IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 199783
WATMnet: A Prototype Wireless ATM System
for Multimedia Personal Communication
Dipankar Raychaudhuri, Fellow, IEEE, Leslie J. French, Robert J. Siracusa,
Subir K. Biswas, Member, IEEE, Ruixi Yuan, Member, IEEE,
Parthasarathy Narasimhan, and Cesar A. Johnston, Member, IEEE
Abstract— A prototype microcellular wireless asynchronous
transfer mode network (WATMnet) capable of providing inte-
grated multimedia communication services to mobile terminals
is described in this paper. The experimental system’s hardware
consists of laptop computers (NEC Versa-M) with WATMnet
interface cards, multiple VME/i960 processor-based WATMnet
base stations, and a mobility-enhanced local-area ATM switch.
The prototype wireless network interface cards operate at peak
bit-rates up to 8 Mb/s, using low-power 2.4 GHz industrial,
scientific, and medical (ISM)-band modems. Wireless network
protocols at the portable terminal and base station interfaces
support available bit rate (ABR), variable bit rate (VBR), and
constant bit rate (CBR) transport services compatible with ATM
using a dynamic time-division multiple-access/time-division du-
plex (TDMA/TDD) MAC protocol for channel sharing and data
link control (DLC) protocol for error recovery. A custom wireless
control protocol is also implemented between the portable and
base units for support of radio link related functions such as
user registration and handoff. All network entities including the
portable, base and switch use a mobility-enhanced version of
ATM (“Q.2931+”) signaling for switched virtual circuit (SVC)
connection control functions, including handoff. In the first stage
of the prototype, the application-level API is TCP/IP over ATM
ABR service class using AAL5. Early experiments with the
WATMnet prototype have been conducted to validate major
protocol and software aspects, including DLC, wireless control,
and mobility signaling for handoff. Selected network-based mul-
timedia/video applications requiring moderate bit-rates (?0.5–1
Mb/s) in ABR mode have been successfully demonstrated on
the laptop PC. Work aimed at a more complete implementation
of WATMnet protocol functionalities, along with performance
optimization and multimedia application software development
is currently in progress.
Index Terms—DLC, MAC, mobility management, multimedia
systems, personal communications, wireless ATM.
for next-generation wireless communication networks capa-
ble of supporting integrated, quality-of-service (QoS) based
HE CONCEPT of “wireless ATM,” first proposed in 1992
, is under active consideration as a potential framework
Manuscript received February 1, 1996; revised June 1996. This paper was
presented in part at the IEEE International Conference on Communications,
Dallas, TX, June 24–27, 1996.
D. Raychaudhuri, L. J. French, R. J. Siracusa, S. K. Biswas, P. Narasimhan,
and C. A. Johnston are with NEC USA, C&C Research Laboratories,
Princeton, NJ 08540 USA.
R. Yuan was with NEC USA, C&C Research Laboratories, Princeton, NJ
08540 USA. He is now with GTE Laboratories, Waltham, MA 02254 USA.
Publisher Item Identifier S 0733-8716(97)00056-5.
multimedia services. A number of R&D activities on wireless
asynchronous transfer mode (ATM) have been initiated over
the last few years, and early results on architecture and
proof-of-concept prototyping have been reported by several
organizations worldwide –. This paper describes an early
experimental wireless ATM network prototype (WATMnet)
which was developed at NEC USA’s C&C Research Labo-
ratories in Princeton, NJ, during the period 1994–1995.
Wireless ATM is intended to provide seamless support for
QoS-based multimedia networking applications running on
both fixed and portable user devices, as shown schematically in
Fig. 1. In this environment, a variety of multimedia computing
devices [ranging from fixed workstations to mobile PC’s or
personal digital assistant’s (PDA’s)] communicate with each
other and/or access information from remote media servers. In
such systems, network-based multimedia services are central
to the application, and should be equally accessible to both
static and mobile users. In order to maximize utility and
performance, it is preferable to have a seamless networking
and software architecture which incorporates both wired and
wireless portions of the system. A general solution should also
provide flexible multimedia integration with QoS control all
through the network protocol and computer software stacks.
ATM network technology provides a useful basis for such
a system since it was designed for service integration and
explicit quality-of-service support. Our conceptual view of
such a multimedia system architecture  based on ATM is
shown in Fig. 2.
Note that the system architecture is centered around a
high-speed network with unified wired
via mobility-enhanced (but otherwise standard) ATM network
and signaling/control layers. Special medium access, data
link, and network control requirements of the radio channel
are accommodated in new wireless-specific layers denoted
as WATM medium-access control (MAC), data link control
(DLC), and control, just above the wireless physical layer. The
figure also shows that a typical multimedia scenario will in-
volve three types of terminal/computer equipment connected to
the network: media server, (fixed) computer/workstation, and
(portable) personal terminal. A new software stack consisting
of a multimedia-capable OS, ATM application programming
interface (API) with QoS control, multimedia/mobile middle-
ware, and object-oriented script applications is envisioned for
the personal terminal.
0733–8716/97$10.00 1997 IEEE
84 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 1997
Fig. 1.Typical multimedia “C&C” scenario.
