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Message Delineation

Identify the start and end of messages by:Identify the start and end of messages by:

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Message Delineation by Flags

Examples of data link protocols using Start Examples of data link protocols using Start and Stop Flags for message delineation:and Stop Flags for message delineation:

Asynchronous Transmission ProtocolsAsynchronous Transmission Protocols Synchronous Data Link Control Protocol (SDLC)Synchronous Data Link Control Protocol (SDLC) High-level Data Link Control Protocol (HDLC)High-level Data Link Control Protocol (HDLC) Serial line Internet Protocol (SLIP) Serial line Internet Protocol (SLIP) Point-to-Point Protocol (PPP)Point-to-Point Protocol (PPP)

Asynchronous Transmission

Each character is transmitted independently of all Each character is transmitted independently of all other characters.other characters.

A start bit (0), and a stop bit (1) are added to each A start bit (0), and a stop bit (1) are added to each character.character.

If you use VT100 protocol, or connect to a UNIX or If you use VT100 protocol, or connect to a UNIX or Linux computer using Telnet, you may be using Linux computer using Telnet, you may be using asynchronous transmission. asynchronous transmission.

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Asynchronous Transmission

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Synchronous Data Link Control (SDLC) Protocol SDLC is a mainframe protocol developed SDLC is a mainframe protocol developed

by IBM in 1972 by IBM in 1972 SDLC and derivatives are still being used SDLC and derivatives are still being used Like many other DL protocols, SDLC has a Like many other DL protocols, SDLC has a

control field included in its frame control field included in its frame What is a control field? What is a control field?

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Control Field

Frame types:Frame types:Information Frame Information Frame (e.g., used for the (e.g., used for the

transfer and reception of messages, frame transfer and reception of messages, frame numbering of contiguous frames) numbering of contiguous frames)

Supervisory Frame Supervisory Frame (e.g., Used to (e.g., Used to transmit acknowledgements)transmit acknowledgements)

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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, HDLC is a formal standard developed by ISO, and is essentially the same as SDLC except for and is essentially the same as SDLC except for few additional features, some of which include:few additional features, some of which include: Having longer address and control fields Having longer address and control fields Having a larger sliding window for supporting Having a larger sliding window for supporting

Continuous ARQ Continuous ARQ

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Point-to-Point Protocol (PPP)

PPP was developed in 1990s as a replacement for Serial PPP was developed in 1990s as a replacement for Serial Line Internet Protocol (SLIP) Line Internet Protocol (SLIP)

Commonly used to dial up from home computers to ISP Commonly used to dial up from home computers to ISP It includes error detection (CRC-16) It includes error detection (CRC-16) Its header includes a protocol field that specifies the Its header includes a protocol field that specifies the

network layer protocol (e.g., TCP/IP, IPX/SPX)network layer protocol (e.g., TCP/IP, IPX/SPX) Message may be up to 1500 bytes in lengthMessage may be up to 1500 bytes in length

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SLIP and PPP Packets - Layout

PPP packet layout

Flag Address Control Protocol Message CRC-16 Flag 1 byte 1 byte 1 byte 2 bytes variable 2 bytes 1 byte

SLIP packet layout

End Message End 1 byte variable 1 byte

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Disadvantages of Using Flags for Message Delineation Problem: Problem:

Solution:Solution:

Transparency ProblemTransparency Problem What is meant by the transparency problem?What is meant by the transparency problem? What is the solution to the transparency What is the solution to the transparency

problem?problem? Example: SDLC (Flag pattern is 01111110) Example: SDLC (Flag pattern is 01111110)

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Other Message Delineation Techniques

Special Signals: Special Signals: Token ring uses special signals to delineate the Token ring uses special signals to delineate the

start and end of messagesstart and end of messages The signals used to start and end these The signals used to start and end these

messages will violate the rules of regular messages will violate the rules of regular transmissiontransmission

Max length of frame is 4500 bytesMax length of frame is 4500 bytes Specifying size of message upfront:Specifying size of message upfront:

Example: Ethernet Example: Ethernet Max length of frame is 1492 bytes Max length of frame is 1492 bytes

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Token Ring and Ethernet Frames

Start Frame Destination Source Message End delimiter control address address variable delimiter

Token Ring format

Access Framecontrol check sequence1 byte 4 bytes

Ethernet format

Destination Source Length Message CRC-32address address 2 bytes variable 4 bytes6 bytes 6 bytes

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Data Link Protocols

Asynchronous and Synchronous ProtocolsAsynchronous and Synchronous Protocols File Transfer ProtocolsFile Transfer Protocols

XMODEMXMODEM XMODEM-CRC (CRC-8)XMODEM-CRC (CRC-8) XMODEM-1K (CRC+1K blocks)XMODEM-1K (CRC+1K blocks)

YMODEM(CRC-16)YMODEM(CRC-16) ZMODEM (CRC-32)ZMODEM (CRC-32) KERMIT (CRC-24)KERMIT (CRC-24)

What are some factors to consider before selecting a data What are some factors to consider before selecting a data link protocol?link protocol?

Transmission Efficiency

Each protocol has both Each protocol has both information bitsinformation bits (to convey the user’s (to convey the user’s message) and message) and overhead/control bitsoverhead/control bits (for error checking, marking (for error checking, marking the start and end of characters or packets etc.).the start and end of characters or packets etc.).

Transmission efficiencyTransmission efficiency (T.E.) is defined as the total number of (T.E.) is defined as the total number of information bits divided by the total number of bits in information bits divided by the total number of bits in transmission. T.E. is typically expressed as a percentage value. transmission. T.E. is typically expressed as a percentage value.

Participation Exercise # 1: What is the transmission efficiency Participation Exercise # 1: What is the transmission efficiency of asynchronous transmission of a character represented by a 7-of asynchronous transmission of a character represented by a 7-bit ASCII code that is enclosed by one start, one parity and one bit ASCII code that is enclosed by one start, one parity and one stop bit? Express T.E. as a percentage value. stop bit? Express T.E. as a percentage value.

Complete participation exercises 2, 3 and 4. Complete participation exercises 2, 3 and 4.

Throughput ThroughputThroughput is the total number of information is the total number of information

bits received per second, after taking into bits received per second, after taking into account the overhead bits and the need to account the overhead bits and the need to retransmit packets containing errors.retransmit packets containing errors. Participation Exercise # 4: Participation Exercise # 4:

Throughput (TRIB) The term transmission rate of information bits (TRIB) The term transmission rate of information bits (TRIB)

describes the effective rate of data transfer.describes the effective rate of data transfer. TRIB = TRIB = Number of information bits acceptedNumber of information bits accepted

Total time required to get the bits acceptedTotal time required to get the bits accepted Calculating the actual throughput of data communication Calculating the actual throughput of data communication

is complex as it depends on several factors such as:is complex as it depends on several factors such as: Packet SizePacket Size Type of Circuit (The use of a shared multipoint circuit, Type of Circuit (The use of a shared multipoint circuit,

rather than a dedicated point-to-point circuit will affect rather than a dedicated point-to-point circuit will affect throughput, because the total capacity in the circuit throughput, because the total capacity in the circuit must now be shared among several computers).must now be shared among several computers).

Time between framesTime 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 characterM = frame length in charactersR = modem transmission rate in characters per secondC = average # of non-information (overhead) characters per frameP = probability that a frame will require retransmission because of errorT = 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.