Bit and Frame Synchronization Techniques Martin Probst , Lars Trieloff Hasso-Plattner-Institute for Software System Engineering [email protected], [email protected] All graphics in this paper are created by the authors unless otherwise stated. Bit and Frame Synchronization techniques are used in order to ensure that signals transmitted from one participant of the communication can be correctly decoded by the receiver. To achieve this goal, certain timing information must be passed to the receiver such as where communication units begin and end.In this article various bit and frame synchronization techniques are presented as well as underlying encoding schemes and applications using the described techniques. Communication Networks Seminar 1-1 Bit and Frame Synchronization Table of Contents 1. Introduction .......................................................................................................... 3 2. Fundamentals of Binary Data Transmission ..................................................... 4 2.1. Binary Data Transfer .................................................................................... 4 2.2. The Need for Synchronization ...................................................................... 6 2.3. Modes of Transfer ........................................................................................ 8 2.4. Characteristics of Transmission Media ........................................................ 8 2.4.1. Conductor Cable ................................................................................ 8 2.4.2. Optical transmission ........................................................................... 9 2.4.3. Electromagnetic Wave Transmission ............................................... 10 2.5. Modulation Techniques .............................................................................. 11 2.5.1. Amplitude Shift Keying ..................................................................... 11 2.5.2. Frequency Shift Keying .................................................................... 12 2.5.3. Phase Shift Keying ........................................................................... 13 2.5.4. Quantization ..................................................................................... 14 2.6. Conclusion ................................................................................................. 17 3. Asynchronous Data Transmission ................................................................... 17 3.1. Return to Zero Signaling ............................................................................ 18 3.2. Non-Return to Zero Signaling .................................................................... 19 3.3. Character or Byte Synchronization ............................................................ 20 3.4. Start- and Stop Bits .................................................................................... 21 3.5. Conclusion ................................................................................................. 22 4. Synchronous Data Transmission ..................................................................... 22 4.1. Manchester-Encoding ................................................................................ 22 4.2. Bit Synchronization .................................................................................... 24 4.2.1. Clock encoding ................................................................................ 24 4.2.2. Digital Phase-Locked Loop .............................................................. 25 4.2.3. Advanced DPLL use ........................................................................ 29 4.3. Conclusion ................................................................................................. 29 5. Frame Synchronization ...................................................................................... 29 5.1. Motivation ................................................................................................... 29 5.2. Requirements ............................................................................................. 29 5.3. Methods ..................................................................................................... 30 5.3.1. Time gap synchronization ................................................................ 30 5.3.2. Start & End Flags ............................................................................. 30 5.3.3. Packet Length Indication .................................................................. 38 5.3.4. Coding Violations ............................................................................. 38 5.4. Conclusion ................................................................................................. 39 6. Real World Applications .................................................................................... 39 6.1. CSMA/CD (Ethernet) .................................................................................. 39 1-2 Communication Networks Seminar 1 INTRODUCTION 6.1.1. Bit synchronization ........................................................................... 39 6.1.2. Frame synchronization ..................................................................... 40 6.1.3. Error Detection ................................................................................. 41 6.2. FDDI - Fiber Distributed Data Interface ...................................................... 41 6.2.1. Bit synchronization ........................................................................... 42 6.2.2. Frame synchronization ..................................................................... 43 6.3. Conclusion ................................................................................................. 44 7. Conclusion .......................................................................................................... 44 Glossary ................................................................................................................... 44 Bibliography .............................................................................................................. 46 1 Introduction If one wants to transfer binary information over a physical medium in oder to establish communication between two computer systems there are several problems to be solved. Among the most basic tasks are choosing the best available transmission media, agreeing on a unambiguous encoding scheme and verifying the integrity of the received data. Bit and frame synchronization techniques are among the most basic problems to be handled at the physical layer. In this paper we are going to describe fundamental problems and their solution in the context of binary data transmission at a very low level. In the next sections we will first dicsuss the basic prerequisites of binary data transmis- sion and later on specific problems of encoding schemes, synchronization issues and their solutions. Digital data will always differentiate between two distinct values 0 or 1, but every physical transmission medium will provide an analogous signal, thus a conversion scheme between digital data and their physical representation must be defined. We will take a look at the most common transmission media and signaling scheme in the introductory section of this article. The next section will feature several basic encoding principles that are used to ensure that the transmitter can identify the borders between two adjacent bits on the transmis- sion medium. This section will also include a description of two simple encoding schemes used for translating bits into physical signals and vice versa. In the fourth section we are going to cover the basics of ensuring synchronization at the level of single bits, which is needed when the system clocks of sender and receiver must be in synchronism. As different systems use different quartz clocks, which tend to run ahead or late it must be possible to set the receiver©s clock to match the transmit- ter©s signal. Using atomic clocks is no solution, because these devices are very expensive and difficult to maintain for a non-scientist. If you try to use the broadcasted clock signal of an atomic clock, you will note that this signal will propagate through space with a limited speed and will most possibly not arrive at the transmitter and at the receiver in the same instant. Additionally the clocks signal must be encoded somehow, and this was exactly the starting point of this problem. Communication Networks Seminar 1-3 Bit and Frame Synchronization We will show how the clock signal can be encoded in the data stream or transmitted on top of it and by which circuits the receiver system can extract the clock signal from the data signal. The fifth section discusses the problems encountered when data is transmitted in bigger blocks (typically about one thousand bytes) called frames or packets. As a re- ceiver needs to distinguish between the single frames a method of frame synchronization is needed. We will explain the common techniques of start & end flags (which brings up the problem of bit or byte stuffing explained thereafter), packet length indication and coding violations. Bit and byte stuffing are techniques of in-band-signalling. With them it is possible to use reserved words or characters on the transmission medium. In the final section, we are going to compare different real-world applications of the principles and techniques described in the sections before. 2 Fundamentals of Binary Data Trans- mission