Fig. 2.Conceptual view of the proposed multimedia system architecture.
II. WATMNET PROTOTYPE ARCHITECTURE
The wireless ATM network under development is a major
component of NEC Princeton’s “Multimedia C&C Testbed”
shown in Fig. 3. This testbed is intended as a proof-of-
concept demonstration of the ATM-based multimedia net-
working architecture in Fig. 2, incorporating a variety of
new hardware and software concepts. The prototype system
started with a small (2.4 Gb/s, 16 port) ATM switch, a media
server, and conventional multimedia workstations, which have
been used to investigate software issues including media
transport, ATM API with QoS support, and distributed object
services for multimedia-on-demand . In the current phase
of work, the prototype is being augmented to incorporate
the wireless ATM network “WATMnet” (shown in the cen-
ter), as well as the network-centric multimedia computing
and communication platform (“MCCP”) shown on the lower
In addition to being a proof-of-concept demonstration, the
WATMnet prototype is intended to investigate and optimize
the design of major software and hardware components of
the system. In the first generation prototype, our focus is on
definition and validation of the key protocol and software
components of the system including radio channel MAC, DLC,
and wireless control; mobility extensions to ATM signaling;
RAYCHAUDHURI et al.: WATMnet: A PROTOTYPE WIRELESS ATM SYSTEM85
Fig. 3. NEC Princeton’s Multimedia C&C Testbed (under development).
base station software architecture; and personal multimedia
terminal software architecture. The important extrinsic fea-
tures/capabilities of the prototype system are summarized as
1) broadband wireless access for microcellular personal
multimedia services (
via high-speed, unlicensed band
2) integrated ATM (i.e., ABR/UBR/CBR/VBR) services
with QoS control (
via customized medium-access
3) high radio link reliability (
protocol layer designed for ATM services);
4) automatic handoff capabilities within ATM network
via mobility extensions to ATM
5) ATM UNI compatible API at portable terminal (
via integration of radio MAC/DLC/control with ATM
6) software framework for distributed multimedia applica-
via QoS API, object script software, etc.).
The hardware/software architecture of the WATMnet pro-
totype is shown at a more detailed level in Fig. 4. The
prototype includes an 8 Mb/s (2.4 GHz ISM band) radio, dy-
namic TDMA/TDD MAC, new DLC protocols, ATM network
mobility enhancements, and object script-based multimedia
applications (with real-time video). As shown in the fig-
ure, the same WATM network interface unit (NIU) with
the radio modem and some wireless protocol functions is
used at both the terminal and base station. At the personal
terminal (which is a commercially available NEC Versa-M
laptop PC), ATM SAR, AAL, signaling, etc., is performed
in software, while the base station contains multiple hard-
ware boards for MAC layer resource allocation, wireless
control, DLC, WATM
ATM cell conversion, mobile ATM
signaling and ATM interface functions. The base stations
are connected via fiber to an ATM switch with mobility-
via data link control
enhanced signaling software required to support functions such
III. PROTOTYPE SUBSYSTEMS
In this section, we describe the major hardware and software
components of the prototype wireless ATM system. More
specifics about the WATMnet protocols implemented will
be given in Section IV; the reader is referred to  and
– for further background on the system’s architecture
and protocol design. Note that the system being described is
a research prototype, so that gradual changes in the design or
implementation approach may be expected in the future.
A. WATMnet Network Interface Unit
The first-generation WATMnet NIU incorporates antenna,
radio/IF, modem, MAC/DLC acceleration, processor/memory,
and bus interface modules, as shown in Fig. 5. The first
generation prototype’s radio uses a low power/unlicensed 2.4
GHz ISM band currently used for wireless local-area networks
(LAN’s), with the recognition that new frequencies (possibly
in the 5 GHz or higher bands)1would likely be allocated
later if broadband wireless services prove to be feasible. Our
approach has been to start prototyping with a reasonably high
bit-rate ( 10 Mb/s) modem operating in currently available
wireless LAN frequency bands, and then focus research efforts
on the validation and optimization of the system’s protocol
architecture and software. The modem may be later replaced
with a higher-speed (e.g.,
25 Mb/s) modem operating in
a suitable broadband PCS or wireless LAN band that may
become available in the future.
The RF/modem subsystem incorporates switched reception
diversity between two polarized antennas. The burst modem
operates at 8 Mb/s using
QPSK modulation and decision
1The U.S. FCC has recently issued a Notice of Proposed Rule Making
(NPRM) document indicating their intent to allocate 350 MHz of 5 GHz
spectrum for unlicensed “NII/Supernet” broadband wireless devices.
86 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 1997
Fig. 4. Hardware/software architecture of the WATMnet prototype.
Fig. 5. WATMnet network interface unit (NIU).
feedback equalization. As currently implemented, the modem
has a sensitivity of
80 dBm, nominal bit-error rate (BER)
and delay spread range 30 ns rms. The burst acquisi-
tion preamble varies with diversity selection and equalization
training requirements, and is in the range of 18–27 bytes.
