Direct Link Networks: Building Blocks (2.1), Encoding (2.2), Framing (2.3) ECPE/CS 5516: Computer Networks Originally by Scott F. Midkiff (ECpE) Modified by Marc Abrams (CS) Virginia Tech courses.cs.vt.edu/~cs5516
Lecture Topics I Physical layer G Examples of direct links (2.1) G Data encoding (2.2) I Point-to-point protocols G Framing (2.3) CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 2
2.1: Hardware Building Blocks CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 3
Links at Physical Layer I Physical media for links: G Twisted pair G Coaxial cable G Optical fiber G Radio waves G Infrared I Media + Electronics + Optics = media properties CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 4
Physical Layer Properties I Bit encoding: How is information -- 1’s and 0’s -- encoded? I Full-duplex versus half-duplex operation Full-duplex: data in both directions simultaneously G Half-duplex: data in one direction at a time G I Data rate: How much info can be sent in unit of time? I Extent: What’s max link length for reliable operation? CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 5
Link Examples Service Bandwidth Distances Category 5 twisted pair 10-1000 Mbps 100 m 50-ohm coax (Thinwire) 10-100 Mbps 200 m 75-ohm coax (Thickwire) 10-100 Mbps 500 m Multimode fiber 100 Mbps 2 km Single-mode fiber 100-2400 Mbps 40 km CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 6
Examples of Leased Links Service Bandwidth ISDN (B-channel) 64 Kbps T1 (DS1) 1.544 Mbps T3 (DS3) 44.736 Mbps STS-3 (OC-3) 155.251 Mbps STS-12 (OC-12) 622.080 Mbps STS-24 (OC-24) 1.244160 Gbps STS-48 (OC-48) 2.488320 Gbps CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 7
Links to Homes Service Bandwidth POTS 28.8-56 Kbps ISDN 64-128 Kbps xDSL 16Kbps-55Mbps CATV 20-40 Mbps CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 8
2.2: Encoding CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 9
Encoding I Encoding determines how information is represented by electrical, optical, or electromagnetic signal Node Node signal Adaptor Adaptor information CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 10
Example of Encoding I The most intuitive… Non-return to Zero (NRZ)… 1 1 0 0 1 0 1 0 I Encode “1” as high voltage level, “0” as low I Others: G Non-return to zero inverted (NRZI) G Manchester G Block codes, e.g. 4B/5B CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 11
Physical Layer “Bit Pipe” I Physical layer defines signal levels and timing to deliver bit stream to Data Link layer I Signal bandwidth determines data rate limit I Timing errors G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift” CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12
Physical Layer “Bit Pipe” I Physical layer defines signal levels and timing to deliver bit stream to Data Link layer I Signal bandwidth determines data rate limit I Timing errors G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift” CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12
Physical Layer “Bit Pipe” I Physical layer defines signal levels and timing to deliver bit stream to Data Link layer I Signal bandwidth determines data rate limit I Timing errors G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift” 10111010001 CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12
Physical Layer “Bit Pipe” I Physical layer defines signal levels and timing to deliver bit stream to Data Link layer I Signal bandwidth determines data rate limit I Timing errors G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift” 10111010001 ??11???00?1 CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12
Asynchronous vs. Transmission Synchronous CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 13
Asynchronous Transmission (1) I Each transmission is synchronized G A start bit begins, stop bit ends transmission G Line stays in an idle state until next start bit I Samples timed from beginning of start bit I Used in applications where performance can be reduced to reduce costs G Modems G PC serial ports start 1 1 0 0 1 0 1 0 stop idle start NRZ encoding: T /2 T CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 14
Asynchronous Transmission (2) I Physical layer can provide characters ( n -bit units) to Data Link layer idle idle n bits n bits n bits CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 15
Asynchronous Transmission (3) I Advantages G Simple timing mechanism G Inherent character framing G Adapts to different data rates (idle serves as fill) I Disadvantages G Timing errors can occur if line is noisy (e.g., missed start bit) G Overhead for stop and start bits CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 16
Synchronous Transmission (1) I Used for high data rates, including T1 and or interoffice digital transmission lines I Information is sent continuously G Receiver & repeater maintain synchronization between incoming signaling rate and local sample clock G Idle or fill characters inserted if line is idle I Signal transitions (high-to-low or positive-to-negative) enable clock recovery, or synchronization G Some minimum occurrence of signal transitions are needed to maintain synchronization (Why?) CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 17
Why Frequent Signal Transitions Needed I Long strings of 0’s or 1’s cause problems. G Receiver synchronizes clock on 0-1 transitions. No transitions = no synchronization G Receiver averages signal it receives to determine hi vs. low signal. No transitions = incorrect average. G NRZ encoding is bad. I Solutions… CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 18
Solutions: Ensuring Signal Transitions I Manchester encoding: G Sender XOrs local clock w/ NRZ encoded data, producing hi/low transitions for every bit G But doubles rate at which signal is transmitted I Dedicated timing bits G Use some bit transmissions just for timing G Example: Dataphone Digital Service N Use every one bit out of eight to guarantee a signal transition G Example: Synchronous modems N Periodically insert a SYNC character CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 19
Ensuring Signal Transitions (2) I Bit insertion G Use bit transmissions for timing only when needed by inserting timing bits into data bit stream G HDLC “bit stuffing” N 01111110 indicates end of a data block (for framing) N Six consecutive 1’s must not be sent as data (may be mistaken as end of a data block) N Sender inserts a 0 after every string of five consecutive 1’s; receiver must strip a 0 after five consecutive 1’s G Disadvantage: extra bits = extra delay CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 20
Ensuring Signal Transitions (3) I Data scrambling G Similar to encryption/decryption G Prevents transmission of repetitive patterns G With high probability, prevents long strings of 0’s (or 1’s) that would not have signal transitions CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 21
Point-to-Point Protocols and Links (1) I Point-to-point protocols involve exactly two peer entities or modules that are connected by some “link” I Modules must interact to ensure proper transfer of information using link “Link” Module at Module at Node A Node B CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 22
Point-to-Point Protocols and Links (2) I For example, link may be: G Physical link (e.g. RS-232 is a point-to-point protocol) G Virtual bit pipe (e.g. at data link layer) G A connection or virtual connection (e.g. at transport or session layer) CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 23
Data Link Control -- DLC (1) I For each point-to-point link in a network re are two data link control (DLC) peer modules, one at each end I DLC modules use a distributed algorithm to transfer packets G Received from and delivered network layer I Usual objective is to deliver packets in order of arrival (from network layer) without errors or repeated packets CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 24
Data Link Control -- DLC (2) I DLC modules must use unreliable “virtual bit pipe” provided by physical layer Network Network DLC DLC Physical Physical CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 25
Data Link Control -- DLC (3) I DLC must: G Detect errors (using redundancy bits) G Request retransmission if data is lost (using automatic repeat request -- ARQ) G Perform framing (detect packet start and end) G Support initialization and disconnection operations CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 26
Data Link Control -- DLC (4) I These functions require that extra bits be added to packet to be transmitted G Header bits are added to front of each each packet G Trailer bits are added to rear of each packet header, packet from upper layer (service data unit), G and trailer form a frame header service data unit trailer frame CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 27
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