SYST5030/4030 ☻Error control☻ ☻ Network architecture ☻ ☻ Protocols ☻ ☻ Transmission Efficiency and Throughput ☻

SYST5030/4030 Causes of errors Errors are caused by: –various kinds of surrounding noise which disturbs the signal going through a medium like copper, coaxial cable, etc. –properties of the medium attenuation distortion (high frequencies lose power more rapidly than low frequencies) delay distortion (different frequencies travel through the medium at different speeds)

SYST5030/4030 Error Prevention Shielding Relocating cables Conditioning (carriers guarantee the maximum number of errors that can occur) –C-type conditioning compensates for attenuation and delay distortions. –D-type conditioning improves signal to noise ratio.

SYST5030/4030 ERROR DETECTION AND CONTROL PARITY CHECKING Single Parity bit: Total number of 1 bits must always be even. V = (7-bit ASCII code). Since, the number of 1’s is even, add a 0 as the eighth bit. Therefore, 8-bit representation of V is Similarly, W = (7-bit ASCII code) Since, the number of 1’s is odd, add a 1 as the eighth bit. Therefore, 8-bit representation of W is (In odd parity system, total number of 1 bits is always odd.) What is the drawback with the single parity method?

SYST5030/4030 CYCLICAL PARITY CHECKING This method requires two parity bits per character. Assuming six bits of code (bits 1 through 6) add two parity bits (bits 7 and 8) such that bit 7 is the parity for bits 1, 3 and 5, while bit 8 is the parity for bits 2, 4, and 6. Again, total number of ones is even in both cases Parity 1 Parity 2 How is this method better than having a single parity bit?

SYST5030/4030 M-of-N Codes The code is designed in such a way that there will always be M 1’s and N-M 0’s in each valid character of the code. Example 4-of-8 Code (from IBM) In this 8-bit code there must be exactly 4 ones and 4 zeros. Valid characters:Invalid characters:

SYST5030/4030 The table above shows how the character is converted into its hamming code equivalent. Even parity is used in this case. Bit 1 checks: 1, 3, 5, 7, 9, 11 Bit 2 checks: 2, 3, 6, 7, 10, 11 Bit 4 checks: 4, 5, 6, 7 Bit 8 checks: 8, 9, 10, 11 Hamming codes Hamming codes are Forward error correcting codes

SYST5030/4030 Cyclical Redundancy Check (CRC) This is applied to an entire block of data in synchronous communication. A 16-bit (or more commonly 32-bit) number is calculated from the entire block, and attached to the end of the block by the sender. The receiver performs a similar calculation and compares the 16-bit value to see if it is the same. If they are not the same, it indicates an error in the transmission. This is a highly reliable scheme with almost 100% error detection capability.

SYST5030/4030 Transmission Efficiency and throughput Transmission efficiency is defined as: In asynchronous transmission, efficiency = 70% In synchronous transmission, efficiency is much higher

SYST5030/4030 Throughput Efficiency = 80% Error rate = 1% Modem speed = 9600 bits per second Throughput = 9600 x 0.80 x (1 – 0.01) = bits per second This is also called transmission rate of information bits. Note: Throughput is less than efficiency. Throughput: number of information bits received per second after also accounting for retransmissions due to errors.

SYST5030/4030 Network Architecture Defines: 1) The way communications functions are divided into layers. 2) Protocols, standards and messages at each layer. Objective of the layered approach: 1) Each layer performs one set of functions. 2) Each layer isolates the layers above it from the complexities below Protocols in each layer are the set of rules agreed to and followed by both parties for successful communication.

SYST5030/4030 Layered Network Architecture Several layers are involved in data communications (7 in OSI, 4 in TCP/IP) The most important layers (and the ones in TCP/IP) are: –Application layer: handles the details of particular applications (e.g., Telnet, Ftp, SMTP, SNMP). –Transport layer:provides reliable flow of data between end system hosts for the application layer. –Network layer: performs addressing and routing. –Link Layer: responsible for error control, flow control, message delineation, link management (media access control). Also called network interface layer. General principle: division of work across layers.

SYST5030/4030 OSI Layered Protocol Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer Network layer Data Link layer Physical layer Network layer Data Link layer Physical layer Host Computer (Intermediate node)

SYST5030/4030 Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer Application Programming Interface What if your Web-browser used an Implementation of the OSI model? 1. Application sends message down to Network API and application layer communicates with application CLIENT *openserver (char ,char 80) Get ( 2. Presentation layer encodes message into commonly used data storage format (for example Ascii) 3. Session layer notifies end- system host of a pending data transmission. Get ( 4. Transport layer splits data into packets and controls flow Get ( 5. Network layer encapsulates message with sender and destination address Get ( TO FRM 6. Data Link layer breaks up message into DL size packets ( bits 8 or 16 bits bits 8 or 16 bits :80) Get TO FRM Begin Flag Address Control 8 bits 8 or 16 bits Information Ending Flag 7. Physical layer transforms bits into physical representation (voltages, sign-waves, pulses of light, etc.) and sends it onto the “wire” between two network devices.

