Simulation of Switched Ethernet

Transcription

1 Technical report, IDE0633, February 2006 Simulation of Switched Ethernet Master s Thesis in Computer System Engineering Kishore Kumar Nachegari Suresh Babu Eadi School of Information Science, Computer and Electrical Engineering Halmstad University

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3 Simulation of Switched Ethernet Master s Thesis in Computer System Engineering School of Information Science, Computer and Electrical Engineering Halmstad University Box 823, S Halmstad, Sweden February 2006

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5 Preface and Acknowledgements This project is a part of the work required for the fulfillment of our Master s degree in Computer Systems Engineering in February 20006, which was carried out in the CC-Lab, Halmstad University, Sweden. We would like to thank our supervisors, Professor Magnus Jonsson and Xing Fan without whose continuous valuable guidance this work would not have been achieved. Kishore Kumar Nachegari & Suresh Babu Eadi Halmstad University, February v

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7 Abstract Switched Ethernet is an Ethernet LAN that uses switches to connect individual nodes. This is popular because of its effective and convenient way of exting the bandwidth of existing Ethernets. Switched Ethernet is being considered by the industry community because of its open standardization, cost effectiveness, and the support for higher data rates up to 10Gbps. Even though many special-purposed solutions were proposed to support time constrained communication over Switched Ethernet, still there were some doubts about the real time handling capability of Switched Ethernet. To achieve reliable transmission guarantees for real time traffic over Switched Ethernet, it is important to measure the performance of Switched Ethernet networks for real time communication. In this thesis work we have observed the average -to- packet delay for real time traffic over a Switched Ethernet by simulation, which is very much essential for real time communication in industrial applications, where the communication is time-deterministic. In our thesis we used FCFS priority queuing in both the source nodes and switch. In this thesis we also discussed about the feasibility analysis for fixed sized frames and some traffic handling methods. We used 100mbp/s single full duplex Ethernet switch for our simulation. Finally simulation analysis and simulation results are discussed. Our purpose of simulation of Switched Ethernet networks is of good importance for the real time industrial applications. Keywords Switched Ethernet, Real time Communication, First Come First Serve (FCFS), Feasibility analysis. vii

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9 Goals The main goal of this project is to investigate the performance of real time Switched Ethernet by simulation on packet level. Our task is to build a software simulator which is used to test the performance of the method over a Switched Ethernet. We need to observe the average to packet delay of the real time traffic over a Switched Ethernet. By using some traffic handling methods and queuing mechanisms, the performance of the Switched Ethernet for real time communication can be determined. ix

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11 TABLE OF CONTENTS PREFACE AND ACKNOWLEDGEMENTS...V ABSTRACT...VII GOALS... IX TABLE OF FIGURES...XIII 1. INTRODUCTION APPLICATIONS RELATED WORK INDUSTRY PROPOSAL Ethercat Profibus Profinet ACADEMIC PROPOSAL Related Research work NETWORK ARCHITECTURE AND TRAFFIC HANDLING REAL TIME COMMUNICATION REAL TIME CHANNEL ESTABLISHMENT REAL TIME ANALYSIS REAL TIME CHARACTERISTICS FEASIBILITY ANALYSIS APPROACH FLOWCHART TO CALCULATE AVERAGE DELAY DESCRIPTION OF THE ALGORITHM SIMULATION ANALYSIS SIMULATION RESULTS CONCLUSIONS REFERENCES APPENDIX A GLOSSARY AND EXPLANATIONS APPENDIX B SOURCE CODE xi

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13 Table of Figures FIGURE 1 CONCEPTUAL DIAGRAM OF ETHERCAT... 6 FIGURE 2 FORMAT OF THE IEEE P EXTENDED ETHERNET FRAME FIGURE 3 STAR TOPOLOGY FIGURE 4 CONCEPTUAL MODEL OF A FULL DUPLEX ETHERNET SWITCH FIGURE 5 MODEL OF FRAME TRANSITION IN AN ETHERNET SWITCH FIGURE 6 ESTABLISHMENT OF REAL TIME CHANNEL FIGURE 7 FLOWCHART 1 AVERAGE DELAY CALCULATION FOR FIXED SIZED PACKETS AND FCFS QUEUING FIGURE 8 AVERAGE DELAY VS TOTAL NO OF REQUESTED CHANNELS AND NUMBER OF ACCEPTED CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 11 UTILIZATION OF REAL TIME CHANNELS VS TOTAL NUMBER OF REQUESTED CHANNELS FIGURE 12 AVERAGE DELAY VS TOTAL NUMBER OF REQUESTED CHANNELS AND NUMBER OF ACCEPTED CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 13 UTILIZATION OF REAL TIME CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 14 AVERAGE DELAY VS TOTAL NUMBER OF REQUESTED CHANNELS AND NUMBER OF ACCEPTED CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 15 UTILIZATION OF REAL TIME CHANNELS VS TOTAL NUMBER OF REQUESTED CHANNELS FIGURE 16 AVERAGE DELAY VS TOTAL NUMBER OF REQUESTED CHANNELS AND NUMBER OF ACCEPTED CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 17 UTILIZATION OF REAL TIME CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 18 AVERAGE DELAY VS TOTAL NUMBER OF REQUESTED CHANNELS AND NUMBER OF ACCEPTED CHANNELS VS NUMBER OF REQUESTED CHANNELS FIGURE 19 UTILIZATION OF REAL TIME CHANNELS VS TOTAL NUMBER OF REQUESTED CHANNELS xiii

