Mount Pleasant, Harare. Canada V6T 1W5. Zimbabwe. ABSTRACT. This paper presents the design of a fibre optic packet multiple access network which ...
FDM PACKET MULTIPLE ACCESS NETWORK USING COHERENT FIBRE OPTIC COMMUNICATION Victor C.M, Leung
Fambirai Takawira
Dept. of Electrical Engineering The University of British Columbia Vancouver, B.C. Canada V6T 1W5
Dept. of Electrical Engineering University of Zimbabwe Mount Pleasant, Harare Zimbabwe
ABSTRACT This paper presents the design of a fibre optic packet multiple access network which employs coherent detection over FDM optical channels. The network consists of a single mode optical fibre arranged in the D configuration, and clusters of stations attached via passive taps to the transmit and receive arms. Each station cluster is assigned one dedicated FDM channel for packet reception, and each station is equipped with a frequency agile transmitter to enable communications with any station in any cluster. Slotted access with collision detection (SCD) is employed for nearly 100% channel utilization. However, SCD favours upstream stations over downstream stations. A window flow control and acknowledgement scheme is incorporated to overcome this fairness problem.
1.
To communicate with another station, the transmitting station tunes to the FDM channel dedicated to the receiving station. The monitoring capability at the transmit fap facilitates carrier sensing and collision detection (CSMA-CD) to enhance the efficiency of the access protocol.
2.
INTRODUCTION
Rapid development of LANs and MANS has been promoted by increased demands for integration of such devices as computers, data terminals, telephones and facsimile. Fibre optics is extremely attractive in LANs because of its low loss, wide bandwidth, better electromagnetic immunity and light weight. Either direct detection or coherent detection techniques can be used. Coherent communication has the advantage of allowing FDM of a large number of optical channels with very narrow frequency separation which permits efficient utilization of the very large bandwidth (30 THz) in single mode systems. It also has the added advantage of greater receiver sensitivity than direct detection communication
[ll.
In this paper we consider an optical fibre network which topology is that of a D network (Figure 1) and uses burst coherent transmission. Stations grouped in Q clusters are connected to the fibre via passive taps. Each cluster has N stations. A l l stations in a cluster receive on a dedicated FDM channel via passive taps on the returning arm of the D network. Each station is equipped with a tunable optical transmitter with monitoring capability, passively coupled to the forward arm of the D network. Currently available lasers have a tuning range of only a few nanometers. Hence to cover the whole passband (about 40nm in the l500nm fibre window) we envisage the use of a few lasers, say 10, within each transmitter.
NETWORK ACCESS PROTOCOL
For bursty traffic, random access techniques generally offer shorter delay than fixed assignment or reservation multiple access techniques. It has been shown [21 that slotted access with collision detection (SCD), which makes use of local information at the transmitter for collision detection, achieves a maximum throughput of close to 100% in a single channel D network. Although the access technique was called slotted CSMA-CD [23, as pointed out in [21, there is really not sufficient time to perform carrier detection at the beginning of each slot before a packet is transmitted. In this case, contention due to slotted access is resolved by collision detection, which favours the upstream transmission. To maximize network throughput, we
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shall apply this access strategy to the proposed multichannel FDM network. Unlike the D network considered in [23 which is a multiple access/broadcast network, each of the different FDM channels in the proposed network represents a logically distinct multiple access network with limited broadcast capability to a small subset of all receivers. In particular, the transmitting station is not able to receive its own transmission on the return arm of the D network unless it is transmitting to another station in its cluster, This does not represent a handicap for the SCD protocol since contention is resolved at each transmitter using local information only. That is, once a transmitter has captured a slot, it is guaranteed that no upstream station is transmitting in that slot, and all downstream stations will refrain from transmitting in that slot, Therefore, acknowledgments, as a means of determining the success of transmissions subject to contentions, are not really necessary. However, our access protocol incorporates an explicit acknowledgment scheme to facilitate recovery of transmission errors (not caused by contention) and to enable the use of windowing flow control to prevent upstream stations from hogging the channels. 2.1
Channel Slotting Scheme
Figure 2 shows the slotting scheme in the FDM channels. There are two types of time slots: information slots and acknowledgment slots. Each information slot accomodates one information packet as well as a short preamble used for collision detection. The acknowledgment slots are used for sending acknowledgment packets only and are therefore much shorter than the information slots. The length of an information slot is equal to M times that of an acknowledgment slot. The slots are grouped into frames each consisting of M acknowledgment slots, numbered 0 to M-1, and M information slots, also numbered 0 to M-1. The frames are further grouped into masterframes, each consisting of K frames numbered 0 to K-1, such that the length of a masterframe (in bits) is greater than B+Rr, where 1 is the bit-length of the optical fibre cable, R is the data rate, and T is the processing time for the receiving station to verify the absence of transmission errors and generate an acknowledgment. All FDM channels in the network are synchronized to this slotting scheme with respect to time slot, frame, and superframe boundaries. 2.2
Information Tranemieeion Procedure
Suppose station A wishes to send an information packet to station B. We assume that the transmitter of station A has already established network synchronization. The transmitter tunes to the FDM channel assigned to station B, and starts transmitting the preamble in the first information slot that passes by. If the
slot is already occupied by a transmission from an upstream station, station A senses, via the monitoring port, the collision during transmission of the preamble and aborts the transmission, so that the information packet in the upstream transmission is left undisturbed. Station A defers transmission to the next information slot, where the above process is repeated. This access scheme favors the upstream stations, but it allows the channel to achieve a maximum normalized throughput close to the theoretical limit of 100%. The fairness issue will be addressed below in the discussion of the acknowledgment scheme. The use of the preamble enables collision to be detected without damaging the information packet already occupying a slot.
