Reliable ATM microwave link and network

A digital radio link suitable for transmitting digital voice or data signals includes an input circuit for ATM or equivalent cells, a processing circuit to encapsulate the input cells received by the input circuit with at least error check bits so that detected error can identify a specific cell or group of cells, a digital transmission circuit for transmitting the encapsulated cells via a wireless link, a reception circuit for receiving encapsulated cells from an opposite side of the digital radio link, a cell decapsulation and error detection circuit connected to said reception circuit and a cell output circuit for connecting said decapsulation circuit to a digital access unit. The system includes structure for allocating more than one type of service, such as low delay service for voice transmission and extensive forward error correction service for data transmission.

FIELD OF THE INVENTION 
This invention relates to the field of telecommunication networks and 
packet switching and in particular to providing reliable radio-based links 
for cell-switched networks. 
BACKGROUND 
Cell-based packet switching networks are becoming widely available. The use 
of small (i.e. "short") packets of information is preferred in modern 
digital networks because it enables the efficient mixing of synchronous 
and asynchronous information, thus providing cost-effective transport of 
digital voice, LAN data and video. Furthermore, short packets, also known 
as "cells", can be switched by integrated circuits, allowing quick and 
economical switching of data in broadband fiber optics networks. This 
concept is known in the telecom industry as "Asynchronous Transfer Mode" 
(ATM). ATM networks are commercially available. ATM protocols have been 
formalized by various international organizations, including the ITU and 
the ATM Forum. ATM networks were specified assuming the use fiber optics 
links for transmission. Due to the very low bit error rate of fiber optic 
links, ATM networks do not provide extra overhead services to guarantee 
end-to-end delivery of cells. Cells are routed through the network, but if 
an error occurs (or a buffer overflows), cells may be discarded. The 
simplicity of "best effort" cell delivery results in a fast and cost 
effective network. 
Typical fiber networks consist of long-haul fiber links interconnecting ATM 
switches. These switches may be connected by fiber optics links to 
customer sites, such as office buildings and homes. In the customers' 
buildings there are network access nodes that combine and convert a user's 
information to ATM cells for transmission over the network. 
Although fiber optics links are becoming the preferred medium for 
terrestrial links, they are not always available. City regulation, 
installation costs, long installation time and legal right-of way issues 
prevent some regions from installing fiber optics links. Some cities may 
have fiber optics links installed, but owned by a monopoly which a service 
provider may wish to bypass. 
Digital microwave radio links can provide an alternative to fiber optics 
links between network access nodes and ATM switches. Frequency bands 
within the range of about 300 MHz to 60 GHz have been allocated for 
commercial communications. Some microwave links in the millimeter wave 
region are unlicensed or licensed for low usage fees by regulating 
governments. Microwave radio links then become a cost-effective and a 
timely solution to the fast deployment of telecommunication links. There 
is a drawback, however, to these microwave radio links. Digital microwave 
radio communication is prone to bit errors, especially under 
weather-induced fading conditions, such as rain. Some forward error 
correction, redundancy and retransmission protocol schemes have been 
devised to improve the performance of microwave links. The problem with 
these approaches is that they are not directly applicable for ATM traffic. 
Retransmission is unacceptable because of the delay it introduces. Forward 
error correction alone does not protect from antenna obstruction or 
antenna failure. Redundancy by parallel links is too costly and still 
prone to common link obstructions such as weather-induced signal 
degradation. 
SUMMARY OF THE INVENTION 
This invention provides very reliable microwave radio-based communication 
links for ATM transmission. 
In accordance with this invention, a cell-based access network is formed to 
connect multiple customer sites to a switching center. The network as a 
whole, and each link in this network, are especially designed to provide 
reliable service under bit-error conditions. 
At the link level, reliable service is provided by subsystems called "Trunk 
Units" (TU) which process information before and after transit through the 
error-prone radio link. At the transmit side, the TU divides the 
information to be transmitted into groups of one or more cells, based on 
the ATM or similar cell structure of the information. Each cell is 
assigned a class of service, based on the nature of the information in 
that cell. Constant bit rate information, such as PCM-encoded voice, is 
transmitted with minimum delay and only small amounts of forward error 
correction. Asynchronous information is transmitted with extensive forward 
error correction, and optionally with retransmission upon error. Thus the 
network has, in one embodiment, the capability of transmitting three 
classes of information: 
1) Information which requires minimum delay and small amounts of forward 
error correction; 
2) Information which requires forward error correction; and 
3) Information which requires forward error correction and retransmission 
upon error. 
