between section end 
points, and a regenerator section level which is a portion of a digital 
section. Of the five levels, the physical layer contains the regenerator 
section level, the digital section level, and the transmission path level, 
while the ATM layer contains the virtual path level and the virtual 
channel level. The OAM cells in the ATM layer virtual path level are 
called F4, while the OAM cells in the ATM layer virtual channel level are 
called F5. In the prior art, all ATM layer OAM cells are terminated at the 
ATM layer. 
As is known by those skilled in the art, the ATM technology is based on the 
fifty-three byte ATM cell, with five bytes of overhead (i.e., a five byte 
header), and forty-eight bytes of data (which may include higher layer 
level overhead). The ratio of overhead to data is rather high for low 
speed applications. Thus, the ATM forum is developing an implementation 
agreement for a frame based user network interface (i.e., a frame UNI or 
FUNI) which passes frame information across the interface over variable 
sized frames between customer premises equipment (CPE) and an ATM switch. 
When information is passed across a FUNI, the frame can include up to 
approximately 64,000 bytes of user data. The format of the data is 
essentially in DXI frame format (now known as the FUNI format), with a 
start byte, a header, the user data, a trailer, and a stop byte. The DXI 
header contains ATM layer information (i.e., VPI/VCI) which is passed from 
the CPE to the switch, and vice versa. At the ATM switch, the DXI header 
and trailer are striped off, the DXI header is used to Generate an ATM 
cell header, and the adaptation data is used to segment or reassemble ATM 
cell data. 
While it would be desirable to pass ATM layer OAM information across the 
FUNI to provide a complete end to end fault management and connectivity 
verification similar to the cell UNI, the F4 and F5 OAM cells cannot be 
passed through the FUNI as they are ATM layer cells and are terminated at 
the ATM layer level; i.e., they cannot be passed over a frame interface. 
Thus, for users utilizing a FUNI, AIS alarms, and loop-back commands 
cannot get to their CPE destination. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a mechanism for 
transferring ATM layer OAM information across a frame based user network 
interface so that a CPE can process, utilize, and act on the OAM 
information. 
It is another object of the invention to provide a mechanism for placing 
ATM layer OAM information into a DXI (now FUNI) frame format with 
indicators for a CPE that OAM information is contained therein. 
It is a further object of the invention to provide a simple mechanism for 
extending OAM features to end users, thereby permitting fault localization 
and continuity testing in a telecommunications system. 
In accord with the objects of the invention, a method for transporting an 
ATM layer OAM cell across a FUNI broadly comprises generating a DXI (FUNI) 
frame from the ATM layer OAM cell, and providing an indication in the DXI 
(FUNI) frame that the frame contains OAM cell data. More particularly, the 
ATM layer OAM cell data is encapsulated with a DXI header and trailer in 
order to construct a DXI frame. The DXI header is preferably constructed 
with bits extracted from the OAM cell header, plus an indication in 
predetermined bit fields which permits the CPE to distinguish the DXI 
frame containing OAM cell data from other DXI data frames. In accord with 
a preferred embodiment of the invention, the previously reserved second 
bit of Octet 1 of the DXI frame header is set to a value "1", while the 
previously reserved third bit of Octet 2 of the DXI frame header is set to 
a value "0" in order to signal that an OAM cell is being contained in the 
DXI (FUNI) frame. The same bits are set to values "0" and "0" in order to 
identify user data information (i.e., a typical DXI frame). In accord with 
a preferred aspect of the invention, one OAM cell is encapsulated in a 
single DX1 frame having a payload of forty-eight bytes. The five byte 
header of the OAM cell is removed prior to encapsulation, with the 
information contained in the OAM cell overhead being used to fill fields 
in the DXI header. 
The objects and advantages of the invention will become apparent to those 
skilled in the art upon reference to the detailed description taken in 
conjunction with the provided figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning to prior art FIG. 1a, a telecommunications network 10 is seen to 
include user equipment (CPE) 20a, 20b, and a switched public network 25. 