2.3 Acknowledgment Procedure When station B receives an information packet from station A in information slot i of frame j, it processes the packet to determine if there are any transmission errors. If no errors are detected, station E tunes its transmitter to the FDM channel allocated to station A and transmits an acknowledgment packet to A in the i-th acknowledgment slot of the j-th frame in the masterframe that follows the one containing the received information packet. This procedure assures that there are minimal contentions for acknowledgment slots. Collision of acknowledgment packets will only occur when two stations in the same cluster transmit to different receivers in different clusters at exactly the same time (identical slot and frame). Since the sending station knows exactly where to look for the acknowledgment for each information packet it has transmitted, the packet can be retransmitted inunediately after the station discovers that the acknowledgment is not there. A sliding window flow control procedure is employed to limit the number of information packets stations can send to each other without being acknowledged by the receivers, so that downstream stations (located near the bend of the D network) are given a fair chance to access the network. In this scheme, each transmitter can send at most one window of information packets to each receiver without acknowledgment. When an acknowledgment is received for the oldest packet in the window, the window slides forward to enable a new packet to be sent. The size of each sending window determines the share of the channel capacity allocated to the station to communicate with a specific cluster, and could be negotiated between the sender and receiver depending on the traffic requirements. To ensure that downstream stations will be able to access the channel, the total allocated capacity should not exceed the total available capacity. Under heavy traffic the proposed access scheme with flow control is equivalent to a round robin scheme where each station is allowed to send one window of packets in turn.
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3.
SOME PHYSICAL SYSTEM CONSIDERATIONS
The value of A is fixed by laser tunability and drift, whilst B is a non-linear function of laser linewidth (v), and thermal noise, and is given by the equation [5]
9
3.1 Synchronization A synchronization scheme similar to the one adopted in Fastnet [3] is used. A frame alignment word (FAW) in the form of a short burst is transmitted by the station farthest upstream, alternating periodically in each of the FDM channels. The FAW is received at each station on the returning arm of the D network (i.e. at the receiver end), allowing the receiver to identify the position of each slot with respect to frame and masterframe boundaries. Knowing the distance of the transmitter end from the receiver end, each station can calculate the position of the slots at its transmitter end for synchronization. 3.2
Collision Detection
The collision detection is done on the basis of power sensing, hence avoiding the need for a coherent demodulator at the transmitter end. At a given frequency, the power splitting produced by a passive tap is as shown in Fig. 3. In order to insert a fraction of signal (1-p)~~' onto a fibre, the same fraction (1-p)~~-of the signal on the fibre must be removed. &en a spurce, i, is transmitting, a signal upstream can be detected by subtracting the locally generated signal Byi from the reflected composite signal zi. To ensure that collision detection occurs at the required frequency, a tunable optical filter is coupled to the passive tap as shown in Fig. 3. The filter however has the disadvantage of restricting the selectivity of the system and hence the number of FDM channels possible. 3.3
Carrier Recovery
The signal arriving at a receiver, at a particular frequency, consists of modulated packet bursts from transmitting stations, one burst per slot. The transmitter optical carriers are not phase locked., The receiver phase locked loop (PLL) has to acquire and track the phase and frequency of each transmitted burst. For high capacity utilization, the time required for carrier recovery must be as small as possible. This time depends on the initial frequency offset between the receiver oscillator and transmitted burst, and the phase locked loop transient response, which in turn is related to the damping factor ( E ) and the PLL noise bandwidth (Bn).