Network access systems with two or more radio links are combined with other 
network access systems or alternate links (microwave relay radios or fiber 
optics links) from a network. The network, in one embodiment, has a mesh 
topology, but not necessarily a full mesh. In other embodiments, single or 
multiple rings are also acceptable topologies. The network links bit rate 
is higher than the total bit rate of the ATM cells transmitted via these 
links. The extra bandwidth of the network links allows protection bits to 
be carried that add protection to the information being transported via 
the microwave network. The ATM cells entering the network are assigned to 
one of several grades of service. One of the grades is synchronous. 
Another grade is synchronous protected. Another one is asynchronous best 
effort. Yet another service grade is called asynchronous protected. Still 
other grades may exist. 
For asynchronous services, packets are appended by forward error correction 
("FEC") check sequence. If a packet passes through a link without error, 
there is no delay in the passage through that link. If an error is found, 
the packet is corrected but at the price of a higher delay. 
This invention will be more fully understood in conjunction with the 
following detailed description taken together with the drawings.

DETAILED DESCRIPTION 
A metropolitan area network in accordance with this invention is shown in 
FIG. 1. The dark arrows 10a through 10h represent wireless links with 
radio transceivers (not shown) at each end. These wireless links connect 
buildings shown as 11a through 11j in a city to a central office 14 also 
called a "point of presence". The point of presence 14 ("POP") includes 
ATM switches, frame relay datacom (X.25) switches and voice switches. This 
invention allows exchange of digital voice and data between these switches 
and makes available ATM developed for fiber optics for transmission by 
digital radio. A network allows the extra benefits of relaying information 
from remote stations such as at location 15, even if the remote stations 
have no direct line of sight to the point of presence 14, as well as 
providing redundant links, and the ability to concentrate information from 
multiple nodes. 
The minimum access node for a point-to-point link of this invention is 
depicted in FIG. 2. This minimum system is sufficient if the advantages of 
a full network are not required. An access unit (AU) is capable of 
interfacing with a variety of local interfaces. In the AU, the signals 
from these interfaces are all converted to ATM cells, which are delivered 
to a trunk unit (TU). A suitable AU can be purchased today from a large 
number of vendors, for example Stratacom of San Jose Calif. An appropriate 
AU can also be designed as a set of electronic cards and software, as 
briefly described below. The ATM cells are delivered to the TU. The TU 
includes the hardware and the software that can identify cell boundaries, 
encapsulate the cells, add FEC and other overhead, including overhead that 
describes the type of service the link should provide to the network, and 
other cell processing functions as described below. The TU is a key 
element of this invention. The TU outputs a serial bitstream to the radio 
unit (RU) which is placed on the outside wall or roof of a building, 
attached to a dish antenna. A twisted pair cable, coax or fiber optics 
link connects the RU to the TU. The RU modulates the bitstream and 
transmits it at the desired microwave frequency. This system operates 
normally in a full-duplex mode; thus the RU also receives a bitstream from 
an opposite access node and delivers this bitstream to the TU. The TU 
processes the cells in this received bitstream, including error correction 
and dropping of cells that are not recoverable. The TU delivers good cells 
to the AU and logs or reports cell loss to a control system (a 
microprocessor circuit not depicted in FIG. 1 but shown as control units 
("CU") 43a and 43b in FIG. 4). 
