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Message Delineation. Identify the start and end of messages by:. Message Delineation by Flags. Examples of data link protocols using Start and Stop Flags for message delineation: Asynchronous Transmission Protocols Synchronous Data Link Control Protocol (SDLC)
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Message Delineation • Identify the start and end of messages by:
Message Delineation by Flags • Examples of data link protocols using Start and Stop Flags for message delineation: • Asynchronous Transmission Protocols • Synchronous Data Link Control Protocol (SDLC) • High-level Data Link Control Protocol (HDLC) • Serial line Internet Protocol (SLIP) • Point-to-Point Protocol (PPP)
Asynchronous Transmission Each character is transmitted independently of all other characters. A start bit (0), and a stop bit (1) are added to each character. If you use VT100 protocol, or connect to a UNIX or Linux computer using Telnet, you may be using asynchronous transmission.
Synchronous Data Link Control (SDLC) Protocol • SDLC is a mainframe protocol developed by IBM in 1972 • SDLC and derivatives are still being used • Like many other DL protocols, SDLC has a control field included in its frame • What is a control field?
Control Field • Frame types: • Information Frame (e.g., used for the transfer and reception of messages, frame numbering of contiguous frames) • Supervisory Frame (e.g., Used to transmit acknowledgements)
SDLC Packet Format SDLC format Flag Address Control Message Frame Flag 8 bits variable 8 bits variable check 8 bits sequence 16 or 32 bits
High-level Data Link Control (HDLC) Protocol • HDLC is a formal standard developed by ISO, and is essentially the same as SDLC except for few additional features, some of which include: • Having longer address and control fields • Having a larger sliding window for supporting Continuous ARQ
Point-to-Point Protocol (PPP) • PPP was developed in 1990s as a replacement for Serial Line Internet Protocol (SLIP) • Commonly used to dial up from home computers to ISP • It includes error detection (CRC-16) • Its header includes a protocol field that specifies the network layer protocol (e.g., TCP/IP, IPX/SPX) • Message may be up to 1500 bytes in length
SLIP and PPP Packets - Layout SLIP packet layout End Message End 1 byte variable 1 byte PPP packet layout Flag Address Control Protocol Message CRC-16 Flag 1 byte 1 byte 1 byte 2 bytes variable 2 bytes 1 byte
Disadvantages of Using Flags for Message Delineation • Problem: • Solution: • Transparency Problem • What is meant by the transparency problem? • What is the solution to the transparency problem? • Example: SDLC (Flag pattern is 01111110)
Other Message Delineation Techniques • Special Signals: • Token ring uses special signals to delineate the start and end of messages • The signals used to start and end these messages will violate the rules of regular transmission • Max length of frame is 4500 bytes • Specifying size of message upfront: • Example: Ethernet • Max length of frame is 1492 bytes
Token Ring and Ethernet Frames Token Ring format Start Frame Destination Source Message End delimiter control address address variable delimiter Access Frame control check sequence1 byte 4 bytes Ethernet format Destination Source Length Message CRC-32 address address 2 bytes variable 4 bytes 6 bytes 6 bytes
Data Link Protocols • Asynchronous and Synchronous Protocols • File Transfer Protocols • XMODEM • XMODEM-CRC (CRC-8) • XMODEM-1K (CRC+1K blocks) • YMODEM(CRC-16) • ZMODEM (CRC-32) • KERMIT (CRC-24) • What are some factors to consider before selecting a data link protocol?
Transmission Efficiency • Each protocol has both information bits (to convey the user’s message) and overhead/control bits (for error checking, marking the start and end of characters or packets etc.). • Transmission efficiency (T.E.) is defined as the total number of information bits divided by the total number of bits in transmission. T.E. is typically expressed as a percentage value. • Participation Exercise # 1: What is the transmission efficiency of asynchronous transmission of a character represented by a 7-bit ASCII code that is enclosed by one start, one parity and one stop bit? Express T.E. as a percentage value. • Complete participation exercises 2, 3 and 4.
Throughput • Throughput is the total number of information bits received per second, after taking into account the overhead bits and the need to retransmit packets containing errors. • Participation Exercise # 4:
Throughput (TRIB) • The term transmission rate of information bits (TRIB) describes the effective rate of data transfer. • TRIB = Number of information bits accepted Total time required to get the bits accepted • Calculating the actual throughput of data communication is complex as it depends on several factors such as: • Packet Size • Type of Circuit (The use of a shared multipoint circuit, rather than a dedicated point-to-point circuit will affect throughput, because the total capacity in the circuit must now be shared among several computers). • Time between frames
Throughput (TRIB) TRIB = Number of information bits accepted Total time required to get the bits accepted TRIB = K(M - C)(1 - P) MIR + T where: K = information bits per character M = frame length in characters R = modem transmission rate in characters per second C = average # of non-information (overhead) characters per frame P = probability that a frame will require retransmission because of error T = time between frames in seconds, such as modem delay/turnaround time on half duplex, echo suppression delay on dial-up, and propagation delay on satellite transmission. This is the time required to reverse the direction of transmission from send to receive or receive to send on a half duplex (HDX) circuit. It can be obtained from the modem specification book and may be referred to as re-clocking time.
