All-optical LAN architectures based on Arrayed ... - CiteSeerX

All-optical LAN architectures based on Arrayed Waveguide Grating Multiplexers Hagen Woesner Technische Universitat Berlin Telecommunication Networks Group

ABSTRACT

The paper presents optical LAN topologies which are made possible using an Arrayed Waveguide Grating Multiplexer (AWGM) instead of a passive star coupler to interconnect stations in an all-optical LAN. Due to the collision-free nature of an AWGM it oers the n-fold bandwidth compared to the star coupler. Virtual ring topologies appear (one ring on each wavelength) if the number of stations attached to the AWGM is a prime number. A method to construct larger networks using Cayley graphs is shown. An access protocol to avoid collisions on the proposed network is outlined.

1. INTRODUCTION

All optical communication has been investigated in the LAN area for a number of years. Most attempts to make use of the high potential of Wave Division Multiplexing relied on a physical star topology that employed a passive star coupler as a broadcast medium.1 Other projects2 favor a physical ring topology with a xed assignment of the wavelengths. Whenever optical star couplers are being used, the wavelength assignment is either xed, requiring a multihop architecture, or it has to be managed by a Medium Access Protocol. A variety of MAC protocols has been proposed in the literature, yet it can be stated that none of these approaches actually reached the market. There are several reasons for that: First, the cost of the laser equipment was too high. Either a bench of xed lasers or rapidly tunable lasers were necessary to ful ll the needs of the speci c MAC protocol. Second, the expectations concerning the rapid deployment of ATM led to a concentration on SONET/SDH equipment to connect ultra-fast ATM switches. Since SONET/SDH lines are connection oriented, the need for a MAC protocol disappeared, leaving behind a rather xed assignment of wavelengths to SONET lines. Third, the enormous bandwidth potential of WDM could never be exploited due to the fact that there was a broadcast medium in the middle of the network and only one station could be allowed to send on a particular wavelength at a time. Within the last few years, all of these problems changed. Laser equipment has become cheaper and more robust, but the much more surprising development took place in the battle eld of IP vs. ATM. Up to now, only very few native ATM applications have shown up. In contrast, fostered by the overwhelming success of the WWW, many IP-based audio and video applications appeared. Therefore the need to transport IP packets over WDM links arises again. Traditionally, the IP over WDM data communication would be IP over ATM (AAL5) over SONET/SDH over WDM. To get rid of the ATM \cell tax" of about 25% bandwidth reduction,3 IP over SONET/SDH was proposed in RFC 1619.4 Since SONET/SDH is used in that case only as a reliable point-to-point connection between two IP-routers/switches, the LINE overhead of SONET/SDH becomes redundant. One might think of dropping SONET/SDH now as well and replacing it by some protocol that uses the advantages of WDM in a better way. The Arrayed Waveguide Grating Multiplexer (AWGM) may be an alternative to the passive star couplers. It oers the n-fold potential throughput compared to a passive star coupler of n in/outputs. In chapter 2 we explain the basic features of the AWGM and the resulting network structure. Ways to interconnect these logical rings are shown in the following chapter, where a method to create perfectly symmetric networks is described. We use the construction method of Cayley graphs to create these networks. Large networks with hundreds of stations can be generated that way, maintaining the highest possible fault tolerance. The introduction of a MAC protocol enables us to reduce the number of wavelengths (=lasers). In addition to this, email: [email protected], WWW: http://www-tkn.ee.tu-berlin.de

bandwidth can be assigned exibly. The proposed MAC protocol for the ring structures is a buer insertion ring, which is a simple version of the MetaRing protocol by Cidon and Ofek.5