For a network, more than a single RU is required per access node. A general 
block diagram of such a system is depicted in FIG. 3. Multiple AUs, RUs 
and TUs are shown. A new block, an interconnect unit (IU) is provided. The 
IU is essentially a cell-switch for exchanging cells among the TUs and 
AUs. The IU may be implemented as a backplane, as is commonly done by 
existing ATM switching and concentrating equipment. A cell switching 
matrix can be added to the system. Such a cell switching matrix could be 
connected to all plug-in electronic cards via a backplane. Such an 
implementation is available using the Prisma 16.times.16 Switch On A Chip 
from IBM Microelectronics, Research Triangle Park, North Carolina (800) 
426-3333. Such a chip is currently the preferred way for implementing the 
interconnect unit of FIG. 3. The IU function is then distributed among the 
modules (AUs and TUs) and implemented in the form of a bus access chip 
set. A control entity (e.g. a network management system 49 ("NMS")), 
external to this system, may establish communications with a local 
microprocessor, that can use the backplane to instruct each unit connected 
to the IU which cell is to be routed to which other unit connected to the 
IU, based on a connection identification number (VPI/VCI in ATM 
terminology). As shown, a TU can also be connected to copper or fiber 
optics links if such are available, where there is an economical advantage 
of doing so. In the example of FIG. 1, some of the links are of fiber 
optics. 
FIG. 4 depicts a preferred embodiment of a large node. An extension unit 
(EU), shown as 40a and 40b, is similar to an AU, except that the EU 
includes a proprietary interface to allow maximum economy of connection 
with other AUs, including AUs without ATM capabilities. The TUs, AUs and 
EUs are all plug-in electronic cards to a backplane, with a distributed IU 
as mentioned above. The RUs are outdoor units, as mentioned above. Control 
units, CU, shown as 43a and 43b, are typically microprocessor cards with 
appropriate connectivity to all other cards in the backplane, as is 
customary in the communications system art. The CUs 43a and 43b control 
the system operation and communicate with external control centers such as 
a network control center or a user console. Standard, existing software 
stacks can be used for the CU control functions, such as SNMP, CMIP and 
TMN. The TU/RU pairs are of two types: wide band (WB), for example 34 
Mbps, and narrow band (NB) for example (2 or 8 Mbps). The wideband TU/RU 
44a through 44d and 45a through 45d can be used as the backbone of the 
network of this invention, or as point-to-point links. The narrowband 
TU/RU pairs 47a through 47d and 48a through 48d are used mostly for 
point-to-point access and as tributary access to the backbone. For 
redundancy purpose, two TU/RU pairs may be designated as "Main", and two 
as "Alternate". The alternate units allow repair and network node 
modifications, such as addition of backbone nodes, without interruption to 
service. Since the backbone bandwidth is higher than the local traffic of 
each node in most situations, a drop/insert capability is designed in, 
shown as a "fast flow through bus" 46. Bus 46 allows data to bypass the 
interface unit (IU) to thus speed up the system in appropriate 
circumstances. The TUs look at the VPI/VCI of each cell and drop (pick up) 
only those cells with a VPI/VCI on the list of a local look-up table 
stored in memory. This table is maintained and updated by local circuitry, 
in conjunction with external network management. A dropped cell is not 
retransmitted to the next network node. Instead, it is replaced by an idle 
cell. Cells for transmission are kept in a TU queue and are sent whenever 
an idle cell is encountered, replacing the idle cell. The network 
management or local flow control techniques can ensure fair distribution 
of bandwidth for packet transmit opportunity. As a minimum, the sum of 
tributary pick transmission rates can by allocated for the backbone, 
obviating the need for such flow control, at the expense of system 
bandwidth utilization efficiency. 
FIG. 5 depicts the topology of the metropolitan area network (MAN) or 
"Network" of this invention. Access nodes 50a through 50h, such as the 
node of FIG. 2, are depicted as circles 50a through 50h. Bold lines 51a 
through 51i depict backbone radio links. Thin lines 52a through 52i depict 
tributary radio links. Since the radio links are full duplex, information 
travels in both directions along the backbone. The network is ATM based, 
and since ATM is connection-oriented, cells are dropped and inserted at 
each of nodes 50a through 50h, as described above. If a backbone link 
fails, data can be routed the opposite direction. At the POP 53, two 
wideband RU/TU serve each side of the backbone. Here, all bandwidth is 
sent to the TU and then to AU and eventually to the external switching 
equipment. The "backbone node" 53 in FIG. 5 includes both the access node 
of this invention, as well as the external switching equipment. Fiber 
links to other cities are not normally connected directly to the access 
node of the MAN of this invention. 
