Busses and the Speed of Light

1. Busses and the Speed of Light CPIS 210 John Beckett

2. What is Faster? Was: Parallel is faster than Serial Now: It depends… We can send data faster over wires than before The problem is not as much how many bits per second as how coordinated the parallel wires are Short distances: Parallel is faster Long distances: Serial is faster • Freeway analogy: more lanes gives you more cars per minute • Transmission line technology was poorly understood, which limited bandwidth per wire

3. Serial Transmission • Earliest: Single line referenced to ground • Problem: Interference from other lines using the same ground (duh) • Balanced Line: Use a twisted pair, which nulls out external signals • The higher the speed, the more crosstalk • Catx: Increasingly tight specifications reducing crosstalk at higher frequencies • Oops: “sniffers” don’t work as well

4. Asynchronous Data Framing • Continuous Mark means we have a live connection • “Start” – a Space bit alerting that a byte is coming • Data is transmitted low-order-bit first • “Space” – a Mark bit confirming that byte has ended

5. Problems with RS232 Asynch • We lose 20% of the available bandwidth from start and stop bits • Different systems have different combinations of how many bits (7 or 8) and parity (even or odd or All or None or Not) • Challenges configuring • No provision for auto-correction of errors • The RS232 specification was designed for 50 foot length, 2400 bps • At SAU we routinely ran it 800 feet at 4800 bps • Lightning damage was common

6. Upgrade to Synchronous • Send “SYN” characters continuously when no data is coming • These are self-framing so we don’t need start and stop bits • Some protocols provided for error correction • Sounds like a precursor to TCP/IP over Ethernet? Yes! • Also a precursor to USB

7. Parallel Data • Used for most on-board communication • Necessary to have a “clock” signal to say when the other lines have useful data • Also need a wire for each bit to be transmitted • Transitional technologies (4004, 8088) would transmit two bits per wire, separated by time

8. Dialup Modems • A way of communicating RS-232 serial over telephone lines • Bell 103: Frequency shift keying (between two frequencies) borrowed from the old teletype technology • Racal/Vadic, Bell 212 and onward: Used more-subtle transitions (4, 8, 16… different phase shifts) to transmit two bits per transition, which meant “baud” was no longer the same as “bits per second” • People still referred to a 56k bps modem as 56k baud • Loss of noise immunity compensated by auto-tuning of the channel at connect time

9. The Lesson from Modems & SATA Sometimes your overall capacity can be more if you sacrifice one characteristic for another • Modems: We sacrificed noise immunity to get more data speed, then restored immunity by tuning the channel each time a connection was established • SATA: We reduced the number of signal and data lines sharply, but used far-better cables to make up the difference

10. Optical Communication • 1790 (Claude Chappe): Transmit semaphore codes using giant paddles controlled by levers • For centuries, optical transmission was possible but wires were always less expensive so nobody paid attention • (skip forward to 1970): Corning developed a fiber that could send light long distances • Single-mode – thin fibers, faster data • Multi-mode – thicker fibers, slower data

11. Business Drivers for Fiber • Increased immunity from interference and damage • Increased speed • Decreased cross-section of cable for a given amount of data capacity (don’t need a bigger hole to hook up more circuits) • Meanwhile copper is improving but will lose market share

12. Fiber Caveats • With more density, you can do more damage with a back hoe • Higher cost of acquisition • Consider whether other factors pay you back. Maybe they do! • Rumored to be trickier to install • Didn’t prove to be true in practice • Termination is more expensive