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15 1. Introduction Ethernet is the most widely used local area network. Since it s inception in 1973, Ethernet has achieved much popularity and success because of its simplicity and very low price. Ethernet is unsuitable for control level network applications under moderate to heavy traffic loads. Ethernet was not originally designed to support real time traffic due to the random strategy employed by Ethernet s Carrier Sense Media Access/Collision Detection (CSMA/CD) Media Access Control (MAC) protocol for resolving access contention to the network media. CSMA/CD used in Ethernet causes collisions and congestions and there is no flow control in it. This is the fundamental limitation of Ethernet for real time applications. However, Ethernet is attractive for real time applications due its popularity, low cost and high performance. Several techniques were developed to make Ethernet s exhibit the real time behavior, some requires specialized hardware, some providing soft real time guarantees and others providing hard real time guarantees with varying bandwidth efficiency. Switched Ethernet eliminate the CSMA/CD collision problem caused by traditional Ethernet networks. The properties like full duplex and flow control has caused the migration to Switched Ethernet networks. Apart from that it is also very cheap and increases the network bandwidth efficiency. This has opened the way to enable time constrained communication over Switched Ethernet networks. Switched Ethernet is a packet switching technology that reduces the network congestion caused by the CSMA/CD over shared media, and it offers guaranteed bandwidth per port and operates at the medium access control sub layer of the network access layer of the TCP/IP protocol. Switched Ethernet is used to support high bandwidth real time communication in process control, factory automation and other real time applications. There are three main architectures of switches, which are store & forward, cut through and fragment free, but the most commonly available switch is store and forward, in which an Ethernet packet is entirely stored and checked by the device before retransmitting in to the proper destination. Corrupted data grams are eliminated and results in the maximum utilization of the bandwidth. In conventional Ethernet, information is exchanged among the nodes over a common communication channel, whereas in Switched Ethernet the nodes are connected to a switch using point to point connection. In Switched Ethernet, switch is an active device that identifies the destination ports. It controls the data to and from the nodes communicating over the Ethernet. This optimizes data bandwidth and improves real time data traffic. The conventional Ethernet uses a half-duplex link, whereas, the Switched Ethernet uses a full duplex link [18]. Switched Ethernet in recent years has grown as an attractive enabling technology for supporting time constrained communication. Process control, factory automation are some of the examples of using Switched Ethernet to support high speed real time communication [18]. This can be seen by HMS (HMS Industrial Networks), one of the Sweden s fastest growing manufacturing companies in the field bus technology sphere has adapted to Switched Ethernet networks [11]. This shows the growing importance of Switched Ethernet for real time industrial networks. As industries migrate towards Switched Ethernet networks for time critical applications still there s some uncertainty about the time behaviors of the Switched Ethernet networks. End to delay is the most important character in real time systems. This is one of the user 1

16 defined QoS metrics. To evaluate the performance of Switched Ethernet networks to achieve reliable guarantees in time critical applications, it is important to observe the average to packet delay for real time traffic over Switched Ethernet. 2

17 2. Applications Real time applications such as industry process control, telecommunication equipment, automation control and other industrial applications require strict time deterministic transmissions. In such systems frames containing the real time information, have to be delivered with in certain time limit. Correct performance is specified by the packet delivery with in a certain delay bound [13]. The properties like reduced cost, flexibility and expandability makes Switched Ethernet a suitable technology for many real-time communications. Switched Ethernet in industrial applications increases network bandwidth and provides network determinism for industrial control applications, and is the cost effective solution for industrial environments. Switched Ethernet networks are suitable for range of high performance parallel and distributed applications such as radar signal processing, cluster computing, computer integrated manufacturing [13]. Switched Ethernet is also used in optical networks. Switched Ethernet in optical networking brings high quality real time performance capabilities for packet switching networks. Switched Ethernet networks are also used for heavy-duty applications such as process control, machine monitoring, remote data acquisition, environmental control and in other real time applications. In academic applications, Switched Ethernet in VPLS (Virtual Private LAN services) provides proactive network monitoring, browser-based reporting, notification and trouble shooting, flexible billing and service options [26]. Switched Ethernet helps you consolidate administration by providing remote locations with high speed access to servers, file servers and shared internet access. Presently Switched Ethernet is predominantly used for enterprise branch office and site to site communication [25]. 3