FIG. 6 shows how ATM traffic might be handled by digital radios of the 
present art. A serial bitstream 61 containing ATM cells is cut arbitrarily 
to blocks 61a, 61c, for example, of a fixed size (this is called block 
coding), and FEC bits are appended. The information is transmitted over 
the radio link at a slightly higher bit rate to allow for the FEC 
overhead. In the receiver, the errors are corrected, the FEC bits 61b, 61d 
removed, and the original bit rate restored. This mode of operation is 
reasonable if the transmission needs to maintain transparency to any bit 
protocol, and therefore this mode is provided as an option in systems 
built in accordance with this invention. However, to provide a more 
reliable link, as well as to provide better statistics of cell losses, it 
is advantageous to align block coding with the cell boundaries, if such 
boundaries exist in the information. This is depicted in FIG. 7, which 
shows the method of FEC encoding in accordance with this invention. The 
cell processing in accordance with this invention may differ from a simple 
first-in-first-out transmission. Cells may be dropped due to uncorrectable 
error. Cells may also be delivered in a different order than sent, as long 
as cells of the same virtual connection are delivered in order. Changing 
the transmission of cells order is desirable for priority transmission, 
allowing voice-carrying cells to be delivered to the destination with 
minimal, low delay FEC, and to perform extensive forward error correction 
on data-carrying cells. Furthermore, it may be desirable to retransmit 
cells, to provide a more reliable or even guaranteed delivery service to 
connections not sensitive to delay, as is customary with non-ATM link 
layer protocols. The processing done for cells in this invention is 
described below in configuration with FIGS. 8 and 9. 
FIG. 8 shows a data flow diagram (DFD) of the transmit side. This DFD can 
be implemented by a combination of hardware and firmware, using a RISC 
processor for running some of the algorithms, dual port RAM for storage 
and manipulation of the various tables, and RAM devices for cell storage. 
Gate arrays can be used to control various queues. A simple transmit side 
may have all of its virtual connection identifiers (VPI/VCI in ATM 
terminology) set up by a management system. A look-up table would list for 
each VPI/VCI the type of service to be performed. The DFD of FIG. 8 
describes a more advanced solution, in which the content of these tables 
is created automatically. This can be done by observing the nature of each 
cell. For example, an ATM cell of a constant bit rate, such as some types 
of voice applications, carries inside the cell adaptation information as 
defined by the ATM adaptation layer 1 (AAL1). An AAL1 subcell (protocol 
data unit or "PDU") has an SAR-PDU header of eight bits, including a 3-bit 
CRC field. By observing a valid CRC field, and verifying that no valid 
10-bit CRC of AAL3/4 exists, the cell can be assumed to be AAL1. To avoid 
a false decision based on random bit pattern resembling AAL1 (it could be, 
for example, AAL5 PDU which in many instances contains no headers in the 
AAL PDU and to which the system defaults if the cell is neither AAL1 or 
AAL3/4), each VPI/VCI is examined for two or more cells. In the embodiment 
of FIG. 8, two cells of the same VPI/VCI are required to decide the 
service type. Similar procedures can be developed for automatic 
identification of proprietary cell-based protocols, which exist in some 
current ATM-like switches that have been developed before ATM formats have 
been standardized. 
The receive side DFD is shown in FIG. 9. Error correction may take two 
phases; phase I is global, done to all cells regardless of service type 
while phase II is more extensive, and is done selectively to critical 
cells. Phase II may require extra bandwidth, reducing the bandwidth 
available for information. The decision as to how much phase II FEC to 
perform depends on a particular application and the bandwidth/performance 
trade-off. Both phases of FEC can be block codes, such as BCH and Reed 
Solomon, or a concatenation of convolutional code for phase I and block 
code for phase II. Furthermore, a retransmission protocol can be 
established for some connections. Since these radio links are short, 
normally less than 5 kilometers, the round trip delay is about 30 
microseconds, which at 34 Mbps (megabits per second) is less than four 
cells. Thus cells can have a second chance, if so desired, with a sliding 
window protocol only four cells deep. The window may actually be a bit 
larger due to processing delay, but the absolute physical limit is small 
enough that retransmission is a practical option. An important feature of 
this invention is the transmission of error performance indications from 
the receive side back to the transmit side of the opposite node. This 
indication can be done by assigning bits as a part of the cell 
encapsulation overhead. 