The user equipment 20a is shown coupled to the public network 25 via a 
frame based user network interface (FUNI) 27, utilizing a DS-1 or E1 
physical line 29. The user equipment 20b, on the other hand, is shown 
coupled to the public network 25 via a cell-based ATM user network 
interface 31, using any physical line 33. In order for the network 25 
(shown as an ATM network) to support the FUNI, the network provides a FUNI 
to ATM conversion function 25a which was discussed above in the background 
section. 
The layers supported by the user equipment 20a, 20b, and the network 25 are 
seen in prior art FIG. 1b. Thus, the cell based user equipment 20b is 
provided with a physical layer 80, the ATM layer 82, an adaptation 
AALx/SAR segmentation and reassembly sublayer 84, an adaptation layer 
AALx/CPCS 86, and appropriate upper layers 88 such as TCP/IP layers. The 
frame based user equipment 20a, on the other hand, simply includes the 
physical layer 80, the FUNI data link layer 85, and upper layers 88. The 
network 25 is shown to perform a mapping function 89 between the layers 
80, 82, 84, 86 of the cell based user equipment 20b, and the layers 80, 85 
of the frame based user equipment 20a. 
It will be appreciated by those skilled in the art that an ATM connection 
between the user equipment and the network may be a logical connection 
which is part of a high bandwidth physical connection. Thus, as shown in 
prior art FIG. 2a, data from a plurality of DS-1 lines 29a, 29b, 29c (line 
29c being utilized as a FUNI for the ATM network) are multiplexed at 
multiplexer 90 onto a DS-3 line 91. The data on the DS-3 line 91 is in 
turn demultiplexed by a demultiplexer 92 onto a DS-1 groomed ATM line 93a 
which is coupled to the ATM network 25, and onto additional lines 93b, 93c 
which are coupled to other network elements (not shown). 
A fractional DS-1 (or E1) FUNI payload may be similarly carried in a DS-1 
(or E1) physical connection. A fractional DS-1 FUNI payload is 
particularly useful for users who wish to transfer information at low 
speeds, e.g., voice, in the same DS-1 physical connection to the network. 
Thus, as shown in FIG. 2b an N.times.64K bps CPE (not shown) is coupled 
via line 129a, along with other fractional DS1 payloads on lines 129b, 
129c to a multiplexer 190 which outputs the data on a DS-1 line 191. The 
data on line 191 is then demultiplexed to provide a fractional payload on 
a DS-1 groomed FUNI line 193a, as well as fractional payloads on lines 
193b, 193c coupled to other network elements (not shown). 
Details of the ATM cell format on the ATM side of the user network 
interface are seen in prior art FIG. 3. The ATM cell has a five byte cell 
header and forty-eight bytes of data. The first byte of the header 
includes four bits of generic flow control (GFC), and four bits of a 
virtual path indicator (VPI). The second byte of the header includes four 
additional bits of the VPI and four bits of a virtual channel identifier 
(VCI). The third byte of the header has eight additional bits of the VCI, 
while the fourth byte of the header has four additional bits of the VCI, 
three bits of a payload type indicator (PTI), and a one bit cell loss 
priority indicator (CLP). The fifth byte of the header is a header error 
control (HEC). It should be appreciated by those skilled in the art that 
the F4 and F5 OAM cells take the ATM cell format shown in FIG. 3 with 
predefined VCI and PTI field values. 
Details of the DXI (now FUNI) frame format on the frame (FUNI) side of the 
user network interface are seen in prior art FIG. 4. The DXI or FUNI frame 
includes a start flag byte, two bytes of a DXI or FUNI header, up to 
64,000 bytes of payload data, two to four bytes of a DXI or FUNI trailer, 
and a stop flag byte. As indicated in FIG. 4, the start and stop flag 
bytes are preferably of the form "01111110". The first byte of the DXI or 
FUNI frame header includes six bits of a frame address, an undefined one 
bit field, and an lsb set to "0". The second byte of the DXI or FUNI frame 
header includes four additional bits of a frame address, a congestion 
notification bit CN, an undefined one bit field, a CLP bit, and an lsb set 
to "1". The DXI or FUNI trailer is a frame check sequence which may be a 
sixteen or thirty-two bit CRC depending upon the length of the data 
string. 