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19 3. Related work 3.1 Industry proposal To satisfy the time constrained communication required by process control, factory automation and other real time applications many field bus protocols have been developed and are successfully implemented. The demands on real time communication in the automation world are determined in large part by the applications, and are often characterized by the response time. Ethercat, Profibus, and Profinet are some of the special purposed real time Ethernet solutions designed to meet the real time communication requirements in the industry. Some of the industrial real time Ethernet solutions are briefly discussed below: The below mentioned technologies are used in the same area of application, which is in automation industry. Ethercat, Profibus and Profinet can meet the requirements needed for real time communication like Switched Ethernet. But Ethercat, Profibus and Profinet are particularly suited for exchange of small amounts of data under time and offers low bit rate. Exchange of large amount of data under time and higher bandwidth can be achieved by using Switched Ethernet while fulfilling the requirements of real time communication. Limitations of these dedicated technologies, possibly formed along the way to Switched Ethernet Ethercat Ethercat, which is defined as Ethernet for control automation technology is developed by the Beckhoff electronic industry [2]. Ethernet at the field level has some drawbacks. It is not very useful for sing frequent, short messages, and the connection costs are high. These can be overcome by using Ethercat that is fast and cheap.in Ethercat, a new approach is used to bring Ethernet to the I/O domain. A significant improvement of the usable data rate can be achieved by using the Ethercat. The field memory management unit in the Ethercat helps utilizing the data rate almost up to 100%. In fact, a high bandwidth and real time capability can be achieved using Ethercat. It does not require an IP address, where the master automatically controls the configuration using simple algorithms. Therefore Ethercat provides a powerful communication basis for the automation industry [2]. Working Principle: Ethercat bus ss and receives Ethernet telegrams. It has large number of Ethercat slaves, which extract the information coming from its predecessor and insert relevant data before passing it on its next successor, where the last slave s the processed telegram to its master. It operates in a full duplex mode. Ethercat supports almost any topology: line, star or bus. The topology of Ethercat is an open ring bus in logical terms because of the master-slave communication. It can also support branches wherever applicable to form a tree structure instead of a line structure. The Ethercat protocol is optimized for processing data for different types of communication and is transported directly within the Ethernet frame. It consists of several Ethercat telegrams. Any type of communication is possible among the slaves. The following are some of the features of Ethercat [2]: Ethercat has a flexible topology, simple configuration and it is cost effective. Therefore, Ethercat enabled filed buses to be used in the applications, where they were not used in the past. The Ethercat protocol is optimized for processing data and is transported directly within 5

20 the Ethernet frame. This new approach is used to bring the Ethernet to the I/O domain, which is the first true Ethernet solution for the field level. Ethercat can access 12,000 digital I/O in 350 micro second or 100 servo axes in 100 micro seconds. For Ethernet point of view, an Ethercat bus is a large device that ss and receives Ethernet telegrams. Ethercat does not contain an Ethernet controller with down stream but contains large number of Ethernet frames. These process the incoming datagram s directly and the relevant user data can be extracted, or the data is transferred to the next slave by inserting data to the telegram. The last Ethercat slave ss the fully processed data to the master, thus the processing is performed while passing. In Ethercat, Ethernet deals separately with the transfers in separate directions (Tx and Rx) and this operates in full duplex mode by loop back through R x pair, transmitted telegrams are returned to the control. By using cross over cable, direct communication without switch can be established like any other Ethernet, thereby creating a pure Ethercat system. With Ethercat technology, switching is done in the network interface, while the Ethernet frame continues through the device [2]. Figure 1 Conceptual diagram of Ethercat There are no microprocessors or software stacks to execute communication functions in Ethercat. Ethercat achieves the high speed by performing all communication functions in hardware. Ethercat, which is indepent of the transmission speed, can be used for 1 Gb or more without any modification. It is so fast that the master, while sing the last bytes of its query, receives the returned response. The configuration of the Ethercat is automatically controlled by the master. Some of the advantages of the Ethercat are optimized telegram structure for decentralized I/O, communication completely in hardware that gives maximum performance and no switch is needed if only Ethercat devices in the network. By using Ethercat available bandwidth is almost fully utilized and the costs are reduced. Ethercat does not require an IP address, and configuration is automatic-controlled by the master. It has Standard hardware for the master. Compatible with legacy CAN Open. It is well suited for cost-effective control applications and has outstanding performance. Despite of all these advantages it also has some drawbacks which include lack of integration with other systems and high installation overhead, and it does not use switching to increase the throughput. 6

