Networks: Notes 9/10/203

Serial, Sockets, Circuit Switching

Review of Networks

The network itself is the smallest part of the system, when you are talking about studying the network

The dynamics he's discussing are the same, whether on or off a network. The network doesn't describe just things with wires, it covers the way groups work as well.

The small worlds model

You can look at the size of the network, not just as the amount of nodes, but the number of connections to get to all the other nodes.

What the small worlds model means is that 4 nodes make a complete network, but when it is incorporated into a larger network, more lines need to be drawn to connect everyone in shorter jumps

The original phone system was created by physically connecting the wires. It created a complicated system Was vulnerable. If one node went down, the whole system went down

1888: Snowstorm shut down phone system. In New York, this also shut down the Trading system. ATT had to bury the phone lines and reduce the amount of phones needed to make all calls.

The original Phone Switching system is called Circuit Switching

After WWII, a robust network was needed to survive a Bomb Redundancy Decentralize Remove manual patching to reroute phone-calls

Packet switching was a solution to the manual Patching. Rather than there being a dedicated circuit between two people, a router which is attached to 2 or 3 or many endpoints, which may be attached to a series of other routers to deliver the packets.

Packet switching gives you flexibility on how you route traffic

1 message may be made out of several Packets. Each Packet may reach its destination through different routes. TCP will wait until they all reach their destination, check their order and deliver the message. UDP doesn't check and delivers them in any order.

In a TCP system, a lost Packet will be recognized and the end router will request the first for that lost Packet

This happens in a store and forward network.

Packet size is standard: defined in the header of the packet

Baran Davies thought the ideal network was a completely connected network. The next best thing, was a decentralized with a series of robust hubs.

Don't let a hub get too big Redundant hubs Sub-hubs

The hubs should know how to get to the end point.

Routing Tables: A list of who's got the most network. This avoids overloading routers

The health of the network is constantly getting monitored.

High Frequency traders are planning on drilling a straight line from Chicago to New York to save 3 milliseconds on the trades. The virtual network has a very real effect on the physical world, as that straight line goes straight through ecosystems and mountains.

Network engineers are constantly trying to build faster, more direct routes between two points.

If you're an ISP that know a lot, but don't speak much to keep network traffic down.

Network traffic pattern dynamics

Nodes of a network that have the highest amount of connections tend to get the most attention.

In a decentralized network, there are no centers, but there are clusters and a hierarchy

Tier 1 companies

Companies like ATT TMobile Verizon Level 3 deal with each other with 'Peer Agreements'. They don't charge each other for sharing of traffic.

There is a hierarchy from the those top companies to smaller hosting companies, and smallerer hosting companies.

IXP: Internet Exchange Point IXP's do peer sharing between lower level companies, connecting directly between them without going back through the larger ISPs

30 avenue of the americas is home to a lot of institutions. They physically connected the floors with switches on each floor.

60 Hudson is the switching station for NYC

Underwater cable companies buy beach front property to bring connections onto land.

DNS: Domain Name Service Dynamic Host Control Protocol: When you plug into a router, you have a physical address for the computer. The MAC address, the Media Access Control address. The router automatically assign the IP address.

An enterprise router DHCP is often turned off, and a permanent address is assigned

PSTN: Public Switch Telephone network POTS: Plain Old Telephone Service

LATA's, LECs, RBOCS, IXPs. Regional operating companies

Smartness on the endpoints

Eisenburg… "The Stupid Network" creates a faster network because it makes the efficiency. Makes the system simple and interoperable in the middle so that the fancy stuff can happen at the ends, upgrades, etc.

Let's go back to serial communication: Serial communication starts with baud rate to keep it Asyncronous ASCII - The protocol as a way of mapping Alpha-neumeric characters to a set of numbers. Origionally it was just 7 bits, which only covered Latin characters, which was shitty for the large part of the world that used other characters. Unicode was created to cover a more universal set of characters.

Osciliscope: To view the voltage over time over an electrical signal.

Logic: from Saleae. A whole bunch of wires that let you look at a whole bunch of signals all at once.

Tom is doing an awesome demo that you're going to miss out on because this is not a multimedia note taking exercise

There's awesome stuff happening as Tom has connected the Osciliscope to the arduino and is displaying a pulse width modulation sketch and is adjusting the ratio of the on time to the off time.

The distance between ratio of on time and off time correspond to 1 and 0, and will define the ASCII characters.

Stop bits and start bits always empty bits at the end of each 8 bit ASCII character.

These show up as hills and valleys on the oscilloscope and in logic, which labels and interprets the signal into an organized gui

I2C communication example: Synchronous Communication Two paths of communication: Data and Clock

USB: Universal, Serial, Bus RS232: Always sending between -3 and 12 volts. -3 represents 1, and 12 represents 0. It is inverted. If each data bit is represented by a voltage, you will have good data, even in a noisy situation. NRZ. Non Return to Zero

DMX USB Ethernet Data Plus line, and Data Minus Line On the data Plus line a message will be sent: i.e. 100110 On the data Minus line the inverse will be sent. 011001 The mirror is used to find an error. There will always be a 0 differential.

Ethernet cables are always 'twisted pair' Ethernet cables have 4 wires, 2 pairs of twisted wires 2 wires - Tx and 2 wires - Rx The physical twisting of the wires stops Induction, as does the mirroring of the signals as well.

At an architectural infrastructure level, this probability of induction would effect the infrastructure.

Thinner wire has a greater resistance. so over longer distances, thicker wire will provide less resistance

Fiber optics: Light Pulse Transmission. Takes advantage of refraction. The angle of incidence is == the angle of reflection The angle of refraction is < incidence With fiberglass, if the light is internal at the correct angle, the light will not refract through the material and not escape. If the glass is nicked, or bent to a point where the surface is effected, light will be lost and part of the signal will be lost. Light will be lost over distance so repeaters are used to increase the signal over long distances.

WTF is an Opto-Isolator? An LED in with a photo-resistor out. They act as a repeater. The only thing limiting it is the speed of the led and the photocell. It is used to avoid voltage spikes, especially in midi devices. It makes the transfer of information very robust.