In order for ATM data to be transmitted across a FUNI, the ATM cell header, 
and the AALx sublayer header and trailer are striped off. In addition, the 
ATM cells must be terminated at the AAL layer, and the data carried in the 
ATM cell must be placed into a DXI (FUNI) type frame. The number of ATM 
cells which are placed into and which constitute the payload of the DXI 
frame is provided by the AAL layer. However, regardless of the number of 
ATM cells which constitute the larger frame, certain bits of the ATM cell 
header (seen in FIG. 3) must be mapped into certain of the bits of the DXI 
or FUNI header (seen in FIG. 4). In particular, and as seen in prior art 
FIG. 5, the two lsbs of the third byte of the ATM cell header are mapped 
into the two msbs of the first byte of the DXI or FUNI header; the four 
msbs of the second byte of the ATM cell header are mapped into the four 
next msbs of the first byte of the DXI or FUNI header; the four msbs of 
the fourth byte of the ATM cell header are mapped into the four msbs of 
the second byte of the DXI or FUNI header; and the lsb (CLP) of the fourth 
byte of the ATM cell header is mapped into the second lsb (CLP) of the 
second byte of the DXI or FUNI header. Additional details of the prior art 
mapping may be seen by reference to the "af-saa-0030 000 Frame User-to 
Network Interface (FUNI) Specifications" document previously incorporated 
by reference herein, and the documents incorporated in that document. 
It should be appreciated by those skilled in the art, that in the prior art 
arrangement, any OAM cells which would be received at the FUNI would be 
terminated at the ATM OAM layer. Indeed, no mechanism is provided in the 
prior art to transmit the OAM cells across the FUNI. However, according to 
the present invention, OAM cells may be transmitted across the FUNI by 
placing the OAM information into a DXI (FUNI) frame format with indicators 
in the DXI (FUNI) header that OAM information is contained therein. In 
particular, and as set forth in flow chart form in FIG. 6, rather than 
terminating the OAM cell, the ATM switch maps the OAM ATM cell header into 
the DXI (FUNI) header at 202 as discussed above with reference to FIG. 5. 
At 204, the second lsb of the first byte and the third lsb of the second 
byte of the DXI header are set to a predetermined value (e.G., "1", "0"). 
At 206, the forty-eight bytes of the OAM cell are placed into a single DXI 
or FUNI frame as a DXI (FUNI) payload. At 208, the DXI or FUNI trailer 
(checksum) is calculated and placed in the frame; and at 210, start and 
stop flags are appended. With the OAM cell now formed as a DXI (FUNI) data 
frame, the OAM cell is passed at 212 across the FUNI to the CPE. At 214, 
the CPE decodes the second lsb of the first byte and the third lsb of the 
second byte of the DXI (FUNI) header. If those bits are set to the 
predetermined value (e-g., "1", "0"), the CPE determines that the DXI 
(FUNI) frame includes an OAM cell, and treats the OAM "data" accordingly. 
If those bits are set to other values, (e.g., "0", "0"), the CPE 
determines that the DXI (FUNI) frame is comprised of "regular" data, and 
processes that data accordingly. 
Because the mechanism of the invention utilizes the previously undefined 
bits of the DXI header, the DXI header for the FUNI is effectively 
changed. Thus, as seen in FIG. 7 (and compring with FIG. 4), the second 
lsb of the first byte of the FUNI header is indicated by "A", and the 
third lsb of the second byte of the FUNI header is indicated by "B". In 
accord with the preferred embodiment of the invention, in order to 
indicate an OAM cell, bit A is set to "1", while bit B is set to "0". On 
the other hand, where a data frame is to be sent across the FUNI, bit A is 
set to "0", and bit B is set to "0". The other two combinations, bit A set 
to "0"-bit B set to "1", and bit A set to "1" and bit B set to "1" are 
preferably reserved for future use. It should be appreciated that whether 
or not the FUNI header contains data as opposed to an OAM cell can be 
signalled by the use of a single bit. Indeed, in the best mode provided, 
it is effectively the second lsb of the first byte of the FUNI header (the 
"A" bit) which is indicating what is contained in the FUNI frame, as the 
third lsb of the second byte of the FUNI header (the "B" bit) is set to 
zero in both cases. 