21 3.1.2 Profibus Profibus, which is a field bus standard for manufacturing automation and process control, uses a token passing protocol on the top of a MAC level [18]. This is one of the most famous field bus standards. Profibus encompasses several industrial bus protocol specifications, which include Profibus-DP, Profibus-PA, and Profibus-FMS. Profibus-DP is a device level bus that supports both analog and discrete signals, used in motor control centers and remote I/O systems. Profibus PA is used for process level instrumentation and is a fullfunction field bus. Profibus-FMS is a control bus used for communication between DCS and PLC systems. Real time behavior of the traffic in the Profibus can be guaranteed by two different approaches. One approach is the unconstrained low priority traffic profile, where the real time requirements for the synchronous traffic are satisfied. In this synchronous message is transmitted per token which is indepent of asynchronous traffic load [17]. Another approach is constrained low priority traffic profile, where all ping real time traffic can be transmitted at each token by controlling the number of a low priority traffic message [20]. Working Principle: The Profibus protocol architecture consists of three layers: physical, data link, and the application layers. The physical transmission characteristics are defined by the physical layer, bus access is defined by the data link layer and the application functions are defined by the application layer. It uses a shared bus topology. Profibus service allows information broadcasting/multicasting. Profibus uses a token-passing protocol on top of a broadcast medium on the MAC level. Profibus MAC is based on token passing procedure, which is used by the master stations to grant the bus access to each other, and on a master slave procedure, which is used by the master to communicate with the slave stations [18]. The interaction model offered by the Profibus is mainly on the master slave polling, which is more suitable to periodic data exchanges [19]. The Profibus token passing procedure uses a simplified timed token protocol. One important parameter of this kind of protocol is the target token rotation time (T TR ), which is set at the network installation time and stands for all the network nodes. After receiving the token, the station may transmit high priority messages for a time period no more than its allocated synchronous capacity (H i ). If the previous rotation time is lower than the T TR, low priority messages can be transmitted [18]. When comparing to the timed token protocol, the main difference of the Profibus token passing consists the absence of synchronous bandwidth allocation. In Timed Token, if a station receives a late token, only one high priority message can be transmitted, whereas, in Profibus, low priority traffic affects the high priority traffic. In fact, if a station receives an early token, and the low priority traffic is not constrained in that station, the subsequent stations may be limited to one high priority message transmission. Real time behavior of the high priority traffic can be guaranteed with the Profibus, despite the absence of the synchronous bandwidth allocation [20]. Profibus addresses the needs of multiple manufacturing domains, from bus power for field instruments to high-speed deterministic control. It is a single integrated system from planning to maintenance. It is used in wide range of applications, particularly in the field of process automation and factory automation. It is suitable for fast, time-critical applications and complex communication tasks. It can handle large amounts of data at high speed and serve the needs of large installations. The Profibus DP, FMS and PA versions collectively address the majority of automation applications. Some of the advantages of Profibus DP-PA- FMS networks are: network support at the device, process control and Ethernet levels, interfaces are available for variable speed drive and motor control applications (Profibus DP), process instrumentation available with Profibus PA devices, gate way devices are available to directly support lower cost sensor bus networks. It offers cost effective solution for industrial automation. Some of its drawbacks are high overhead to message ratio for small amounts of 7

22 data, no power on the bus, slightly higher cost than some other buses, Profibus DP does not support intrinsically safety installations, and no control in the field capabilities Profinet Profinet is the industrial Ethernet based communication system devised by Profibus International (PI) [3]. It can simultaneously handle standard Ethernet transmissions and realtime transmissions at high speeds. Profinet is developed to allow Profibus communication across Ethernet networks. It is divided into three classes: Profinet CBA, Profinet RT, and Profinet IRT. Profinet CBA is used for controller communication in component based systems, uses TCP/IP. Profinet RT is used for I/O device communication in soft real-time systems and is well suited for use with distributed I/O, and the Profinet IRT is used for motion control applications in hard real-time systems. Profinet is an integrated and comprehensive standard of automation technology from the field level to the management level, from the standard control to high motion control with safety and security functions integrated. It can support all the general network topologies or even a mixture of these structures. It is based on industrial Ethernet, uses TCP/IP and IT-standards. It offers an open interface to the field buses. Working principle: The real-time channel is established for time critical messages, which is implemented by software based controllers, whereas, the non-time critical messages are carried over TCP/UDP and IP. An Isochronous real time communication is established for motion control applications. Profinet CBA supports non real-time communication over TCP/IP and RT class 1 protocol for medium performance real time communication. The Profinet IO supports non RT based on UDP/IP, and RT class 1, RT class 2 for hard real time applications. It supports a multi-layer diagnostics that helps efficient error location and elimination. To carry all the traffic classes on the same channel time division multiplexing is employed [4]. Profinet IO and Profinet CBA use the same real time protocol to distribute real time data. Real-time data and other data are scheduled by the driver inside the device to keep the real-time at high priority. The components of the Profinet are mapped during run time in the form of DCOM objects whose communication is ensured through the mechanisms of the object protocol. This identifies the object and the method with the associated interface and parameters. The DCOM protocol is part of the run time software. The service definition and protocol specification of real time communication are provided by the Profibus I/O device, based on Ethernet, IP and UDP for the field bus domain. Service communication between I/O controllers and I/O devices can be defined by the Profibus I/O. Profinet defines a non-real time protocol and two classes for real time communication to enable real time and TCP/IP traffic. Each protocol has a specific target time. Profinet CBA focuses its efforts in defining an object oriented environment, where the engineering phase can be simplified [4]. The biggest advantage of using Profinet for communication is real-time and TCP-based IT communications can coexist on a single line. It allows seamless integration of underlying field bus systems thereby reducing costs for product development. It supports both pure Ethernet applications, and combined field bus and Ethernet applications. It supports standard IT communication mechanisms for normal use in the office environment. The use of the same communication mechanisms in the office and automation environment permits total integration of different corporate levels. Since it builds on the world-class and highly popular Profibus standards, it can be applied on very wide range of applications like remote access, wireless communication, and high performance motion control applications. The disadvantage of using Profinet is that, it cannot be ported to other architectures. It overrides the standard 8

