Published online 18 June 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/dac.944
Performance evaluation of CSMA
/ID MAC protocol for IP over
WDM ring networks
Jih-Hsin Ho
1,∗,†, Wen-Ping Chen
2, Wen-Shyang Hwang
2and Ce-Kuen Shieh
3 1Department of Computer Science and Information Engineering, Diwan University, Taiwan2Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan 3Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan
SUMMARY
In this paper, a packet pre-classification media access control protocol based on a carrier sense multiple access with idle detection (CSMA/ID) scheme is investigated for supporting IP packets over all-optical WDM ring networks. The purpose of the protocol is to increase throughput and to decrease the packet transmission delay of IP packets over optical networks in a metropolitan area network. This protocol avoids both packet collision and packet fragmentation. In order to improve the utilization of the network, the packets transmitted from a local area network are first pre-classified into various class queues of an access point (AP) according to their length. After checking the available space based on the wavelength received by the receivers of the AP, the packets in the queues are transmitted. An analytical model is developed to evaluate the performance of the protocol, with simulation results showing good network efficiency. The proposed network has short-term variations that introduce unfairness conditions. This problem could be overcome by assigning a quota on individual queues to allow all queues fair access. Copyrightq 2008 John Wiley & Sons, Ltd.
Received 1 April 2006; Revised 5 March 2008; Accepted 29 March 2008
KEY WORDS: IP over WDM; CSMA/ID; packet pre-classification; analytical model; fairness
1. INTRODUCTION
With the explosion of information traffic due to the rise of the Internet, electronic commerce, computer networks, voice, data, and video transmission, the need for a medium with the bandwidth capabilities for handling a vast amount of information is paramount. Recent advances in solid-state and photonic technologies have delivered bit wavelengths of 2.5, 10, and 40 Gbp/s. The data can be sent over optical fibers, a transmission medium that permits light to travel through it without amplification for hundreds of kilometers. Currently, the total bandwidth of an optical fiber
∗Correspondence to: Jih-Hsin Ho, Department of Computer Science and Information Engineering, Diwan University, Taiwan.
exceeds 2 Tbit/s (200×10Gbit/s), 2.4Tbit/s (120×20Gbit/s), 3Tbit/s (300×11.6Gbit/s), and 3.2Tbit/s (80×40Gbit/s) [1–3]. Research has demonstrated that the number of wavelengths per fiber could increase to more than 1 000[4]. This indicates that WDM can be a solution for the ever-growing bandwidth demand.
Owing to the growing number of services and users on the Internet, IP packets dominate data networks. These packets are transferred, switched, and manipulated through complex protocol stacks, such as IP/ATM/SONET/WDM and IP/HDLC/SONET/WDM. How to merge and collapse the middle layers to reduce cost, complexity, and redundancy is an important research issue[5–7]. Additionally, since many WDM systems have been deployed in wide area networks (WANs), the bottleneck of communications will be pushed from the backbone networks to local access networks. As a result, how to apply WDM to local and metropolitan area networks has attracted a lot of research interest[4–8].
A number of papers have examined WDM ring networks. Cai et al. proposed the MTIT access protocol for supporting variable size packets over WDM ring networks based on a fixed-transmitters-and-fixed- receivers architecture [8]. To achieve all-optical communications, MTIT adopts the source removal policy [6] for dropping packets from networks to prevent packet re-circulation. Shrikhande et al. developed HORNET as a testbed for a packet-over-WDM ring metropolitan area network (MAN)[7]. To facilitate signal regeneration and destination removal, HORNET utilizes opto-electronic and electro-optic conversion, which may constrain the trans-mission rate of the network. Although the IP standard allows a packet length of between 40 and 64 kbytes, a measurement trace from one of the MCIs backbone OC-3 links shows a discrete packet-size distribution, from 40 to 1500 bytes (see Figure 1)[8]. The smallest packet of 40 byte corresponds to TCP ACK packets and the 1500-byte packets are Ethernet’s maximum transfer unit (MTU). Figure 1 shows that almost 5.34% (byte volume) of the packets are 40 bytes long, 27.58% (byte volume) of the packets are 41–552 bytes long, and 67.08% (byte volume) of the packets are 553–1500 bytes long. Hwang et al. proposed an all-optical media access control (MAC) protocol based on avoiding packet collision using a fragment packet scheme for all-optical WDM multi-rings with a tunable transmitter and fixed receiver (TT-FR) [9]. However, avoiding packet collisions using this fragment scheme creates a large number of fragmented packet and introduces
complexity. For this reason, we propose a new MAC protocol that avoids packet collision without a fragment scheme. In this paper, the WDM ring network architecture, carrier sense multiple access with idle detection (CSMA/ID) protocol, and transmission algorithms are presented in Section 2. Analytical models for evaluating the average packet delay performance are developed in Section 3. Then, Section 4 validates the accuracy of the proposed model by comparing the analytical results with those obtained using simulations. Finally, Section 5 contains the conclusion.
