Advances in Photonic Packet Switching: An Overview <s> HEADER AND PACKET FORMAT <s> All-optical updating of subcarrier multiplexed headers and the simultaneous wavelength conversion of baseband payload is demonstrated. Updating is achieved by utilizing the low-pass frequency response of wavelength conversion via cross gain modulation in semiconductor optical amplifiers to suppress the original subcarrier multiplexed header while efficiently converting the baseband payload. A new header is then inserted by optically modulating the output of the wavelength converter with the new subcarrier encoded data. Updating of a pseudorandom 100 Mb/s header coded on a 16.5-GHz microwave subcarrier is achieved with wavelength conversion of a 2.5 Gb/s pseudorandom baseband payload. Crosstalk measurements are presented to determine cascadability of this technique and indicate that subcarrier suppression of 16.9 dBe followed by 20.2-dBe remodulation overcomes degradation due to residual original subcarrier and remodulation of the baseband payload. A key feature of this technique is the preservation of all-optical baseband payload transparency up to the header subcarrier frequency. <s> BIB001 </s> Advances in Photonic Packet Switching: An Overview <s> HEADER AND PACKET FORMAT <s> Summary form only given. The objective of the present work is to estimate the on-line BER on candidate routes and wavelengths before setting up a call. Note that the existence of other calls currently in progress, i.e., traffic variation, will affect the BER estimate (because they will affect the cross talk in cross connect switches (XCSs) and the wavelength dependence and saturation of gains and ASE noise generation in EDFAs). <s> BIB002
In electronic networks, packet header is transmitted serially with payload data at the same data rate, such as IP packets and ATM cells. Electronic routers or switches will process the header information at the same data rate as the payload. In an optical network, the bandwidth is much larger than their electronic counterparts. A typical wavelength channel has the line speed of 2.5 Gb/s (OC 48). Although there are various techniques to detect and recognize packet headers at Gb/s speed, either electronically or optically, it is still difficult to implement electronic header processors operating at such high speed to switch packets on the fly at every node. Among several different proposed solutions, packet switching with subcarrier multiplexed (SCM) header is attracting an increasing interest. In this approach the header and payload data are multiplexed on the same wavelength (optical carrier). In the current that modulates the laser transmitter, payload data is encoded at the baseband while header bits are encoded on a properly chosen subcarrier frequency at a lower bit rate, as shown in Fig. 13 . The header information on different wavelengths can be retrieved by detecting a small fraction of the light in the fiber with just a conventional photodetector, without any type of optical filtering. In the output current of the photodetector various data streams from different wavelengths jam at baseband, but the subcarrier remains distinct and the header can be retrieved by electrically filtering out the desired subcarrier (Fig. 14) . Since the laser and photodetector electrical frequency response must extend as far as the highest subcarrier frequency, it is important to keep the subcarrier frequencies as few, as low and as closely spaced as possible. Since the minimum subcarrier spacing cannot be less than twice the header bit rate, it is also important to keep the header bit rate low. On the other hand, if the header bit rate is too low, it would take longer to transmit and receive the header information, thus causing longer delays for the payload data. A nice feature of subcarrier multiplexed header is that the header can be transmitted on top of the payload data and it can take up the whole payload transmission time, since it does not interfere with the payload. Of course the header can also be transmitted serially with the payload if so desired. One potential pitfall of subcarrier multiplexed header is its possible limit on the payload datarate. If the payload datarate is increased, the baseband will expand and might eventually overlap with the subcarrier frequency, which is limited by the microwave electronics. In many of the routing and switching protocols packet headers have to be updated at each node. There have been several approaches proposed on all-optical same-wavelength header replacement for headers transmitted serially with the payload datastream. All-optical header replacement could be done by blocking the old header with a fast optical switch and inserting the new header, generated locally by another laser, at proper time. One important issue here is that in WDM networks the new header should be precisely at the same wavelength as the payload data, otherwise serious problems could arise because of dispersion, nonlinearity or wavelength sensitive devices in the network. It has been proposed that the header updating be done by transmitting the payload and header on separate wavelengths, demultiplexing the header for optoelectronic conversion, electronic processing and retransmission on the header wavelength. This approach suffers from fiber dispersion, which separates the header and payload as the packet propagates through the network. Subcarrier multiplexed headers have far less dispersion problems since they are very close to the baseband frequency. SCM header could be removed by narrow-band optical filters, but it would be extremely sensitive to wavelength drift. Previous practical SCM header replacement schemes had been limited to full optoelectronic conversion of the entire packet followed by electronic filtering, remodulation and retransmission on a new laser. BIB001 has proposed a technique to update the SCM header with simultaneous wavelength conversion of baseband payload using SOAs. It involves a two-stage process: First, simultaneous SCM header suppression and wavelength conversion of the baseband payload is achieved due to the low-pass frequency response of cross-gain modulation in the SOAs, then header replacement is achieved by optically re-modulating the wavelength converted signal with a new header at the original subcarrier frequency. Packet length is another issue of concern for network designers. A short packet might not give a good throughput because more percentage of the bandwidth is given to the header or guard time between time slots. On the other hand, a long packet would need longer optical buffers, and not provide a granularity that is fine enough. From a physical point of view, balancing the PER between payload and header is very important. PER is different from bit error rate (BER); it is the probability of the entire packet received in error. PER increases with BER and the number of bits contained in the packet. For efficient network operation, the PER for payload and header should be about the same, in order to deliver the packets as successfully as possible BIB002 . Payload usually contains many more bits than the header. If header is updated at every traversed node, the bits in payload will have to suffer more physical impairment than the bits in the header. Another fact is that if SCM is used, the header is usually transmitted at a lower bit rate than the payload data. All these facts lead to a big advantage of lower BER for header bits over the payload bits. Therefore it is imperative to optimize the amount of power to be tapped from the packet at each node and the packet length in order to achieve a balanced PER for payload and header at the destination node.