Applied Data Communication Lecture 14

Transcription

1 Applied Data Communication Lecture 14 Character oriented Data Link Character-oriented data link control Asynchronous Synchronous Kristjan Sillmann reaalajasüsteemide õppetool TTÜ automaatikainstituut character-oriented Xmodem Zmodem Kermit Others bit-oriented character-oriented example: Binary Synchronous 1 2 Asynchronous File Transfer Used on: Point-to-point asynchronous circuits Typically over phone lines via modem (ex: Bulletin Board System) Computer to computer for transfer of data files Designed to provide error detection and correction facilities for reliable file transfer. Large amount of data to be transmitted i.e. makes sense to group data together into blocks to transmit at same time Asynchronous File Transfer Characteristics of async. file transfer : Group data (binary or text) into blocks to be transmitted (rather than sending character by character), Supply error detection (CRC) symbol(s) with each block Implement ARQ scheme for error correction (resend the blocks which had errors) Xmodem Takes data being transmitted and breaks into blocks Each block has: Start-of-header (SOH) character 1-byte block number 128 bytes of data 1-byte checksum for error checking 3 4

2 Start of Header Xmodem One of the oldest async file transfer Uses stop-and-wait ARQ. Advantages Universally available Copes well with noisy lines Disadvantages Small packet size = high overheads = low efficiency No batch facility SOH Block # Block # compl. (128 bytes) Checksum 5 Xmodem frame With asynchronous, characters are sent one after another, with variable gaps between them. SOH One character header Data: 128 bytes CRC 6 extensions of Xmodem extensions of Xmodem Xmodem-CRC improves error checking by using CRC-8 (instead of checksum) Xmodem-1K: Xmodem-CRC + increased efficiency by using 1024 bytes for data Zmodem: Newer protocol than Xmodem Incorporates features of several Uses CRC-32 with continuous ARQ (sliding window) Dynamically adjusts packet size according to communication circuit conditions Usually Zmodem is preferred to Xmodem. 7 ZModem Advantages Large packet size = low overheads = high efficiency Batch Transfers Auto receive Disadvantages Relatively susceptible to noisy lines Not available in all comms packages Kermit: Supports different packet data sizes and error detection methods Typically uses 1 KByte packets with CRC-24 size adjusted during transmission to optimize efficiency 8

3 Data Link Protocols Protocol Size Error Detection Retransmission Medium Access Asynchronous Transmission 1 Parity Continuous ARQ Full Duplex File Transfer Protocols XMODEM bit Checksum Stop-and-wait ARQ Controlled Access XMODEM-CRC bit CRC Stop-and-wait ARQ Controlled Access XMODEM-1K bit CRC Stop-and-wait ARQ Controlled Access ZMODEM * 32-bit CRC Continuous ARQ Controlled Access KERMIT * 24-bit CRC Continuous ARQ Controlled Access Synchronous Protocols SDLC * 16-bit CRC Continuous ARQ Controlled Access HDLC * 16-bit CRC Continuous ARQ Controlled Access Token Ring * 32-bit CRC Stop-and wait ARQ Controlled Access Ethernet * 32-bit CRC Stop-and wait ARQ Contention PPP * 16-bit CRC Continuous ARQ Full Duplex * Varies depending upon the message length. 9 Communications (BSC or BiSync): Supports EBCDIC, ASCII, and Transcode (rare 6-bit code) Uses Stop-and-Wait ARQ (- the major drawback of BSC) Error Checking A block check character (BCC) in data frame contains error checking information CRC is the most popular method for calculating the BCC The transmission is based on the sending of special characters to control the data link. 10 Control Function ASCII Code EBCDIC Code Character Hex * Character Hex ACK 0 DLE, 0 10, 30 DLE, '70' 10, 70 ACK 1 DLE, 1 10, 31 DLE, / 10, 61 DLE DLE 10 DLE 10 ENQ ENQ 05 ENQ 2D EOT EOT 04 EOT 37 ETB ETB 17 ETB 26 ETX ETX 03 ETX 03 ITB US 1F IUS 1F NAK NAK 15 NAK 3D PAD DEL FF 'FF' FF SOH SOH 01 SOH 01 STX STX 02 STX 02 SYN SYN 16 SYN 32 SYN (Synchronous Idle) Provides the hardware recognizable bit pattern rquired to establish character synchronization at the receiving adapter ENQ (Enquiry) Recognized as a request for a response, or a bid for line control. In some cases it may be used to signify an abnormal end of text or message 'abort'. SOH (Start of Header) Indicates the inclusion of auxiliary data preceding the message text. WACK DLE, ; 10, 3B DLE,, 10, 6B 11 12