A (63, 51) BCH code is used for forward error correction
after demodulation. (Note: the early prototype uses a fixed
antenna without equalization, pending further investigation of
channel characteristics and adaptive algorithms for diversity
The MAC/DLC accelerator hardware is intended to sup-
port time-critical wireless protocol operations. In the present
WATMnet prototype, this module will accelerate functions
such as dynamic TDMA/TDD MAC framing, packet trans-
mit/receive functions, and the cyclic redundancy code (CRC).
Higher level MAC functions (such as contention access logic
slot mapping) and the DLC protocol will be per-
formed in the V.53 processor. In the early stages of the
implementation, the processor is used for packet I/O and
modem control functions, with most of the MAC logic and all
the DLC implemented in software running on the host device
to which the NIU card is attached (i.e., portable terminal or
base station). At present, the ATM SAR and AAL functions
are also performed in software on the portable host computer.
A gradual migration of the MAC, DLC, and ATM functions
into the NIU is planned as performance requirements and
hardware/software partitioning are better understood.
B. WATMnet Base Station
The prototype base station is responsible for bridging the
wireless ATM link with the fixed ATM network, in terms
of both data and control/signaling protocol stacks. The initial
demonstration system contains two identically configured base
stations supporting adjacent microcells. Each base is a VME
chassis containing an i960 processor card, an ATM interface
card, and a VME to PCMCIA adapter card, which connects
to the WATMnet interface card described in Section III-A.
The i960 processor runs a real-time embedded operating
system called pSOS, which also supports multiprocessing with
additional i960 cards where needed. An Ethernet port on the
i960 card is used only at boot time to download the base
RAYCHAUDHURI et al.: WATMnet: A PROTOTYPE WIRELESS ATM SYSTEM87
Fig. 6. Base station software architecture.
Two communication stacks are used in the base station
as shown in the software outline given in Fig. 6. One stack
supports the wired ATM network, while the other supports
the wireless link segment. The ATM stack consists of a
TAXI ATM interface, AAL3–5 convergence layer, and ATM
signaling (similar to Q.2931). The ATM signaling imple-
mentation matches that of the ATM switch to which the
base station is connected, incorporating microcell to microcell
hand-off capabilities required for mobility support. Note that
the ATM AAL function is required for signaling messages
originating at the base, while ATM cells to and from the
mobile terminal would normally be interfaced at the cell
level (in our initial implementation, data was interfaced at the
packet level above AAL due to constraints of the commercial
ATM interface card). The wireless protocol stack consists
of a high-speed radio interface, media access control, data
link control, AAL3–5, ATM signaling, and a wireless control
C. Personal Terminal
The mobile personal terminal is an NEC Versa-M/100 laptop
PC running the Linux Operating System, into which is plugged
the WATMnet network interface card (NIC) via the PCMCIA
interface. Architecturally, the WATMnet support software is
divided into two major components: a loadable device driver
which runs when the WATMnet card is inserted into the Versa
and user-level software which controls the initial configuration
and the intake for base station handoff, etc.
The device driver runs within the Linux kernel. It uses
the standard Linux Card-Services manager to allocate system
resources and to handle card insertion and removal events. The
driver also exports a network interface to the standard Linux
IP and TCP code. The interface is managed using the usual
ifconfig program to set addressing and routing parameters. It
thus appears to the system just like any other network interface.
Finally, the driver provides a character-mode interface which
is used to download the initial configuration files to the
WATMnet card. This same interface also provides the API
function (as an ioctl) to cause the mobile-initiated base station
handoff. The application-level (user-space) program uses the
character mode interface to program the Xilinx arrays and
download the NEC V.53 software running on the interface
card. The data for these programs are located in the Linux
Two modifications were made to the Linux kernel to support
the prototype WATMnet card. The system clock was changed
to generate an interrupt every ms (rather than every 10 ms)
and a kernel routine was added to specify an I/O polling
routine to be called on every timer interrupt. These changes
were made to the source of the Linux kernel. With the above
exceptions, the mobile runs a standard version of Linux. TCP,
IP, and Card Services were not changed at all to support
88 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 1997
Fig. 7.Personal terminal software outline.
The WATMnet device driver developed for this prototype
implements three vertical stacks between standard system
components and the network interface hardware. As illustrated
in Fig. 7, these are as follows.
a) The Card Manager Interface uses configuration data
programmed into the WATMnet board to identify the
card. Based on card identification, read when the card is
inserted into the PCMCIA slot, the Configuration System
initializes the device driver.
b) The open/close/read/write/ioctl operations used to pro-
gram the WATMnet board are supported by the character
interface. The various state transitions: reset, Xilinx-
boot, V.53 boot, relocate, MAC-download, and MAC-
start, are controlled through the ioctl interface from the
application. Bulk data transfer is done through write
c) The driver also exports a network interface. This pro-
vides the address of a routine within the driver to be
called by the IP code to transmit an IP packet. In the
reverse direction, the standard routine netif rx is called
by the driver.
During normal operation, only the network interface is used.
The character interface is used to initiate the handoff request.