23 Ethernet medium access control, requires strict synchronization and also higher in cost compared to other existing standards. However, these standards have their own drawbacks when we demand more real time capability than they offer. To achieve higher bandwidth, flexibility and scalability many field bus standards include Ethernet networks as a low layer communication system. These networks are soon replaced by the Switched Ethernet to achieve real time handling capability. Because of the complex load of real time applications and different network context, there is a growing demand for the use of Switched Ethernet in these applications. Switched Ethernet has grown to be more and more attractive enabling technology for supporting time constrained communication. To ensure a guaranteed delivery of the real time traffic using Switched Ethernet in industrial applications, we analyze the real time handling capability of Switched Ethernet networks by observing the average to packet delay. 3.2 Academic proposal There are different networks with different real time capabilities, aiming at different applications. Switched Ethernet is one of the approaches to improve Ethernet s behavior to support real time communications. The real-time Switched Ethernet network approach is presented in Ethereal [23], SIXNET Industrial Ethernet Switch [24]. Ethereal is a real time fast Ethernet switch architecture that provides bandwidth guarantees to distributed multimedia applications without any OS and hardware modifications Related Research work There are also several research areas to support real time communication over Switched Ethernet. Research work is carried out to improve the real time behavior of Ethernet networks using traffic smoothing [15]. In this the author talks about fuzzy traffic smoothing. To achieve statistical delay bound on packet delivery over Ethernet networks, an Adaptive traffic smoothing technique at the field level is employed. This technique was chosen because fuzzy smoother outperform the other adaptive smoothers proposed earlier. Fuzzy traffic smoothing is a soft computing based technique to perform adaptive traffic smoothing and address the optimization of the fuzzy smoother through genetic algorithms. Some further work needs to be done for traffic smoothing dealt with fuzzy smoothers for Switched Ethernet networks. In order to support quality of service (QoS), a FTT-Ethernet protocol, which supports hard real time communication in a flexible way, seamlessly over shared or Switched Ethernet is proposed in [22]. This protocol satisfies the requirements of dynamic real time applications. It includes online admission control to guarantee continued real time operation under dynamic communication requirements together with data structures and mechanisms that are used to support dynamic QoS management. The implementation of FTT-Ethernet over micro segmented Switched architectures needs to be addressed further to solve some of the problems that affect the real time performance of Ethernet switches. Research work is going on Switched Real Time communication for industrial applications in Halmstad University by Dr. Magnus Johnson and Xing Fan [10] [1]. The research work is going on how to form methods to support industrial real time services, how they can be implemented in the switches without changing the underlying protocol, how it should be supported by the existing higher level protocols in the mean while. The aim of the research is to implement such methods in the industrial applications. Other aim of the research 9

24 is how to form methods to increase analyzability and what degree of throughput, latency can be expected from these communication systems. For industrial applications it requires minimum latency. The main goal of this research is to develop and analyze methods to support real time traffic. To get as much functionality and performance these methods can be implemented in the switches or network. By including more number of switches, the system performance can be improved and the possibility to offer real time services can also be increased. Some research work was already done in the field real time services which are used to introduce real time services on the top of the LAN technology, and the traffic with different priority classes has been developed [11]. For the industrial applications we require shorter response times. Some work on real time communication over Switched Ethernet has done but they mainly focused on the bandwidth guarantees for multimedia traffic not on the industrial applications with shorter response time. 10

25 3 Network architecture and traffic handling Switch handling: We consider a network with a star topology, single Ethernet switch and number of nodes. Every node is connected to other node via switch. A direct logical line of communication is established between two points for each frame by the switch. The switch is assumed to be store and forward, contains three main components: queuing model, switch implementation and switch fabric [8]. The queuing model refers to the buffer and congestion mechanism in switch, and switch implementation is the decision making process, whereas the switch fabric is the path that moves the data from one port to other [8]. All recent Ethernet switches are operating with wire speed and nonblocking. Wire speed means all ports of a switch can simultaneously transmit or receive at their full bit rates. In nonblocking, a message can be forwarded to the destination port as long as that port is free, while a blocking one may not able to forward the message to the destination although the destination port is free. Buffering and buffering delay exist in the switch. Buffering delay occurs when the output port cannot forward all the input messages in time and this corresponds to the burst traffic arrival. Buffering delay analysis deps on the input traffic pattern. The true number of the buffering switch with each output port bit rate is higher than/or equal to the sum of all input ports [8]. The contents of the Switched Ethernet frame is shown in the below [8]: Figure 2 Format of the IEEE p Exted Ethernet frame Preamble: A sequence of 56 bits that have alternative 1 and 0 values used for synchronization. Start Frame Delimiter (SFD): The bit sequence , which indicates the actual start of the frame. Destination Address (DA): It may be a unique physical, global or group address, in which inted frames are specified by the stations. Source Address (SA): It specifies the station that sent the frame. Tag Protocol Id (TPID): TPID carries the tag field. In an Ethernet frame if the TPID filed value equals to 8100 then it carries tag IEEE 802.1Q/802.1P. 11