(3)
where a2 is the variance of PLL phase and P the normalized signal power, i.e. number of signal electrons released from the photodetector. For typical values of laser linewidth (300KHz < v < 10MHz) and frequency offset (A < 1 GHz), the carrier recovery time can be significant, unless Bnis judiciously chosen. 4. PROTOCOL PERFORMANCE
The maximum normalized throughput for each FDM channel achievable with the proposed access protocol is
'max
provided
(2)
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(4)
With the sliding window flow control, consider station i with sending window wi on a given FDM channel. It takes about two masterframes to receive an acknowledgment, during which time w packets could be sent. Therefore, the maximum thoughput of station i in the given channel is approximately
It can be shown r41 that for a second order E = l/iT the irequency acquisition (Tf) and p h a s e a c q u i s i t i o n ( T p ) times are approximately given by
1.5 Bn
M ( 1-a) M+I
where a is the ratio between the slot overhead time and the total slot time. A typical slot is given in Fig. 4 showing the overheads. The total overhead time consists of the guard period, carrier recovery and preamble. The guard period is necessary due to imperfect frame synchronization. Close to 100% throughput may be obtained for large M and small a.
loop with
Tp =
=-
where stations are numbered such that i station i is upstream of station j.
< j if
fair capacity allocation strategy is to divide the maximum throughput, Smm,. of each FDM channel equally among the QN stations in the network. This could be accomplished by choosing the same sending windown w for all stations, i.e. A
'KM w i = w = - QN
, i = 1 , ,..., ~ QN
5.
CONSLUSION
In this paper we described the SCD protocol for a multichannel FDM fibre optic LAN which uses coherent detection. We have shown that the throughput of the protocol can be close to 100% if tight carrier and frame synchronization is achieved. We have considered alternatives in addressing the fairness issue that is inherent in the SCD protocol.
(6)
The maximum throughput of each station in each FDM channel is then References
T. Kimura, "Coherent Fiber Transmission," IEEE J. Lightwave Technology, Vol. LT-5, No. 4, April 1987, pp. 414-428. N.F. Maxemchuk, "Twelve random access strategies for fibre optic networks." IEEE Trans. Comm., Vol. 36, Aug. 1988, pp. 942-950.
This capacity allocation strategy, although fair to all stations, may result in inefficient utilization of the overall channel capacity, since many of the stations may have no use for the capacity allocated to them in certain FDM channels when they are actively communicating over other channels.
J.O. Limb and C. Flores, "Description of Fastnet - A unidirectional local area network," BSTJ, Vol. 61, No. 7 , Sept, 1982, pp. 1413-1440.
Alternatively, a capacity management approach could be employed. A specific station in each cluster is designated the channel manager which keeps track of capacity allocation in the FDM channel it receives in. Normally a small amount of capacity, say w = 2, is allocated to each station. If a station expects to generate a lot of traffic in a particular channel, it can negotiate for a larger window with the channel manager.
J. Spilker: Digital Communications by Satellite. Prentice Hall, Englewood Cliffs, NJ, 1977. L.G. Kazovsky, "Performance analysis and laser linewidth requirements for optical PSK heterodyne communication systems," IEEE J. Lightwave Technology, Vol. LT-4, No. 4, April 1986, pp. 415-425.
FORWARD ARM (PASSIVE TAPS)
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RETURN ARM (PASSIVE TAPS) CLUSTER1
1
2
I***l
Q
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TRANSMISSION
FIGURE 1 CONFIGURATION OF THE D NETWORK
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0 . .
M-1
M INFORMATION SLOTS
-i
FIGURE 2 SLOTTING SCHEME FOR FDM CHANNEL PASSIVE FIBRE TUNABLE OPTICAL FILTER
1 POWER DETECTOR
FIGURE 3 CARRIER SENSING ARRANGEMENT PREAMBLE PREAMBLE
1
2
INFORMATION PACKET
GUARD T,ME
PREAMBLE 1: FOR COLLISION DETECTION PREAMBLE 2: FOR CARRIER AND TIMING RECOVERY
FIGURE 4 CONTENT OF THE INFORMATION SLOT 345.1.5. 1652