When an IP packet is received for a new IP address, the
following steps are taken.
i) An ATM-ARP message is transmitted to resolve the
requested IP address.
ii)When the ARP reply is received, a signaling message
is sent to the resolved address to establish an ATM
Fig. 8. WATMnet protocol architecture.
iii) Once the connection is established, the saved IP pack-
ets are transmitted.
Subsequent IP packets are transmitted over the same con-
nection. The IP packets, ATM-ARP, and signaling messages
are transmitted over wireless VC’s and pass through modules
implementing the data link control (DLC) and medium-access
control (MAC) software.
D. ATM Switch
An off-the-shelf 2.4 Gb/s local-area ATM switch was used
in the WATMnet prototype. The signaling software on the
switch was modified to support basic mobility functions such
as handoff. In the present version, the modifications were made
to a custom Q.2931-type signaling implementation on the
Fore ASX-100 switch (with a UNIX/Sparc control processor).
Migration to standard Q.2931 (
mobility control) on an NEC
RAYCHAUDHURI et al.: WATMnet: A PROTOTYPE WIRELESS ATM SYSTEM89
Fig. 9. Data cell and ACK/control packet formats used in WATMnet.
Model 5 switch running a real-time OS is planned at the next
The current implementation extends the switch signaling
software by adding two new functions to the conventional
ATM signaling. The original software structures and codes
are not changed, making it backward compatible. The added
functions are written separately for easy maintenance. Since
during the handoff process, the ATM connections needs to
be modified rather than deleted, several new connection/route
manipulation routines are provided. However, whenever possi-
ble, the new functions attempt to reuse the lower-level function
calls provided by the original software.
IV. PROTOCOL IMPLEMENTATION
The WATMnet system uses an extended ATM protocol
architecture as shown in Fig. 8. It can be observed that the ap-
proach is to incorporate wireless channel specific MAC, DLC,
and wireless control layers into the ATM stack, while adding
mobility support functions to higher layer ATM protocols such
Data and wireless control cell formats corresponding to the
above protocol stack are shown in Fig. 9. Wireless link cells
consist of a 48 byte payload, 4-byte compressed ATM header,
2-byte wireless header, and 2-byte CRC (for a total of 56
bytes). The control packet consists of the 2-byte header, 4-
byte control payload, and the 2-byte CRC (total of eight bytes),
and are used for both data link ACK’s and wireless control
In the rest of this section, we describe the implementation of
MAC, DLC, wireless control, and signaling protocols running
on the terminal, base station, and switch platforms outlined
in Section III. For more details on protocol functionality, the
reader is referred to –.
A. Medium-Access Control
Fig. 10 shows the dynamic TDMA/TDD channel format
used to support the traffic classes outlined above. The down-
link transmission is a single burst consisting of modem pre-
amble, frame headers, control, and data. Uplink consists of
a contention access (slotted ALOHA) control subframe, fol-
lowed by allocated ABR/UBR, VBR, and CBR data slots.
Data slots are allocated by a supervisory MAC process run-
ning in software at the base station; a range of allocation
algorithms for ABR/UBR/VBR/CBR and QoS support are
supported by the system but have not been finalized at this
Fig. 11 shows the architecture of the wireless link protocols
in WATMnet. Observe that each service VC from the ATM
interface is associated with a dedicated DLC process, which
in turn communicates with the MAC layer for transmission
over the wireless link. The MAC functions can be decomposed
into a supervisory MAC for higher level functions such as slot
allocation and QoS control, along with a core MAC for high-
speed functions such as multiplexing and transmit/receive.
Supervisory MAC: The S-MAC processes control infor-
mation and builds a “schedule table” for the C-MAC in
each frame. At the base station, the S-MAC is responsible
for slot scheduling, both uplink and downlink, for all the
VC’s (ABR/UBR/VBR/CBR) in the system based on requests
received via control meta-signaling. Subject to the fulfillment
of QoS requirements of each VC, the S-MAC arrives at a
policy of scheduling data transmissions and acknowledgments
(to enable error recovery by the DLC). Call admission control
(CAC) functions for the wireless link is also part of the base
station S-MAC. At the remote station, the S-MAC processes
the control information received at the start of each frame and
builds a schedule table for the C-MAC. It is also responsible
for scheduling the transmission of the uplink control messages
in the slotted ALOHA mode.
Core MAC: The C-MAC is the interface between the DLC
and the wireless physical layer. Based on the schedule table
supplied by the S-MAC, it multiplexes/demultiplexes trans-
mission and reception for the corresponding VC’s. A schedule
table entry, contains service type (transmission/reception),
message type (data/control message), the VCI, the position,
and the duration of the service. The functions of the C-MAC
at both the base station and the remote station are more or
less identical. It is noted that the C-MAC is a candidate for
high-speed implementation in hardware.
CBR VC’s are assigned slots periodically according to their
bit-rate. A group of K frames (e.g.,
a MAC superframe, which is used to obtain a reasonable
granularity for bit-rate assignments (e.g.,
90 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 1997
Fig. 10.Dynamic TDMA/TDD medium-access control frame format.
