 Level 2
- layer in the model of networks
- in fact, the lowest layers... what actually connects the machines to
  each other
- the foundation for these subnets
- unlike other parts of the Internet, there are many different types and
  they're continually evolving

Characteristics
===============

Geography
* LAN (Local)
* MAN (Municipal/Metro)
* WAN (Wide Area)
-> delay

Topology
* Point-To-Point
  - really simple, just use a wire or fiber or whatever
  - or one for each direction
  - limited to two machines
* Bus
  - easy to organize since just connect any machines in a row
  - traffic goes to all of them, so easy to pick out, but also insecure
  - easy to partition into two pieces
* Ring
  - perhaps harder to partition, if plan carefully
  - similar to bus, but closed
* Star
  - single point of failure
  - but, easy to isolate misbehaving sources
  - can control what different nodes see

Reservations
* Firm
* Ad hoc
* Contention-based

Timing
* Asynchronous
  - easier to do (doesn't require clock signals)
* Synchronous
  - can go faster, since clock provides implicit)

Specific Technologies
=====================

PPP/Serial Lines
================

Ethernet
========

- basic idea is give nodes a common channel (wire, radio) to talk on,
  and let them fight it out to get their messages through
- if two people happen to send packets at the same time, the packet
  wont get through. they'll have to retransmit

Evolution
  Aloha
  - original protocol at University of Hawaii for packet radio
  - was then adapted to wire
  CSMA: Carrier Sense Multiple Access
  - add carrier sensing
  - this means you listen before you send, to make sure no one else
    is sending
  CSMA/CD: /Collision Detection
  - add collision detection
  - this means you notice when you're sending at the same time as
    someone else, and you can stop right then, instead of going on
  10base2, 10base5
  - based on a bus topology
  - machines physically connected to the same wire
  - 10 means 10 megabits/second
  - base is for baseband 
  - 2 or 5 mean 2 or 5 hundred meter limit
  Repeaters
  - connect multiple buses together
  - electrically (level 1) regenerate the signal
  - helps overcome length limits
  10baseT
  - T is for Twisted Pair (departure from scheme)
  - now wires have to go into a hub, so physically it's a star
  - however, hub just repeats electrical signal, so still a logical bus
  Bridges
  - like repeaters, but at level 2 instead of 1 (i.e. read the bits and
    recreate them)
  - some are intelligent, and only forward the traffic to segments with
    the appropriate hosts on them
  - others just use a spanning tree to forward everywhere without loops
  - see: level 2 need not be dumb
  Switches
  - don't need to change cards
  - totally different, but keeps interface the same

  100Mbps Ethernet

  Gigabit Ethernet

 Mapping from IP to Ethernet Address
  ARP:
  RARP:

FDDI
====

- based on idea of token ring

Frame Relay
===========

ATM
===