26 Tag Control Info (TCI): TCI filed is the header field that is having following 3 fields level format network User Priority: This is a 3 bit field. This field can be used to store the priority for the frame. Canonical Format Indicator (CFI): This is a one bit field denoting whether MAC addresses in the frames are in canonical format. This is called the canonical indicator. Virtual Local Area Network (VLAN) ID: This is a 12 bit virtual local area ID. Length: Indicates the length of the data field in bytes. Type: This is Ethernet type field, which is used for identifying the contents of the data field. Data: The data which we s is IP data grams. Maximum length of the data field is 1492 bytes. Frame Check Sequence (FCS): This is a 32 bit cyclic redundancy check, based on all fields except preamble, SFD, FCS. The length of the Ethernet frame excluding SFD and preamble is between 64 and 1518 bytes. Traffic can be classified by addling 802.1P header to the frame and the classified traffic can be put into different priority queues. Network Topologies: In our work we have preferred star topology. The star topology is the simplest one for the real time communication because between any communication points only one switch is needed to be traversed. It is very easy to maintain and configure. This topology is suitable for small and some special purpose networks. It is a cost effective way to achieve real time services [12]. Figure 3 Star Topology 12

27 Traffic Handling: A network with a star topology, where nodes are connected by switch is examined. The IEE 802.1p queuing feature, which enables layer 2 switches to set priorities for traffic and perform dynamic multicasting to distinguish between different traffic classes. In our work we have considered hard real time traffic for our simulation. The generated traffic can be put into the corresponding queue before entering into the network interface card. The generated traffic is stored in the queue according to the FCFS. In the same way, there are buffering queues for each output port in the switch, where all queues in the switch follows the FCFS scheduling [5]. Generally a full duplex Ethernet switch model of n ports is given below [5]: Figure 4 Conceptual model of a full duplex Ethernet switch All N physical ports are in full duplex mode, which has n input ports and n output ports. Here, switch is running with 2n times of the line speed, where the switches are wired speed switches, which provides the bit rate of 100mbps with output buffering. Arrival packets can be classified and Switched according to the TDMA principle. The switching program periodically polls the n input ports according to the TDMA, and the arrived packets can be transmitted according to the transmission buffer that is also called as output buffer. Buffering and buffering delay exists in the switch. Output buffering overflow can be minimized by using shared memory queuing. The worst case time corresponds to the sum of the transmission time and interframe spacing time, where worst case response can be evaluated by using fixed priority scheduling. [5] Here we are considering the star topology of Switched Ethernet and nodes. Realtime traffic guarantees can be achieved by adding RT layer to the Switched Ethernet and nodes. Nodes are connected via the switches, which establishes the logical line of communication between two points in each frame and the nodes are connected virtually. In this network, nodes have the capability of controlling the traffic from the nodes, using the FCFS algorithm at the source node and the switch [9]. Frame Transition Model: In a fully Switched Ethernet there is only one switch per switch port, using full duplex switches to delay can be minimized by decreasing the 13

28 message buffering at maximum [8]. A frame traveling through switches with out any buffering has the minimum delay. Total delay introduced by a switch consists of switching latency, frame forwarding latency, and the buffering delay [8]. Policy of service used in the input queue is FIFO. The frames transition for all the input queue is done towards CPU. The objective of the CPU is route frames towards the output according to the routing table. Here also FIFO policy is used. In the output queues, routing is done towards a port of another switch or system [8]. Frame transition in the Switched Ethernet with output buffering is shown in the figure below. Figure 5 Model of Frame transition in an Ethernet switch 14

29 4 Real time communication 5.1 Real time channel Establishment Before real time traffic is delivered, real time channel must be established. Real time channel establishment consists of request and acknowledgement communication where the source node, destination node and the switch agree on the channel establishment. When a node wants to establish an RT channel, it ss a request frame to the switch. Switch performs the admission control and ss a response frame. If the switch finds any feasible link then it forwards the request frame to the destination node [9]. Destination node responds with a response frame to the switch and intimates whether the channel is accepted or not. After taking the response from destination node, switch forwards the response frame to the source node. If the switch did not find any feasible schedule then the response frame is not forwarded to the destination, instead switch ss that requested frame to the source node by intimating about the rejection. After the channel is established, nodes use that channel. Both the switch and nodes have RT layer which shapes the real time traffic on the channel. When an RT channel is established, the network guarantees to deliver each generated message with a bounded delay. Following figure shows the establishment of real time channel [9]. Figure 6 Establishment of real time Channel 5.2 Real time Analysis The to delay bound is one of the user defined QoS parameter. By adding real time channels dynamically, time constraints can be guaranteed, which are characterized by certain parameters. A real time channel is the unidirectional virtual link between the two nodes, which provides the real time guarantee for hard real time traffic. A real time channel with index i is characterized as [10]: {Sourcei, Dest i, T period, i, C i, T deadline, i } Where Source i indicates the source node, Dest i indicates the destination node, Period i is the period of the data generation, C i is the amount of data per period (capacity), T deadline, i defines 15

