Layer 1 Networking5/21/2015 Warner

Link on last slide

Example – 56K modem

Telephone system

Central office digitizes voice w 8 bit codec 2⁸ = 256 discrete amplitude values Quantization leads to power S/N = 65536:1 ln(S/N) = 16

Modem speed

CODEC sample rate 8000/sec Filter limits frequency response to 3.5 KHz So C = 3500 * 16 = 56K

Overall theme

We want to use cheap media along with fancy electronics to get the best

performance possible. Communications integrated circuits

are ultimately printed at low incremental cost while expensive

wire has a cost per foot.

ADSL

Asymmetrical Digital Subscriber Loop Works like dial-up, except... BW is greater – 2 Mb/s versus 3.5 Kb/s BW falls with CO distance Performance is noise-limited by crosstalk

rather than quantization

Crosstalk

Each signal path is a pair of wires twisted together

Many pairs are bundled together into a cable

Individual pairs act like transformers and couple energy into each other

Higher frequencies have more crosstalk

ADSL distance

Yet higher speeds . . .

ADSL2 is not the end of the road VDSL up to 52 Mb/s but distance 300 M At that scale, useful for large buildings

10base5 Yellow cable Ethernet

Repeaters

10 and 100 Mb/s Ethernet permits repeaters subject to a rule that information about

collisions needs to propagate to all stations in the time required to send a minimum length packet. This is required to guarantee that all

stations will detect a collision while the sender is still transmitting. Four repeaters in the

longest path across the network are permitted in 10 Mb/s systems.

Repeaters

Regenerate the amplitude of signals that may have attenuated over distance

Re-time signal transitions to remove accumulated jitter

Replace lost preamble bits at the front of each packet

10base2 Thinnet

Structured cabling

10base5 and 10base2 have a daisy chain architecture

Generally, this leads to a mess To be supportable, all communications

paths should connect radially to a serving cable closet.

100 meter link distance was selected as a workable but somewhat arbitrary standard.

Thin repeater

10 base T

802.3i-1990 Uses 8P8C connector a.k.a. RJ-45 100 meter cat 3 cable Uses two pairs: one transmit, one receive

Line code: a digression

This code is called NRZ A “1” is a positive voltage A “0” is a negative voltage

NRZ = Non Return to Zero

How does the receiver find the bit cell boundaries?

Long runs of all 0s or 1s have no transitions

Early answer: send bit clock on a spare wire pair

Not a great answer. . .

Manchester coding

Direction of transition determines value Lots of transitions for clock recovery No DC content

Twisted pair Ethernet

Signals are driven differentially over their respective pairs.

Because each pair is twisted together external fields excite the same noise in both of the pair conductors.

Receivers reject common mode signals, providing low cost noise immunity.

10 Mb/s tw-pr uses manchester code

Category 3 & 5e

Category. . .

Anixter, an electrical distributor, cooked up the category classification to solve what they saw as major marketplace confusion among customers who just wanted to know what to buy.

The EIA/TIA fell into line behind Anixter and helped with the classification.

The IEEE generally does not bother with things like this.

100 Mb/s tw-pr Ethernet

802.3u-1995 100 meters over category 5 wire 4B/5B encoding MLT-3 line code results in 31.25 Khz Max one repeater in a collision domain Uses two pairs, same as 10 base T

4B/5B, what is that??

There are 32 different codes that are five bits long. The 16 values that can be taken on by 4-bit nibbles are selected from the 32 5-bit

possibilities so that codes are rich in 1's content. This coding guarantees a transition

density sufficient for the clock to be extracted from the data stream. The cost to make the

stream self-clocking is 25 percent.

Category 3 vs 5

Category 5 wire has tighter twisting so to rejects noise to higher frequencies.

Each pair has a different pitch to its twist. This reduces pair-to-pair coupling.

Category 3 wire can have PVC insulation. PVC has poor frequency characteristics caused by dielectric loss. Cat 5 wire uses better insulation that gives lower loss.

Category 5 vs 5e ??

The “e” stands for enhanced. Not much difference. It is the same stuff

made the same way. Category 5e has some additional tests to

assure it can be used bi-directionally. Category 5 has disappeared from the

market.

Gigabit over fiber

802.3z 1998 There are no repeaters. All links are point-to-point Short range (SR) to 100 meters uses

multimode fiber at 850 nM (deep red) Long range (LR) uses single mode fiber at

1300 nM. Distance limit is 0-10 KM.

Multimode – Single mode

Why did they do that ?

The 9 micron core in SM fiber requires a very intense light source. Lasers are needed. $$$$

A larger core diameter permits a cheaper light source. $$$$ Favors multimode.

Graded index MM fiber shapes the refractive index profile so that the rays that travel farther go faster. This partially compensates for the effect on speed.

Fiber bandwidth

Single mode: 10's of terabit/sec per core. Limitation is non-linear effects like heating that are hard to quantify.

Multimode: OM3 – a type of 50 micron multimode – has 2000 MHz-km

At 1 KM, the BW is 2 GHz At 100 M, the BW is 20 GHz

Gigabit over twisted pair

1000baseT IEEE802.3ab 1999 Designed for same wire as 100 Mb/s Uses all 4 pairs in both directions

4x improvement in capacity Each byte sprays across all four pairs 5 level PAM5 code replaces 3 level code Symbol rate is the same as 100 Mb/s, but

cleaner transmission is required for PAM5

What happened to Cat 6 ?

Category 6 tests the wire to 250 MHz and has tighter parameters than 5e. It is not sufficient for 10 Gig-E.

All dressed up with no place to go – Cat 6 wire was designed for a 2-pair Gig-E standard that was a commercial failure. The incremental cost of Category 6 wire has earned it some popularity since it is better than 5e and backward compatible.

OPTICAL TRANSPORT

Glass

MSA Modules

10 Gigabit/s over twisted pair

10GbaseT IEEE 802.3an 2006 Requires category 6a cable to reach 100 m Uses PAM16 for more bits per symbol Full duplex operation only Uses BW to 450 MHz Thruput: 4 pr x 833 Mbaud x 3 bits/symbol

10G Warts

Alien crosstalk – signals from nearby cables induce noise too complex to be compensated electronically

Aliens require shielding that makes the cable bigger and heavier

Power consumption is high, making for high operating costs. This will come down.

Direct connect is cheaper and faster.

10 Gb/s applications

Data center, when power consumption improves

Wave 2 802.11ac WiFi access points

What aboutWiFi?

WiFi

Channel Bandwidth Signal to Noise Modulation Line codes

Stream count

20, 40, 80 MHz 20 dB = 100:1 OFDM From 1 bit per Hz to

8 bits per Hz MIMO

Http://noc.ucsc.edu/docs/Layer-1.pdf