4 STX (Start of Text) Indicates the beginning of data in a block. STX may be preceded by a header. Directly behind the STX is the first character of the data field. NAK (Negative Acknowledgement) Indicates that there was an error in a data block. Also used as a response to a bid for line control to indicate a 'Not Ready' condition. DLE (Data Link Escape) Multiple usage as a control character modifier. ETB (End of Transmission Block) Indicates an end of data block, but more will follow. Is used to instruct the receiving unit to perform error checking and acknowledge. ETX (End of Text) Same as ETB, only no more blocks will follow. ITB (End of Intermediate Transmission Block) Same as ETB, except that the receiving statio will not acknowledge after the error checking. EOT (End of Transmission) Indicates that a station has no data to transmit frame formats ACK0, ACK1 (positive Acknowledgements) ACK0 ackowledges 'even' numbered blocks (and as a positive response to a line bid) and ACK1 acknowledges 'odd' numbered blocks. DLE EOT (Mandatory Disconnect) Used on a dial-up line to indicate that the dialing unit is hangup and an instruction for the receiving unit to do the same. 15 Message blocks of BiSync have the following format: Control Frame Data Frame Every package that is transmitted is packed between an STX and an ETX character, followed by one (LRC-check) or two (CRC-check) BCCs (Block Check Character). After this (and the check on the receiving side) the receiver will send an ACK. 16

5 synchronization The beginning of a transmission can be started with a leading PAD ( ) character. What must follow are at least 2 SYN characters. For bit synchronization: uses PAD character before each block of characters (frame). For character synchronization: uses two or more SYN ( ) characters before each frame. 17 Receiver detects SYN character Direction of transmission SYN SYN SYN Receiver in character synchronization Time Receiver enters hunt mode STX Frame contents Once the receiver has obtained Bit synchronisation it enters what is known as the Hunt Mode. It starts to interpret the received bit stream in a window of eight bits as each new bit is received. In this way it checks whether the last eight bits were equal to a SYN character (to find the start of next byte) 18 synchronization transmission modes Once in Byte Synchronization, the receiver checks for a STX or SOH character, indicating start of text/header. Once synchronisation has been achieved, SYN ( ) characters will be continuously sent when data is not being transmitted to ensure that the sender and receiver remain synchronised at both bit and byte level. The trailing PAD character assures that the transmission is held long enough for the last critical character to have been received (usually an "FF all 1 s character). 19 The sending of Before information is send it might be necessary to send a couple of that sets the device in the right mode, like a printer. To indicate that this 'special' information is being send it is preceded by a SOH character. The sending of data Next to the sending of plain ASCII or EBCDIC text, there is also the possibility of sending binary data. To realize this, data is sent in a so called 'transparency-mode'. Which means that every transmission control character is preceded by a DLE character. 20

6 point to point control frames Control Frames Question: PAD SYN SYN ENQ PAD (can I send now?) Answer: PAD SYN SYN ACK0 PAD (if ready to receive) PAD SYN SYN NAK PAD (if not ready to receive) PAD SYN SYN WACK PAD (temporarily busy, try later). The requesting station will reply with another ENQ After data is sent, line is freed up by sender with PAD SYN SYN EOT PAD Three seconds later, either party can ENQ multipoint control frames Polling & Selecting: master sends PAD SYN SYN (addr) ENQ PAD addr is address of terminal or cluster controller The polled/selected station can respond several ways: If anything waiting to send to host: PAD SYN SYN SOH (Header) STX (Data) ETX BCC BCC PAD PAD SYN SYN EOT PAD (Finished sending) PAD SYN SYN STX ENQ PAD (give me 2 seconds to get ready and then I'll start sending, but don't give the line to someone else) multipoint control frames The polled/selected station can respond several ways: If no data waiting (in the station) to send to host: PAD SYN SYN ACK0 PAD (ready to receive) PAD SYN SYN NAK PAD (not ready to receive) PAD SYN SYN WACK PAD (temporarily busy, try again later) other transmission controls Receiver delay If the slave can't cope with the speed of the master is can send a WACK to delay the sending of data to prevent buffer overflows. Sender delay If the master station can't present it's data as fast as the receiving end can it uses a TTD (Temporary Text Delay) character to hold the line (to overcome the non-activity time-out of the slave). Actually a TTD is an illegal sequence of characters (STX, ENQ) on which the slave responds with a NAK

7 other transmission controls Abort To abort a block that is already partially sent, the master sends ENQ, on this 'block abort' the slave responds with a NAK and drops all already received data (from this block). If the master sends a EOT before a ETX is received by the station it means a 'station abort. The slave then has a possibility to become master, otherwise the connection is terminated. 25