2. NETWORK ARCHITECTURE AND CSMA/ID MAC PROTOCOL
2.1. The network architecture
The network architecture used in this paper is a single, unidirectional fiber ring network, which connects an N number of nodes. The optical fiber is composed of W data channels
(1,2,3,...,w), as shown in Figure 2. The network scope is assumed to cover a metropolitan area (i.e. a ring circumference of about 100 km); therefore, the system is referred to as a WDM metro ring. The access points (APs) connect local area networks (LANs) to the MAN ring network, while PoP connects the MAN to the WAN. Each data channel makes use of one specific wavelength to convey the optical signal. Therefore, using WDM technology, channels can work independently without interfering with each other. Logically, the network can be treated as a multi-ring network.
The node structure of the network is shown in Figure 3. Each node has one tunable transmitter and W fixed receivers, one for each data channel. For the optical signal sent from upstream nodes, a splitter is used to tap off a small portion of the optical power from the ring to the receivers. Every receiver detects the optical signal carried in its corresponding wavelength within the output
Figure 3. The node structure of the network.
branch from the splitter for node address identification. If the destination address in the incoming packet header matches the node address, the packet data are sent to the host. Meanwhile, the MAC control scheme is signaled to activate the opening of the on–off switch for the corresponding data channel in order to remove the received packet carried in the major portion of the optical signal through the delay line. If the node is not the packet’s destination, the detected packet is ignored and the process of scanning the next packet is started.
2.2. CSMA/ID MAC protocol
The downstream AP recognizes an incomplete IP packet by the presence of the sub-carrier signal and pulls it off the ring. The carrier sense can check the available channel length (ACL) to notify the Tx to transmit the packet to the queue. Based on the protocol, each node monitors the wavelengths and detects the corresponding ACL, provided that there are IP packets for transmission. If an IP packet is being transmitted to a target channel while the node is detecting another IP packet arriving on the same channel at its input, a ring access problem (an access collision) will occur. Such collisions are due to the fact that the node cannot know if the opening is long enough to accommodate the packet. With the carrier access scheme, to guarantee the correctness of the protocol operations, the delay line inside the nodes must be used to delay the incoming packet. In addition, the delay line should be long enough to cover the maximum IP packet length (1500 bytes) so that unnecessary fragmentation can be avoided along with packet collision, thus improving the utilization of the bandwidth. The fiber delay line inside the AP is also responsible for processing IP packet time. Figure 4 shows the CSMA/ID MAC protocol flowchart. The MAC protocol decides whether the packet in the queue can be transmitted or not according to idle channel messages, transmit packet lengths, and the transmission algorithms. This protocol has three features: (1) it is a fully distributed, asynchronous protocol that does not need a centralized controller or a separate control channel to harmonize and synchronize the operations of nodes; (2) a transmitting packet will not collide with an incoming packet on the same wavelength because the FDL length (1500 bytes) is long enough to avoid this; and (3) it supports variable-length IP packets without complicated segmentation and reassembly, which becomes harder as the line speed of optical wavelengths increases.
Figure 4. The CSMA/ID MAC protocol flowchart. 2.3. Transmission algorithms for CSMA/ID MAC protocol
2.3.1. First in first out (FIFO) queueing scheduling. The packet that first arrived will be the first
to be served in different Tx-queues (Q1, Q2, and Q3). The MAC controller monitors all channels’ ACL in the delay line, which is larger than or equal to the length of the packet in the FIFO queue. We select one queue based on the ACL algorithms and transmit the packet to the corresponding delay line; otherwise, the packet has to remain in the buffer (electronic memory) of the TX-queue until sufficient ACL is available. In this manner, the packet collision problem can be avoided; however, the head-of-line will decrease the throughput performance.
2.3.2. Pre classification queueing (PCQ) scheduling. The process is as follows:
1. IP packets are pre-classified into three kinds of queues (Q1, Q2, and Q3) by the buffer selector. The three kinds of queues are for storage of 553–1500, 41–552, and 40 byte packets, respectively. The MAC controller reads the IP packet size storage message and sends it to the appropriate queue.
2. Since each node is equipped with a receiver for its corresponding data channel, an IP packet can be transmitted via a corresponding available data channel to its destination node. The receiver is responsible for checking the destination address of incoming IP packets and detecting available channels to notify the MAC controller.