30 relative deadlines used for the admission control. T period, i and T deadline, i are expressed in microseconds, C i is expressed as the number of bytes of the complete message. One message can have several Ethernet frames. Before performing feasibility analysis, we need to calculate the total amount of traffic per period including data and header for each logical real-time channel. The length of the Ethernet frame T ef is 1526 bytes, if only Ethernet header is considered. Minimum length of the data filed in an Ethernet frame T mind is 10 bytes. Maximum length of the data field in an Ethernet frame T maxd is 1492 byes. If the data field is not less than 38 bytes then the length of the header T h is 34 bytes. Otherwise a pad field is added to meet the minimum frame size required by IEEE standard [10]. The amount of traffic including the data and header per period for each real time channel i is expressed as number of bytes C i, where C i can be derived as [27]: capi Tef T max d + ((cap i mod T maxd )) +T h If ((cap i mod T maxd )) T mind; C i = capi Tef T max d +72 If 0< ((cap i mod T maxd )) <T mind ; capi Tef T d max If ((cap i mod T maxd )) = 0; 5.3 Real time Characteristics A. Switch architecture: Functionally, switch is considered as a multi port bridge. Switch is more powerful than a multi port bridge due to its ASIC based hardware architecture and ultra rapid simultaneous multiple access memory. Switch can have an IP address and as many as MAC addresses. Ethernet switch provides one collision domain per port. Collisions can be completely eliminated, if it is a full duplex switch. There is no standard for Ethernet switch. There exist many kinds of internal architectures called switch fabrics. Three main architectures are matrix, bus and shared memory [8]. Matrix based switch fabrics, also called as crossbars. This type of architecture has great number of ports that neither bus nor shared memory can achieve. But matrix based architecture is more problematic, when broad cast or multicast or unicast occur simultaneously. When a broadcast or multicast taking place, no unicast can be transmitted. In bus based architecture it has very high speed core bus, shared by input and output ports. Bus architecture is based on TDMA access control, when compared to the matrix architecture, bus based architecture naturally supports broad cast traffic. This is an advantage for bus based architecture. But the problem with this architecture is when many inputs are forwarded to the same output port then out put buffer overflows. 16

31 Third widely used architecture is shared memory architecture which is based on ultra rapid simultaneous multiple access memory, shared by all ports. Here data entered into the switch is stored in memory. And the data can be forwarded by ASIC engine which looks up the MAC destination address in the forwarding table, finds it and ss to the correct output port. To avoid the head of line blocking, output buffering is used instead of input buffering. Using shared memory, output buffering can be minimized since the buffer size is dynamically adjusted. Buffer over flow can be reduced when all output ports, share the same global memory compared to per port queuing. All recent Ethernet switches are announced to be working with wire speed and non blocking. In wire speed switches, all ports of a switch can simultaneously transmit or receive at their full bit rates. This requires the switch fabric to operate at a bit rate that equals to the aggregate speeds of all the ports. A switch is non blocking when it can forward a message to the destination port as long as the port is free, while blocking one may not forward the message to the destination although the destination port is free. B. Full-Duplex: In a full duplex mode, both the ser and receiver can transmit information simultaneously. It is only possible when a single LAN station is connected to an Ethernet Switched port. Shared hubs do not support full duplex mode. Server performance can be improved in full duplex mode. Bandwidth is also improved because both transmission and reception can occur simultaneously. The other factor, signal attenuation has the major effect on the cabling scheme. Attenuation is nothing but weakening of the signal and this is a crucial factor in LAN design. Full duplex is primarily useful to ext the distance of an Ethernet, where the limitation length is due to propagation delay not attenuation. Full duplex doubles the Ethernet speed. Full duplex cabling scheme is a point to point, where all clients communicate with the server [8]. C. Bit rate improvement: A switch can support different bit ports: 10 Mbps, 100Mbps, 1Gbps and even 10 Gbps. Switch can adopt to the bit rates of their ports according to the connected equipment. Scalable bandwidth can be achieved by switch. Higher the bit rate, shorter the delay of communication is achieved [8]. D. Forwarding Model: A switch decreases the data transmission delay by forwarding a frame before it is completely received by the switch [8]. Normal Switch Model: 1) Store & Forward mode: It waits until the reception of the complete frame and stores it before forwarding it to the output port that corresponds to its destination address. Through CRC calculation, error can be detected and causes long frame transmission latencies. 2) Cut through mode: It waits until the reception of the destination address and then forwards it to the output port. This reduces the frame forwarding latency but this model some times transmits erroneous frames, since CRC is not calculated before forwarding. 3) Fragment free mode: This mode eliminates the small fragments caused by collisions. This mode is same as cut through mode but waits until the 64 th byte before forwarding. 4) Auto select mode: This mode is the integration of store and forward, cut through and fragment free mode. This mode begins with store and forward and if no error occurs then it shifts to fragment free. If always no errors then it shifts to cut through. 17