An example of a VPI/VCI table for automatic updating is shown in FIG. 10. 
The first column shows VPI/VCI in hexadecimal format. The second column 
shows service class as assigned manually or automatically. Unassigned 
VPI/VCI are marked by a class F that represents a connection not yet 
encountered for a long period of time. The third column represents a time 
stamp, that is periodically incremented, unless the same VPI/VCI cell is 
read, in which case the time stamp is reset. An FF represents an expired, 
or never encountered connection. Although this table lists all of the 
possibilities of VPI/VCI combinations with 12-bits, there are 24 bits 
maximum in a standard ATM UNI (User-Network Interface); thus one strategy 
may be to erase expired connections and to keep a rather small table of, 
say, up to 256 rows with active connections. In a preferred embodiment, 
this is implemented by first having a multi stage look-up table that 
checks if a VPI may exist in a subset of the memory space, and then point 
to the table entry itself. Similar techniques have been used for 
hardware-based, self-learning bridges and IP routers. 
The above DFDs are implemented as a combination of hardware and firmware 
residing in the TU. The hardware is shown in FIG. 11. A Cell I/O 110 
circuit receives cells from the IU and places them in a queue (RAM) 111. 
This I/O circuit may consist of an off-the-shelf ATM switching integrated 
circuit, or made of an ASIC. A queue and QoS Controller 113 is a fast 
processor (e.g. RISC) that manages the queues, hosts the look-up tables as 
external or on chip memory, and performs the service identification 
procedures. Processor 113 also controls the cell encapsulation and FEC 
block 112. The queue controller and QoS controller 113 takes cells and 
places them in various queues in cell queue 111 to reflect the type of 
service which is to be given to the cells. For example, cells which 
reflect voice data will be placed in a queue in cell queue 111 which will 
be given low delay and minimal error correction. On the other hand, cells 
representing data will be placed in a queue in cell queue 111 which will 
have extensive forward error correction. Alternatively, a queue can be 
provided for retransmission of cells which are transmitted with an 
unacceptable level of errors. 
The VPI/VCI table of FIG. 10 is implemented in the queue and QoS controller 
113 in FIG. 11. 
Processor 113 is connected to the Control Unit ("CU") by appropriate 
interconnection circuitry. The CU provides system-level control, which 
includes external instructions to update the VPI/VCI table, as well as 
drop/insert tables in the drop/insert mux 115. The transmitted cells are 
combined in a drop/insert mux 115 that sends the cells to the cable modem 
117. This can be a copper-FDDI of copper ATM baseband modem, which is 
essentially a line driver. Such devices are commercially available. The 
modem 117 sends the data to the RU for radio transmission. Data received 
from the RU by the modem 117 is first sent to the drop/insert mux 115, 
that in conjunction with the fast flow-though bus 118a, 118b decides how 
to route cells. The drop/insert mux 115 also performs phase I FEC 
correction, to improve the chance of correct routing decisions on cells 
not fully corrected. The cells are then decapsulated by an ASIC combined 
with phase II FEC, and are placed in a queue for output to the IU. 
FIG. 12 depicts an RU. The RU has a cable modem, similar to the TU. A local 
microprocessor (MP) monitors operation, voltages, and synthesizers 
function. A modulator converts the bitstream to transmit RF signal at a 
microwave frequency, such as roughly 6 GHz. This can be an FSK or QPSK 
modulator. The modulator frequency is controlled by a synthesizer to set 
the precise frequency. This signal is then up-converted to the desired 
frequency, such as 38 GHz. A preferred converter is a frequency multiplier 
by a factor N. A diplexer allows simultaneous transmit/receive in the same 
antenna. The receive signal goes through double conversion and 
demodulation and then is sent to the cable modem and finally to the TU. 
Other embodiments of this invention will be obvious to those skilled in the 
art in view of this disclosure.