Fig. 11. Architecture of wireless link protocols and interface to ATM.
positions of the assigned slots within a MAC superframe are
maintained relatively static in order to facilitate operation of
low complexity telephone terminals and also to reduce the
control signaling load on the wireless link. ABR/UBR virtual
circuits (VC’s), are handled on a burst-by-burst basis with
dynamic reservation of ABR/UBR slots and unused CBR/VBR
slots in each frame. Contiguous allocation of ABR/UBR slots
is preferable in view of the lower overhead. A round-robin
slot allocation policy may be used to prevent long bursts
from overloading the channel and increasing the delays for
the other VC’s. VBR VC’s are assigned slots with the aid of a
usage parameter control (UPC) based statistical multiplexing
algorithm. In order to maintain high channel utilization, this
algorithm is augmented by an “instantaneous” bit-rate esti-
mation at the base station over an estimation interval whose
size is of the order of a MAC superframe discussed above.
A detailed discussion of the VBR slot allocation scheme is
beyond the scope of this paper. Note that CBR and VBR calls
may be blocked, while ABR/UBR virtual circuits are always
accepted, subject to applicable source rate flow controls.
The initial version of the WATMnet prototype implements a
simplified MAC layer supporting ABR VC’s. This MAC layer
has first been verified in the point-to-point TDD mux/demux
mode, with work on the full TDMA/TDD mode currently in
progress. CBR and VBR capabilities will be added later after
basic operation of the ABR system has been validated (note
that this is appropriate because current ATM local area network
are still primarily oriented toward ABR traffic). At the base
station, the MAC layer is implemented as an independent task
in pSOS. It communicates with the DLC tasks using message
queues. A Tx/Rx cycle begins with the C-MAC transmitting
the data present in its transmission buffer. It then wakes up the
S-MAC to begin the allocation for the next downlink frame.
The data/ACK sent by the DLC tasks are entered into the
transmission buffer. At the end of the uplink frame, the C-
MAC de-multiplexes the received data and notifies the DLC
tasks of any received data/ACK. At the remote station, the
MAC is a part of the loadable device driver for the WATMnet
card, which runs within the Linux kernel. The communication
between the MAC and the DLC is through function calls. Apart
RAYCHAUDHURI et al.: WATMnet: A PROTOTYPE WIRELESS ATM SYSTEM91
Fig. 12. Multithreaded DLC architecture at base station.
from these differences, the implementation follows the same
lines as at the base station.
B. Data Link Control
The data-link layer protocol is critical to WATMnet opera-
tion since it limits the propagation of radio channel errors into
the ATM network (which is not designed to support high cell
loss rates). The DLC implemented in our prototype serves the
1) It provides error free (or reduced error) services to the
higher layer protocols. It also ensures sequential cell
delivery to the ATM/AAL layer after recovering from
physical layer transmission errors.
2) It implements the interface between MAC and higher
layer protocols needed to support a demand driven
medium-access strategy for multiservices ATM traffic.
As for the MAC layer, the first DLC implementation sup-
ports ABR service only, and will be extended to include CBR
and VBR traffic types in the next phase of work. The DLC
uses the 2-byte wireless header shown in Fig. 9 for sequence
numbering of WATM cells and the ACK packet (also shown
in Fig. 9) for group acknowledgments of up to 20 cells in
the current implementation. Since a zero cell loss data-link is
implemented for ABR traffic, no specific time limit for error
recovery is imposed. At the transmission side, when a burst
of ATM cells arrives at DLC, an access allocation request is
sent to the MAC and upon reception of a reply, the receiver
DLC sends a group acknowledgment back after receiving the
whole burst and the acknowledged information is used at the
transmitter to retransmit the erroneously transmitted cells. This
requires the transmitter DLC to buffer cells until a group
acknowledgment indicating proper reception at the other end is
received. Also note that the receiver needs to buffer the WATM
cells in order to ensure sequential delivery to the ATM layer.
Two different implementations of the data-link protocol
were necessary for the present system. At the base station
(which runs a real-time OS, known as pSOS, on an i960
processor) a separate pair of transmission and reception tasks
are used for the DLC processing of each virtual circuit
(see Fig. 12). All the data-link tasks communicate with a
fixed ATM cell relay task and a MAC task using a fairly
straightforward set of inter-task communication primitives
(messages queue and shared memory). Both the transmission
and reception DLC tasks for a VC are created during its setup
time within the base station. This per VC DLC task allocation
scheme aids the implementation of quality-of-service (QoS) by
altering the allocated buffer space and setting the priority of a
DLC task based on the QoS requirement of the corresponding
VC. Since all these tasks run in a single address space in pSOS,
the expense of frequent context switching is small enough to
be outweighed by the advantages of QoS-based scheduling.
Linux, a multitasking operating system, is used at the
personal terminal side where the data-link and MAC software
are integrated within the kernel as a dynamically reloadable
device driver. Since Linux kernel does not support multiple
tasks within the kernel, a single threaded version of the
same DLC protocol is implemented where the communication
between MAC and DLC is realized as a function call interface.