Turning now to FIG. 8, a block diagram of an apparatus 25a (at the switch) 
which generates the FUNI frame from the OAM cell in accord with the flow 
chart of FIG. 6 is seen. As seen in FIG. 8, the apparatus 25a includes an 
OCx interface 302, a switch fabric 304, an ATM interface 306, memory 308, 
a microprocessor 310, a framing processor 312, and a DS1/E1 interface 314. 
Thus, ATM data which is carried in a SONET frame and which is received 
from the optical network over OCx physical optical lines, is provided to 
the OCx interface where the SONET overhead is typically striped. After 
being switched in the switch fabric 304, the ATM data is provided to the 
ATM interface or slot controller 306 (details of which may be seen with 
reference to commonly owned U.S. Pat. No. 5,436,893 to Barnett) which 
includes its own microprocessor. The ATM interface 306 terminates the ATM 
cell, forwards the ATM cell data and at least certain of the overhead 
bytes into memory 308, and flags the microprocessor 310. The 
microprocessor 310, upon receiving an indication that an ATM cell (or 
cells) has been received, uses the data and overhead information which has 
been forwarded into memory (including certain AALx overhead), and maps the 
ATM cell or cells into a DXI or FUNI type frame. In particular, the 
microprocessor 310 generates a DXI (FUNI) header from the ATM cell and 
AALx overhead and either provides the header directly to the framer 
processor 312, or writes the header to the memory 308 and flags the framer 
processor to indicate that the header is available in memory 308. In turn, 
the framer 312 takes the data from the memory 308 (and the microprocessor 
310), generates a DXI or FUNI checksum (CRC) trailer which is appended to 
the data, and provides start and stop bytes. The DXI (FUNI) frame which is 
formed in that manner is then forwarded to the DSI/E1 interface 314 which 
generates the appropriate overhead bytes for the DS1 or E1 format so that 
the FUNI frame can be forwarded over a DS1 or E1 line to the CPE 20a (see 
FIG. 1). 
Based on FIGS. 6-8, those skilled in the art should appreciate that OAM 
cells received by the switch are effectively filtered at the ATM layer and 
sent to the OAM layer. The OAM layer of the switch can then either utilize 
and terminate the information received in the OAM cells (if they are not 
intended for the CPE), or can pass the OAM cells transparently to the 
OAM-to-FUNI mapping process which will put the forty-eight bytes of 
payload from the OAM cell into exactly one DXI (FUNI) frame payload. The 
mapping process of the DXI header from the OAM cell header is described 
above with reference to FIGS. 5-7. In passing the OAM cells to the 
OAM-to-FUNI mapping process, the AAL layer is preferably bypassed 
completely and not used in the OAM information transfer. At the CPE, the 
DXI (FUNI) frame payload is extracted and provided the CPE layer that 
processes the OAM information. 
There has been described and illustrated herein an apparatus and method for 
transferring OAM cells across an ATM data frame user network interface 
(FUNI). While particular embodiments of the invention have been described, 
it is not intended that the invention be limited thereto, as it is 
intended that the invention be as broad in scope as the art will allow and 
that the specification be read likewise. Thus, while a particular high 
level circuit for the switch which generates the DXI frame from the OAM 
cell was shown, it will be appreciated that other circuits could be 
utilized. Also, while particular preferred bit values for the second lsb 
of the first byte and the third lsb of the second byte of the DXI header 
were described as indicating an OAM cell, it will be appreciated that only 
one bit need be utilized to distinguish "regular" data from OAM data, and 
that the single bit could be either the second lsb of the first byte or 
the third lsb of the second byte of the DXI header as desired. It will 
therefore be appreciated by those skilled in the art that yet other 
modifications could be made to the provided invention without deviating 
from its spirit and scope as so claimed.