latency: the time it takes for information to transfer
the million-fold improvement in latency helped the modern world move at the speed of light on fiber optic cables across the world
as the machine world became more accessible, it became increasingly useful to share data and resources and the first networks of computers appeared
Networks
first computer network appeared in the 1950s and 60s, generally used within an organization like a company or research lab to facilitate the exchange of information between different people and computers
faster and more reliable than the previous method of having someone walk a pile of punch cards, or a reel of magnetic tape to a computer on the other side of a building - which was later dubbed a sneakernet
second benefit was the ability to share physical resources
e.g. instead of each computer having it’s own printer, everyone could share one attached to the network - it was also common on early networks to have large, shared, storage drives, one too expensive to be attached to every machine
these relatively small networks of close-by computers are called Local Area Networks or LANs
it could be as small as two machines in the same room, or as large as a university
Ethernet
although many LAN technologies were developed and deployed, the most famous and successful was Ethernet
developed in the early 1970s at Xerox PARC and still widely used today
in its simplest form:
a series of computers are connected to a single, common ethernet cable
when a computer wants to transmit data to another computer, it writes data, as an electric signal onto the cable
of course, because the cable is shared, every computer plugged into the network sees the transmission, but doesn't know if data is intended for them or another computer - to solve this problem the Ethernet required that each computer has a unique Media Access Control Address or MAC Address
the unique address is put into a header that prefixes any data sent over the network
so computers simply listen tot the ethernet cable, and only process data when they see their address in the header
every computer made today comes with its own unique MAC address for both Ethernet and WiFi
The general term for this approach si Carrier Sense Multiple Access or CSMA for short. The carrier is any shared transmission medium that carrier data - copper wire in the case of etherenet, and the air carrying radio waves for WiFi
many computers can simultaneously sense the carrier, hence the “Sense” and “Multiple Access” and the rate at which a carrier can transmit data is called its Bandwidth
Exponential Backoff
using a shared carrier has one big drawback
when network traffic is light computers can simply wait for silence on the carrier and then transmit their data
but as network traffic increases, the probability that two computers will attempt to write data at the same time also increases ⇒ Collision ⇒ data gets all garbled up
fortunately, computers can detect these collisions by listening to the signal on the wire - computers stop transmitting, wait for silence, and then try again - the problem is other computers on the network are going to try that too, and this just leads to more and more collisions
the fix:
when transmitting computers detect a collision, they wait for a brief random period before attempting to re-transmit, however, if it collides again, which suggests network congestion, instead of waiting the same period, it will double the time every time until it’s successful
this was the rate of collision went down and data started moving again, freeing up the network using an exponentially growing wait time
Collision Domain
to reduce collision and improve efficiency we need to shrink the number of devices on any given shared carrier ⇒ Collision Domain