C. Wireless Control
In general, the wireless control layer is responsible for
management of all radio related network operations. These
functions include mobile-ID assignment at startup, MAC layer
controls, microcell handoff, frequency and power manage-
The wireless control protocol implemented in the first
prototype system has two distinct functions. The first is the
initial meta-signaling interaction with the base station to obtain
92 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 1997
Fig. 13.Signaling sequence for basic handoff control in WATMnet.
a mobile identifier. The second is to initiate handoff to the base
station serving the applicable physically adjacent microcell.
Wireless control messages are transmitted in-band within
data-link frames, via meta-signaling with formats as shown in
Fig. 9. They are distinguished from regular traffic by the type
field in the upper 4-b of the first octet of the 8-byte sequence.
Control messages are scheduled by the data-link process but
do not pass through the AAL and SAR processing portions of
the DLC/ATM protocol stack.
“WHO AM I”
“YOU ARE” message from the base station, giving a mobile-
ID number to be used in subsequent connection-oriented
messages. The mobile responds with an “ID CONFIRM”
message.Boththe “WHO AM I”
until the three-way handshake is complete. The data in a
“WHO AM I” message is a random number generated by the
mobile, and is used to match the corresponding “YOU ARE”
response which is also sent in broadcast mode.
The transfer to another base station is initiated by the
mobile transmitting “HO REQUEST” message on the wireless
control channel. This causes the current base station to transfer
control to the new base station (may be determined via mutual
negotiation of radio parameters, MAC layer status, etc.). The
response is a “YOU ARE” message from the new base station,
giving the mobile-ID number to be used for the new link. The
mobile responds with an “ID CONFIRM” acknowledgment.
The old base station passes the random number associated with
the original “WHO AM I” message to the new base station
so that the mobile can correctly receive the “YOU ARE”
message. If the mobile does not receive a “YOU ARE”
message within the timeout period, it restarts by transmitting
a “WHO AM I” message to locate an active base station.
D. Mobile ATM Signaling
Mobility support in ATM – requires extensions
to control plane protocols for support of terminal mobility
(i.e., handoff, authentication, location management, etc.) The
signaling functions in the prototype system consist of three
parts. First, a limited set of regular ATM signaling messages
is used for normal connection management between the mobile
terminal and other ATM workstations. Second, a wireless
meta-signaling protocol is used to establish a control connec-
tion between the mobile terminal and the base station. Third,
a handoff control protocol between the base station and ATM
switch is used to support the mobile terminal during handoff
between base stations. In the first stage of the prototype, a
relatively simple set of these protocols has been implemented
to support handoff between base stations on a single switch.
A more complete and scalable implementation of signaling
and location management as described in  is currently in
The current mobile ATM signaling operation for hand-
off (as shown in Fig. 13) may be summarized as follows.
During signaling sequences, the base station intercepts all
the signaling messages between the mobile terminal and the
switch. It fills in the appropriate ATM address in the signaling
message and maintains a connection table that includes all
the active connection associated with the mobile terminal.
When a “HO REQUEST” message is received through the
meta-signaling channel, the current active base station sends
a wireless control message to the new base station, upon
receiving the acknowledgment from the new base station, the
RAYCHAUDHURI et al.: WATMnet: A PROTOTYPE WIRELESS ATM SYSTEM93
Fig. 14. Photograph of the WATMnet personal terminal.
current base station sends an ATM “HO REQUEST” message
to the switch for each active connection associated with the
mobile terminal. This “HO REQUEST” message informs the
switch at the new base station address, connection identifier,
as well as the desired VPI/VCI for the new connection.
Upon receiving the “HO REQUEST” message from the
base station, the switch changes the information elements
associated with the specific connection. In particular, it updates
the routing table so that the cells can be routed to/from the new
base station. Afterwards, the switch sends a “HO RESPONSE”
and signal to the new base station, instructs it to install
an active connection with the specified connection identifier,
VPI/VCI, and AAL type.
V. EXPERIMENTAL RESULTS
In this section, we briefly report on preliminary experimental
results obtained with the prototype WATMnet system during
the first several weeks of use after it became operational
in July 1995. Photographs of the experimental WATMnet
personal terminal and base station are given in Figs. 14 and
15, respectively. As indicated earlier, the mobile terminal is
an off-the-shelf NEC Versa PC together with the WATMnet
NIU and associated software. The base station consists of i960
processor card, ATM interface, and PCMCIA adapter card on a
VME chassis. Observe that the same WATMnet NIU module is
used at the base station, interfacing with the VME equipment
via the PCMCIA adapter. The output of the ATM interface at
the base station is a TAXI fiber which connects to a port on
the ATM switch (not shown in the photograph).
