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Channel Access Protocol for Multi-Wavelength Fiber-Optic Network

IP.com Disclosure Number: IPCOM000100286D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 164K

Publishing Venue

IBM

Related People

Chen, MS: AUTHOR [+3]

Abstract

The network under consideration is a multi-channel fiber-optic metropolitan area network (MAN). The channels are realized by using wavelength division multi-access (WDMA), as shown in Fig. 1. Each transmitter is provided with two lasers, one of them at a wavelength unique to that transmitter, and the other at a common wavelength shared by all the stations. The output of the two lasers is coupled into a 2x1 combiner, whose output is connected to one of the inputs of the NxN star coupler by means of an optical fiber. Thus, at each output port of the coupler, all the N wavelengths, as well as the common control wavelength, are available. A fiber runs from each output port to the corresponding receiver. At the receiver, the input optical signal is split into two parts by means of a 1x2 splitter.

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Channel Access Protocol for Multi-Wavelength Fiber-Optic Network

       The network under consideration is a multi-channel
fiber-optic metropolitan area network (MAN).  The channels are
realized by using wavelength division multi-access (WDMA), as shown
in Fig. 1.  Each transmitter is provided with two lasers, one of them
at a wavelength unique to that transmitter, and the other at a common
wavelength shared by all the stations.  The output of the two lasers
is coupled into a 2x1 combiner, whose output is connected to one of
the inputs of the NxN star coupler by means of an optical fiber.
Thus, at each output port of the coupler, all the N wavelengths, as
well as the common control wavelength, are available.  A fiber runs
from each output port to the corresponding receiver.  At the
receiver, the input optical signal is split into two parts by means
of a 1x2 splitter. One part goes to a fixed optical filter which
passes only the control wavelength, and the other output goes to a
tunable optical filter,  which may be tuned to pass any one of the
data wavelengths.  The star configuration is chosen because it
supports more stations than a bus or ring under the same power budget
considerations.

      Problem and Solution Approach: At high data rates (1
Gb/s/channel), and MAN distances of 50 km, with typical packet
lengths of 256 bytes, the maximum end-to-end propagation delay is
about 250 microseconds, while the duration of a packet is about 2
microseconds, two orders of magnitude smaller.  The performance of
typical random-access schemes, such as CSMA/CD (carrier sense
multiple access/collision detection) and BTMA (busy tone multiple
access), is close to ALOHA and is not acceptable.  Similarly, because
of the star configuration, token passing and buffer insertion schemes
are not adequate because the distance between any two stations can be
as large as the network diameter.  On the other end of the spectrum,
static bandwidth allocation using time division multi-access (TDMA)
provides a guaranteed bandwidth and delay, but is not suitable for
bursty traffic.

      We assume the existence of a common clock, obtained either by
distributing a clock to all the stations, or by means of some self-
clocking mechanism inherent in the data. When a station joins the
network, it uses its unique wavelength to measure its offset to the
hub, i.e., the one-way propagation time between the station and the
hub. Time is measured relative to the hub, and the end-stations have
to adjust their clocks appropriately.  Time is split into slots.
Slots on the data channels are called data slots and contain the
actual data packets.  Slots on the common control channel are called
status slots because they carry only the status information about the
packet and the transmitter.  A status slot is divided into N
mini-slots, one mini-slot assigned to each transmitter. There are
three fields in a mini-slot (as shown in Fig. 2(a)).
1.   Address (A)
2  ...