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Credit Determination of Fibre Channel in Avionics Environment View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Chinese Journal of Aeronautics Chinese Journal of Aeronautics 20(2007) 247-252 www.elsevier.com/locate/cja Credit Determination of Fibre Channel in Avionics Environment LIN Qiang*, XIONG Hua-gang, ZHANG Qi-shan School of Electronic and Information Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China Received 8 May 2006; accepted 16 November 2006 Abstract Fibre channel (FC) is the main candidate architecture for “unified network”. Flow control deals with the problem in which a de- vice receives frames faster than it can process them. Credit is an important service parameter for fibre channel flow control. Configuring the credit reasonably can avoid buffer overflow in nodes. This paper derives the mathematic relationships among credit, bandwidth and message sets under real-time condition according as three main topologies of fibre channel, and proposes the credit determination and the optimal credit for typical message sets. This study is based on the features of hard real-time communications in avionics environ- ment. Keywords: fibre channel; topology; login; credit; hard real-time condition 1 Introduction* ple topologies. Candidate COTS network standards for “unified network” must provide sufficient For more than two decades, MIL-STD-1553B bandwidth, low latency, a range of media and to- has served as the baseline standard for the integra- pologies, the need for guaranteed delivery, time dis- tion of the “federated” avionics architecture. Feder- tribution, and the need for broadcast and multicast ated systems use multiple buses in the avionics ar- services. chitecture and bring both technical and cost prob- One such candidate is fibre channel (FC). FC is lems. In the 1980’s, seven interconnection systems a new serial communication protocol approved by (HSDB, TM, PI, DN, SDDN, VDDN and 1553B) ANSI. It is widely used in the domains of network were proposed in “integrated” avionics architectures and high speed bus gradually with its good-com- to meet different and more challenging requirements patibility, high-speed and long-distance. FC defines of data communication. An avionics “unified net- three topologies, namely point-to-point, arbitrated work” takes the place of almost all the buses of loop, and fabric. networks in “integrated” architecture now to sim- The original FC protocol cannot be directly plify the design, eliminate the multiple networks, exploited for avionics real-time systems because its and improve the performance. The objectives of the extension ensures the delivery of messages rather unified network include the goal of a single protocol than guaranteeing a deterministic latency. Sugges- running over either the single topology or the multi- tions have been made by FC working group mem- bers on how to extend the FC to support the *Corresponding author. Tel.: +86-10-81414980. real-time applications in avionics environment E-mail address: [email protected] Foundation item: National Natural Science Foundation of China (FC-AE), but the problem of providing real-time (10477005) · 248 · LIN Qiang et al. / Chinese Journal of Aeronautics 20(2007) 247-252 capability in FC network for general purpose is not sages do not have any real-time requirement. In solved satisfyingly and remains to be answered. avionics systems, the primary aim of the real-time Flow control is the FC-2 level control process communication system is to guarantee timely the to pace the flow of frames between Nx_Ports, an delivery of synchronous messages. Nx_Port and the fabric and within the fabric to pre- It is assumed that there are n streams of real- vent overrun at the receiver. FC flow control is time messages marked S1, S2, ···, Sn in the network based on the concept of credit. Credit is an impor- which form a message set M, i.e. tant service parameter in FC, which is defined as the M = {S1, S2, ···, Sn} (1) number of receive buffers allocated to transmitting The message streams S are described as fol- FC_Port. There are two types of credits used in flow i lows: control: one is end-to-end credit(EE_Credit) be- ① Message inter-arrival time P : the message tween communicating Nx_Ports, the other is buffer- i S generating period, for non periodic message, it to-buffer credit(BB_Credit) between adjacent FC_ i represents the minimum inter-arrival time of the Ports. Configuring the credit reasonably can avoid message. buffer overflow in receive buffers of each node and ② Message length C : the amount of time to improve the reliability of the network. The number i transmit the message S . of receive buffers is closely relative to the message i ③ Message deadline D : the maximum al- transmission bandwidth allocation. The standard of i lowed message delay for S , from its arrival at the FC