Experiments were conducted to verify basic operation of
all major subsystems including personal terminal software
and wireless interface card, base station software, and ATM
switch software. Our early testing confirms correct operation
of the MAC (simplified version), DLC, wireless control (basic
version), and mobile ATM signaling protocols described in
this paper. During normal operation with “PING” or FTP
applications running on the Versa laptop, we were able to
Fig. 15. Photograph of the WATMnet base station.
confirm the utility of each of these new protocols for the
wireless environment. In particular, we observed that the ABR
DLC protocol was essential for smooth operation of the FTP
application in view of occasionally high cell error rates on
the radio channel (typically
ized modems, depending on antenna positioning, distance,
multipath conditions, etc.). At the same time, basic handoff
functionality using a combination of wireless control meta-
signaling and mobile ATM signaling was verified over the
radio channel by successfully migrating the laptop from one
base station to another without interrupting the application
(such as TCP).
Extensive traffic monitoring tools were implemented on the
base station i960 processors, and several network monitoring
screens are available for observing various throughput, delay
and wireless link status indicators. Initial nonoptimized end-
to-end throughputs (i.e., delivered to an application on the
Versa) in the range of 0.5–1.0 Mb/s were obtained during
early operation, with corresponding ABR delays in the range
of tens of microseconds. The current throughput constraints
may be attributed to several factors including modem hardware
I/O limitations, MAC layer overheads, radio channel errors
causing DLC retransmissions, DLC and MAC layer software
processing times, etc. Further work on hardware improvements
and software optimization is currently under way with the
objective of reaching our design goal of
throughput. Observed handoff latencies (i.e., delay between
initiation of handoff by portable to resumption of ATM stream
on new base station) were typically under 50 ms. Handoff
related cell loss events were observed occasionally, but the
effect on the present local network TCP-based application was
minimal. Further details on handoff performance including
cell loss and latency measurements will be reported in a
forthcoming paper .
In terms of applications, in addition to the testing with PING
and FTP reported above, we have successfully ported some ex-
ample ATM/TCP/AAL5-based video/multimedia-on-demand
applications  to the Versa. As indicated in Section II,
our software approach uses a script-based distributed ob-
ject technology called Jodler  for flexible and modular
1–5% with the first unequal-
2–4 Mb/s terminal
94 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 1, JANUARY 1997
implementation of network-based multimedia applications. Al-
though video frame size/rate are currently limited by network
throughput and Versa software decoding speed capabilities
, the system can support QSIF software decoded MPEG
video at about 10 frames/s or SIF Cell-B video (which has a
significantly lower decoding complexity) at 20–30 frames/s. It
is observed that during the first phase of work, our emphasis
has been on validating software compatibility of ATM appli-
cations through a WATMnet API, rather than on quantitative
performance which will be addressed in the next phase of
work. A more detailed report on experimental measurements
and application performance will be given in a future paper.
VI. CONCLUDING REMARKS
The architecture and implementation of the WATMnet pro-
totype developed at NEC USA’s C&C Research Laboratories
in Princeton, NJ, has been described in this paper. This
prototype first became operational in July 1995, with ba-
sic implementations of all the new wireless ATM protocols
described here including MAC, DLC, wireless control, and
mobile ATM signaling. The prototype system initially supports
a range of high-speed data and multimedia applications to a
standard laptop PC (NEC Versa) running a Linux operating
system with WATM driver software in ABR mode at service
bit-rates in the region of 0.5–1 Mb/s. During the next phase of
work, we plan to work on speeding up the protocol software
and network interface card hardware, with the objective of
achieving terminal throughput in the range of
After key performance issues have been addressed, more
complete protocol implementations supporting a full range of
ATM services with QoS will be implemented. We are also
pursuing parallel efforts on personal new multimedia terminal
architectures and distributed multimedia software based on
WATMnet communications capabilities. Overall, we believe
that wireless ATM networking is viable, and will serve as a
key enabling technology for ubiquitous personal multimedia
communication and computing applications.
The authors wish to acknowledge the assistance and
guidance of their colleagues, including J. Li, V. Bansal, D.
Reininger, and M. Ott of NEC USA.
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networks: System design issues,” in Proc. 3rd Workshop Third Gener-
ation Wireless Inform. Networks, Rutgers University, New Brunswick,
NJ, Apr. 1992, pp. 259–288.
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in Proc. SPIE Conf. Visual Commun. & Inform. Proc. (VCIP’95), May
F’95) received the B.Tech. degree (with Honors)
from the Indian Institute of Technology, Kharagpur,
India, in 1976, and the M.S. and Ph.D. degrees
from the State University of New York, Stony
Brook, both in electrical engineering, in 1978 and
He was with the David Sarnoff Research Center
(formerly RCA Laboratories), Princeton, NJ, as a
Member of the Technical Staff from 1979 to 1987,
a Senior Member of the Technical Staff from 1988
to 1989, and Head of Broadband Communications Research from 1990 to
1992. At the David Sarnoff Research Center, he worked on a range of R&D
topics including: very small aperture terminal (VSAT) satellite networks,
direct broadcast satellite (DBS), packet video, digital HDTV, multimedia
communication, and wireless data networks. From 1990 to 1992, he led
the project team responsible for system design and specification of the
“Advanced Digital HDTV” prototype tested by the U.S. FCC in 1992. Since
January 1993, he has been with the NEC USA C&C Research Laboratory,
Princeton, NJ, where he is currently Department Head, Systems Architecture,
with research focus on multimedia networking technologies including ATM
interface hardware/software, wireless ATM, and distributed multimedia
software. He is a Technical Editor for the IEEE/ACM TRANSACTIONS ON
NETWORKING, and the IEEE MULTIMEDIA MAGAZINE and is currently serving
as a Distinguished Lecturer for the IEEE Communications Society. He has
authored approximately 75 technical papers and holds eight U.S. patents.