32 E. Congestion control: Switched Ethernet is a packet switching technology that reduces the congestion caused by CSMA/CD. Congestion can be reduced by dynamically routing frames across network. A switch can achieve congestion control by preventing buffer over flow. Two techniques can be used for this: 1. Back pressure: In a full duplex mode, the switch can simulate a collision, if the amount packets received on the port is more than the switch can handle. 2. IEEE 802.3x pause command : A pause command can be s by the switch to the ser if the port in full duplex mode receives more traffic than it can handle [8]. 5.4 Feasibility Analysis Feasibility test can be done by the admission controller. Feasibility test can be implemented in an node or switch. In our thesis we discuss the feasibility analysis, only using FCFS priority queuing in both the source nodes and switch. In our work, feasibility is done by the switch. Feasibility test includes two steps, utilization constraint and delay constraint. Utilization for the physical link, U should be less than or equal to the 100% [10]. Ci U = 100% T i period, i The delay bound constraint is the sum of the worst case delay in the source node T d1,i and the worst case delay in the switch T d2,i. It should be less than or equal to deadline T deadline, i. T d1, i + T d2, i T deadline, i Worst case delay in the source node is the same for those logical real time channels who have the same source, this can be expressed as: T d1, i = T d1, j = D node n if Source i = Source j = n Where D node, n is the worst case delay, for the packets from the source node n. Worst case delay in the source node can be calculated when all real time channels start their period at the same time and maximum capacity of each real time channels is used [10]. D node n calculated as D node n = i C i Lr Where Lr k is the data bit rate of the link. k Worst case delay in the switch is the same for those logical real time channels who have the same destination, this can be expressed as T d2, i = Td2, j = Dport k if Dest i = Dest j = k. Where D port k is the worst case delay in the switch for the packets through output port k and D port k can be calculated by the following algorithm [10]: 18

33 1. Initialization Input (Nchs, Nports, k); T=0;Q=0; D portk = 0; plink = zeros[1...nports] 2 Find out D portk is the worst case delay in the switch for the packets through out put port k. 2.1 Find out how many packets on the way for each incoming physical link for j=1.nchs If (Dest j = = k) && mod (t, T period, j ) = =0 then Plink[source j ] = Plink[source j ] + C j ; if for 2.2 S packets to the output buffer in the switch at most one packet from each physical link for j = 1.Nports If Plink j >0 then Plink j = Plink j -1; Q = Q + 1; if End for 2.3 Remove one packet from the output buffer If t>0 then Q = Q 1; if 2.4 Keep track of worst case queuing delay If Q> D port k then D port k = Q if 3 If the output buffer is not empty then repeat step 2; 4 Return (D port k ); Table 1 Algorithm 1 Dportk calculation for fixed size frames and FCFS queuing In the algorithm Nchs denotes the number of logical real time channels in the system and Nports denotes the number of out put ports in the switch. After establishing the real time logical channel we have to generate the traffic in the source nodes. We have to calculate the average delay for the total number of transmitted packets. 19

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35 5 Approach To support the real time communication over Switched Ethernet and to increase the performance, an Ethernet switch and nodes combines the several short messages into an Ethernet frame. The generated traffic must be checked by the admission control in the switch whether they can be accepted into the network or not. We have employed FCFS scheduling in the source node and the switch, as FCFS scheduling guarantees real time transmission. FCFS priority queuing in the switch gives hardware support for switching at wire speed and limits the switch processing overhead [15]. End to delay bounds can be guaranteed by the feasibility analysis. Steps for building a simulator: At first traffic should be generated. The admission controller located in the switch performs the feasibility analysis. The FCFS scheduling algorithm is used at both the source nodes and the switch. In order to put the generated traffic in a queue at each node, the maximum number of packets has to be checked at every time point t. For each input packet, we need to observe average delay of the generated traffic for a successful transmission of the generated packets. Assumptions: Some assumptions are made for performing the simulation. The network architecture is star topology. Each incoming frame has its destination address. The simulation is done by taking hard real time traffic into consideration. We consider fixed size frames for simulation. 21

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37 7. Flowchart to calculate average delay Figure 7 Flowchart 1 Average delay calculation for fixed sized packets and FCFS queuing 23

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39 8. Description of the algorithm The calculation of average delay is shown in flowchart 1.We use Nch to denote the number of channels, N denotes the number of nodes, Nports denotes the number of ports in the switch, simt denotes the simulation time and max queue length denotes the maximum size of the queue. In the simulation, each real time channels having randomly generated source and destination nodes. We have to generate the period, deadline and capacity for each real time channel. In the simulation logical real time channels are added one by one and checked whether the channels are accepted or not. This can be done by the admission control. To check whether the channel is accepted or not, we have to check the utilization constraint and delay bound constraint. If the utilization constraint and delay bound constraint are met, the new logical real time channel can be accepted. If the channels are accepted then keep track of total number of accepted channels. If the channel is accepted and the period starts, then generate the packets in the source nodes and the generated traffic should be kept in the FCFS sorted queue of the source nodes. In our simulation we used FCFS priority queuing in both the source nodes and switch. When the packet is generated, then keep track of the time when they were generated and store the packets in the queue according to the position. If the queue is full don t s any more packets. After this, move at most one packet, which is in the first position of the source node queue to the switch. After this, move the packet from the switch to its corresponding destination node. Then calculate the delay for each transmitted packet. After the transmission of all packets for a given simulation time, calculate the average delay. Average delay is the sum of the delay of all packets divide by total number of transmitted packets. Delay for each packet is given by finish time minus start time. Finish time is the time at which the packet reaches its destination and start time is the generated time of the packet. We have done this up to large simulation time to observe the average delay. 25