framing and signaling only provides the default i source to be transmitted to the destination. login value of credit[1]. ④ Minimum message deadline Dmin: the mi- 2 Hard Real-time Model of FC nimum of message deadline Di. The requirement of system designment is Avionics is one of the typical hard real-time systems which are required to complete their works Dmin≤Di≤Pi [2] and deliver their services on a timely basis. The pe- Each message stream Si can be represented as riodic task model is a well-known deterministic Si = (Ci , Pi , Di) (2) workload. With its various extensions, the model characterizes accurately many traditional hard real- Let Xi(t) denote the minimum amount of trans- time applications. We adopt a simple operational mission time available for the message stream on definition: the job is a hard real-time job, the user node i during any time interval of length t. Any requires the validation that the system always meets message in the message set M should have enough the timing constraints. amount of transmission time before its deadline, that is, for message stream Si: 2.1 Message model Xi(Di)≥Ci (i = 1, 2, ···, n) (3) One of the results of avionics system design is the forming of message streams among sub-systems 2.2 Network model or modules. The messages are regulated by interface The point-to-point topology consists of two control document (ICD) specification and can be and only two FC devices connected directly together. divided into two classes: synchronous and asyn- The transmit fibre of one device goes to the receive chronous. The example of synchronous message in fibre of the other device, and vice versa. There is no avionics may be messages from radar or other sen- sharing of the media, which allows the devices to sors, control and status message etc. Synchronous enjoy the total bandwidth of the link. Every node in messages are periodically generated and subjected arbitrated loop is connected to the network via to real-time constraints while asynchronous mes- L_Port which is assigned unique local loop physical LIN Qiang et al. / Chinese Journal of Aeronautics 20(2007) 247-252 · 249 · address during initialization. When a node attempts 3 Bandwidth Allocation for Shared Media to access the network, it must win an arbitration. Environment The arbitration relies on priority of each node. To The FC-AL protocol describes the algorithm on prevent that the lower priority L_Ports cannot gain how to solve the problem of MAC in the shared access to the loop, the fibre channel arbitrated loop media environment. Every node in FC-AL is con- (FC-AL) standard proposes access fairness algo- nected to the network via L_Port which is assigned rithm which sets up an access window in which all unique local loop physical address during initializa- L_Ports are given an opportunity to arbitrate and tion. FC-AL standard proposes access fairness algo- win access to the loop. Switched fabric is used to rithm which sets up an access window in which all connect many (224) devices in a cross-point L_Ports are given an opportunity to arbitrate and switched configuration[3]. The benefit of this topol- win access to the loop. When all L_Ports have an ogy is that many devices can communicate at the opportunity to access the loop once, a new window same time[2]. is started. An L_Port may arbitrate again and even- Flow control is managed using end-to-end tually win access to the loop in the new access credit, end-to-end credit_CNT, ACK_1, buffer- window. If a port has won an arbitration and ac- to-buffer credit, buffer-to-buffer credit_CNT, and cessed the network, it cannot arbitrate for loop ac- R_RDY along with other frames. The buffer to cess again until all other ports have had an opportu- buffer flow control model for FC is shown as nity to access the loop. Only when the network be- Fig.1[1]. The ideas used for BB _Credit in this paper comes idle can the next access window start[4]. are also suitable for the EE_Credit determination. Let Dmin denote the upper-bound of the access During login, a new node exchanges service fairness window, that is, it will not be exceeded with parameters including credit with node to which it total message transmission time and system over- intends to communicate. Node service parameters heads. Let θ denotes the overhead time, one has are then saved and used whenever the node initiates n ≤ a communication. ∑ fi Dmin −θ (4) i=1 According to the characteristics of avionics, it During any time interval of length t, the mini- is assumed that both the transmitting and receiving mum amount of time available for node i in FC-AL network layers are always ready, the rate at which can be represented as data are transmitted is fixed and the processing time ⎧00<≤ t Dmin accords for hard real-time requirement, and the ⎪ ⎪(1)mftmii−+− iDmin communication channel is error free.
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