Leslie J. French received the B.A. degree in natural sciences, the M.A.
degree, and the Ph.D. degree in theoretical chemistry, all from Pembroke
College, Cambridge University, Cambridge, U.K., in 1979, 1982, and 1983,
He has worked in the computer industry since 1982. For the past two years,
he has been working at the NEC USA C&C Research Laboratory, Princeton,
NJ, where he is currently a Senior Research Staff Member, researching
high-speed ATM networks, wireless ATM, and multimedia operating systems.
RAYCHAUDHURI et al.: WATMnet: A PROTOTYPE WIRELESS ATM SYSTEM95
Robert J. Siracusa received the M.S. degree in
computer science from Johns Hopkins University,
Baltimore, MD, in 1981.
From 1972 to 1981, he worked as a Spacecraft
Evaluation Engineer for RCA. In 1981, he became
a Member of the Technical Staff at the David
Sarnoff Research Center, Princeton, NJ. At the
David Sarnoff Research Center, he made signifi-
cant contributions in the following areas: teletext
protocols and prototypes, a satellite multi-access
protocol experimental testbed, a test apparatus for
a teleconferencing video CODEC, and the transport protocol used today
with the RCA/Thompson DSS MPEG-based television system. In 1993, he
joined the Systems Architecture Department of the NEC USA C&C Research
Laboratory, Princeton, NJ, as a Senior Research Engineer, where he has been
engaged in research in multimedia transport protocols and system software
prototyping for both wired and wireless ATM networks. He has five U.S.
Subir K. Biswas (S’92–M’94) received the B.E.
and M.E. degrees from Jadavpur University, Cal-
cutta, India, both in electronics and telecommuni-
cations, in 1987 and 1989, respectively, and the
Ph.D. degree in computer Science from Cambridge
University, Cambridge, U.K.
From 1989 to 1990, he was with the Design Cen-
ter for Semi-Custom IC’s in the Jadavpur University
Electronics Department. In 1990, he joined the
Cambridge University Computer Laboratory. Since
1994, he has been a Research Staff Member in the
System Architecture Department of the NEC USA C&C Research Laboratory,
Princeton, NJ. During the academic year of 1992/1993, he worked as a
Network Software Engineer with Olivetti Research, Ltd., Cambridge, U.K. His
current research interests cover various aspects of wireless networks, including
multi-access protocols, handoff, location management and multimedia traffic
support over wireless ATM networks.
Ruixi Yuan (S’89–M’90) was born in Jiangxi
Province, China, in 1965. He received the B.S.
degree in physics from the University of Science
and Technology of China in 1985, the M.S. degree
in physics and the Ph.D. degree in electrical
engineering from Texas A&M University, College
Station, in 1986 and 1991, respectively.
He is now with GTE Laboratories, Waltham,
MA, where he conducts research on wireless
communication networks, mobility management,
signaling protocols, and the Internet.
Parthasarathy Narasimhan received the B.Tech.
degree in electrical engineering from the Indian
Institute of Technology, Madras, India, in 1987, the
M.S. degree in computer science from the New
Jersey Institute of Technology, Newark, in 1990,
and the Ph.D. degree in operations research from
Rutgers University, New Brunswick, NJ, in 1996.
From 1992 to 1995, he was associated with
WINLAB, Rutgers University, New Brunswick, NJ,
working on performance analysis of multi-access
protocols for wireless networks. He is with the
NEC USA C&C Research Laboratory, Princeton, NJ. His interests include
modeling and performance analysis of communication networks, wireless
ATM, multiaccess protocols, and queueing theory.
Cesar A. Johnston (M’96) received the B.S. and
M.S. degrees from Polytechnic University, Brook-
lyn, NY, both in electrical engineering, in 1986 and
In 1987, he became a Member of the Technical
Staff of the Applied Research Division of Bellcore,
Morristown, NJ. He was involved in pioneering
ATM/SONET experiments in the area of packet
video and broadband customer premises networks.
He was also the main designer of the first-known
ATM integrated circuit, which proved the feasibility
of ATM in 1989. Before leaving Bellcore, he led the architectural design
and system integration of a 2.5 Gb/s HIPPI-ATM-SONET (HAS) terminal
interface for the Nectar Gigabit Testbed and was the system architect of HAS
prototypes at STS-12c and STS-48c data rates for the National Information
Infrastructure (NII). In February 1995, he joined the NEC USA C&C Research
Laboratory, Princeton, NJ, as a Research Staff Member. He is involved in
the development of wireless ATM networks and is currently responsible for
prototyping efforts of the high-speed network interface for WATnet. He has
over 20 published technical papers and holds three U.S. patents.
Dr. Johnston is a member of Eta Kappa Nu and Tau Beta Pi.