2. WAVELENGTH TRANSFER MATRIX

The Arrayed Waveguide Grating Multiplexer and possible applications of it have been described by Tachikawa et.al.6 The device can be logically seen as a combination of n demultiplexers and n multiplexers, even though it is essentially an analog grating based element with severe limitations in its size due to crosstalk properties. Each wavelength on an input of the AWGM appears only on one output. The advantage of such a solution is that this passive device oers n times the bandwidth of a passive star coupler and is completely collision free. All routing is done by the selection of the input port and the input wavelength. For this paper, we use a similar notation to the one presented by Oguchi.7 The output matrix Om is a product of the Wavelength Transfer Matrix (WTM) Lm;n and the input matrix In :

Om = Lm;n  In

(1)

The product of the elements of the WTM and the input wavelengths is de ned as follows: k  k =  k l   k = 0 (l 6= k)

(2) (3)

For an AWGM with m=n=5, i.e. 5 inputs and 5 outputs, the WTM is the following:

0 BB 12 L5;5 = B B@ 3 4

5

2 3 4 5 1

3 4 5 1 2

4 5 1 2 3

5 1 2 3 4

1 CC CC A

(4)

Numbering the inputs from A to E leads to the following input matrix I5;5 (Ak = k on input A):

0A BB B11 I5;5 = B B@ DC1 1 E1

A2 B2 C2 D2 E2

A3 B3 C3 D3 E3

A4 B4 C4 D4 E4

A5 B5 C5 D5 E5

B2 A2 E2 D2 C2

C3 B3 A3 E3 D3

D4 C4 B4 A4 E4

E5 D5 C5 B5 A5

Equation 1 now gives the output matrix O5;5 :

0A BB E11 O5;5 = B B@ DC 1 1 B1

1 CC CC A

(5)

1 CC CC A

(6)

If we now multiply O5;5 with an appropriate selection matrix S5;5 from the left, which means that we simply change the rows of the output matrix, the resulting output matrix looks like:

01 BB 0 S5;5  O5;5 = B B@ 00

0 0 0 0 0 1

0 0 0 1 0

0 0 1 0 0

0 1 0 0 0

1 0A CC BB E11 CC  BB D1 A @ C1 B1

B2 A2 E2 D2 C2

C3 B3 A3 E3 D3

D4 C4 B4 A4 E4

E5 D5 C5 B5 A5

1 0A CC BB B11 CC = BB C1 A @ D1 E1

B2 C2 D2 E2 A2

C3 D3 E3 A3 B3

D4 E4 A4 B4 C4

E5 A5 B5 C5 D5

1 CC CC = O5;5 A 0

(7)

The last step means nothing but an exchange of the outputs of the AWGM, but it leads to an interesting conclusion, when we assume that a station A is connected to the rst input/output pair, station B to the second and so on. Wavelength 1 is always routed back to the station where it came from, so it can not be used for the transmission to other stations, but the other four wavelengths form unidirectional rings with all stations connected to all rings. This is shown in gure 1. Note that the rings on wavelength 2 and 5 are counterdirectional as well as 3 and 4 . The resulting connectivity pattern can be seen as a fully meshed interconnection, too. On the other hand, one may start up with a single receiver/transmitter pair per station and then put in additional lasers (i.e. wavelengths) when more bandwidth is required. A transmission from station A to B may take place not only on the "direct" wavelength 5 but also on 3 , when a multihop scheme is introduced and station D works as a relay station for A and B. We see that potentially all of the wavelengths can be used for a transmission between a given pair of stations. Therefore the overall user data rate for an AWGM with n inputs (that is, n stations in maximum) is (n-1) times the bandwidth of a single channel. E

A

λ5

E

B AWGM

λ4 λ3

D

C

λ2 Figure 1.

D E D E D E D

Α

Β

C Α

Β

C Α

Β

C Α

Β

C

Connections in a network of 5 stations using 4 wavelengths.

The routing decision (output wavelength k ) at the sending station is based on:





k = x  n ?mdistance + 1 mod n (de ne: if k=0 then k=n)

(8) (9)

for integer numbers m,n and x with: m = hop number (1