SYST5030/4030 Network layer Data Link layer Physical layer Network layer Data Link layer Physical layer (Intermediate node) 1. Physical layer receives physical representation of bits turns them back DL packets 2. Data Link layer re- assembles NL packet ( bits 8 or 16 bits bits 8 or 16 bits :80) Get TO FRM Begin Flag Address Control 8 bits 8 or 16 bits Information Ending Flag 3 Network layer receives packet and calculates best port to get to end- system – then sends packet back down to DLL Get ( TO FRM 4. Data Link layer breaks up message into DL size packets ( bits 8 or 16 bits bits 8 or 16 bits :80) Get TO FRM Begin Flag Address Control 8 bits 8 or 16 bits Information Ending Flag 5. Physical layer transforms bits into physical representation (voltages, sign-waves, pulses of light, etc.) and sends it onto the “wire” between two network devices.

SYST5030/4030 Get ( 2. Presentation layer encodes message back into format used by application layer 3. Session layer notifies sending system that message has been received, and terminates connection Get ( 4. Transport layer re-assembles message (if needed) Get ( 5. Network layer removes packet/addressing information and sends information up to TL Get ( TO FRM 6. Data Link layer re- assembles packet and sends it up to NL ( bits 8 or 16 bits bits 8 or 16 bits :80) Get TO FRM Begin Flag Address Control 8 bits 8 or 16 bits Information Ending Flag 7. Physical layer receives physical representation of bits from the wire, and sends bit-patterns up to DLL SERVER Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer Application Programming Interface 1. Application layer sends message through Network API up to the server application CLIENT *openserver (char ,char 80)

SYST5030/4030 OSI Layered Protocol Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer Network layer Data Link layer Physical layer Network layer Data Link layer Physical layer Host Computer (Intermediate node)

SYST5030/4030 The Level 3 Approach Network Layer Primary Attribute Addressing QoS Multiplexing Low Error Rate Fault Tolerance High Capacity Physical Medium 1 2 3

SYST5030/4030 Architecture Example The actual flow of data is between WWW applications. The apparent flow of data is peer to peer across the network. The operation of the application processes is independent of the underlying communications and network technologies -- hence a communications architecture.

SYST5030/4030 Layers in protocols Source: Kurose and Ross (2001), “Computer Networking: A Top-Down Approach Featuring the Internet”

SYST5030/4030 Implementing an Architecture Each layer appends its own header to the application data. At the receiving end, each layer strips off the corresponding header.

SYST5030/4030 Data Link protocols BSC (Binary Synchronous Communications) SDLC (Synchronous Data Link Control) HDLC (High Level Data Link Control) Protocol Features and Issues Communications line control (polling/selecting) Framing Addressing Synchronization Data transparency Error control Flow control Fragmentation and reassembly Need a reliable way of exchanging information at data link layer

SYST5030/4030 Multipoint SDLC network

SYST5030/4030 SDLC Frame Begin Flag Address Control 8 bits 8 or 16 bits Information Variable length16 bits Frame Check Sequence Ending Flag

SYST5030/4030 How Data Link Protocol Works (Automatic Repeat Request - ARQ - method) Stop and wait ARQ: –Sender stops and waits for response from receiver after each packet –Receiver sends ACK if no errors in message –Receiver sends NACK if errors in message. This is a half-duplex method used in BSC protocol.

SYST5030/4030 A B frame 0 Ack1 frame 1 Ack0 Stop and wait ARQ In this case, there are only two frames numbered 0 and 1

SYST5030/4030 Protocol for error correction (Automatic Repeat Request) Continuous ARQ: –sender does not wait for response from receiver after each packet –receiver asks for retransmission of erroneous packets. This is a full-duplex method. It is also called sliding window protocol. It is used in SDLC protocol.

SYST5030/4030 A B frame REJ(5) Continuous ARQ Frame 0 received okay Frame 1 received okay Frame 2 received okay Frame 3 received okay Frame 4 received okay Frame 5 not received Frame 6 received okay Frame 7 received okay Frame 5 received okay Frame 6 received okay Frame 7 received okay 5 Note: Received Receipts will go back from B to A (not animated)