3. Using the information from (1) and (2), the above MAC controller delivers a message to the active buffer selector to transmit Q1, Q2, or Q3buffer packets. Figure 5 illustrates the MAC controller model flowchart and Figure 6(a) illustrates an example of the MAC controller scheme. If the maximum ACL is 552 bytes and the three kinds of queue storage for the first IP packet are 1200, 512, and 40 bytes, respectively, then the MAC controller transmits a message to a tunable transmitter, which transmits Q2buffer’s packets. Figure 6(b) illustrates another example of the MAC controller scheme. If the receivers detect that the ACL is smaller than the first packet for the buffer selector, then the MAC controller does not transmit a message to the TX, and the TX will not transmit any packets to the fiber.
Figure 5. The MAC controller model flowchart (with IP packets pre-classified into three kinds of queues).
The downstream AP recognizes an incomplete IP packet by the presence of the sub-carrier signal and pulls it off the ring. The carrier sense can check the ACL to notify the TX to transmit the packet from the Q1, Q2, or Q3buffers.
2.3.3. Pre classification queueing with quotas (PCQ quota) scheduling. IP packets are
pre-classified into three kinds of queues (Q1,Q2, and Q3) by the buffer selector. The three kinds of queues are for storage of 553–1500, 41–552, and 40 byte packets, respectively. The classification is based on the previous long-term measurement, as in PCQ scheduling. The PCQ quota scheduling is designed specifically for short-term variations that may cause fairness problems. Each queue is assigned a quota (in bytes), which it is allowed to transmit. If the queue is still transmitting at the end of the quota, the MAC controller is preempted and given to another queue.
2.4. The frame format
To support the carrier access scheme, a frame format is adopted, as shown in Figure 7. The carrier sensing mechanism for finding transmitted packets in an optical fiber can be based on sub-carrier signaling[10] or receiver monitoring. For sub-carrier signaling, each wavelength is associated with a carrier frequency. When a node transmits a packet, it multiplexes the corresponding sub-carrier frequency. The nodes determine the occupancy of all wavelengths in parallel by monitoring the sub-carriers in the RF domain. In addition, since each receiver extracts the optical signals from the corresponding data channel, receiver monitoring can be used to determine the occupancy of all wavelengths. It seems natural that the receivers should be associated with an auxiliary function to monitor the status of the optical ring network. Today, the cost of such receivers is still too high for them to be economical to manufacture, but a cheaper process may be developed in the future. The start delimiter (SD) and the end delimiter (ED) mark a physical data frame conveyed in data channels for packets. The source address (SA) and the destination address (DA) serve as the address information in the network. To prevent possible errors midway through the transmission, the cyclic redundancy check (CRC) is employed. The flag (FG) field is reserved for extended protocol functions.
5. CONCLUSION
In summary, in this paper we proposed a novel MAC protocol for all-optical WDM ring networks. The protocol supports the transmission of IP packets directly over WDM from LAN to MAN. How to merge and collapse the middle layers between IP and WDM for the next generation optical LANs/MANs was investigated. This protocol can avoid packet collision, reuse wave-length, and lacks a fragment packet scheme. In the verification, the simulated results closely match the analytical values, demonstrating the performance of the network. The throughput char-acteristic of the network is almost proportional to the number of channels in the network. With regard to the utilization of bandwidth of all the optical ring networks, our protocol displays the excellent characteristics of high throughput, low delay, and a good fairness index for all-optical communications.
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AUTHORS’ BIOGRAPHIES
Jih-Hsin Ho received his BS degree in computer science and information engineering from Tatung University, Taipei, Taiwan, in 1993, and the MS and PhD degrees in Electrical Engineering from National Cheng Kung University, Taiwan, in 1998 and 2007, respectively. He is currently an assistant professor teaching at the Department of Computer Science and Information Engineering, Diwan University, Tainan, Taiwan. His current research interests include performance evaluation, WDM networks, Internet QoS.
Wen-Ping Chen received the BS degree in electrical engineering from National Taiwan Institute of Technology, Taiwan in 1992, MS degree in electrical engineering from National Sun Yat-sen University, Taiwan in 2000, and the PhD researching in computer network from National Kaohsiung University of Applied Sciences, in 2003. From 1993 to 2003, he worked for National Kaohsiung University of Applied Sciences as a teaching assistant in the department of electrical engineering.
Wen-Shyang Hwang received BS, MS and PhD degrees in Electrical Engineering from National Cheng Kung University, Taiwan, in 1984, 1990 and 1996, respectively. He is currently a professor of Electrical Engineering, and the chairman of department of computer science and information engineering in National Kaohsiung University of Applied Sciences, Taiwan, Republic of China. His current research interests are in the fields of storage area networks, WDM Metro-ring networks, performance evaluation, multimedia wireless communications, software design on embedded system, Internet QoS and Internet applications.
Ce-Kuen Shieh is currently a professor teaching at the Department of Electrical Engi-neering, National Cheng Kung University. He received his BS, MS and PhD degrees from Electrical Engineering Department of National Cheng Kung University, Tainan, Taiwan. His current research interests include distributed and parallel processing systems, computer networking and operating systems.
