Overview

Data Link layer

the layer below IP
the layer above the physical layer
concerned with issues such as
- framing: combining bits into packets
- access control: for shared media, orderly sharing of the shared medium
- error checking/correcting, using CRCs
usually originally designed for only a single hop, though often multihop

Ethernet
802.11
modems with PPP or SLIP
amateur packet radio
parts of ATM (the rest may be classified as the network layer, or as the data link layer, or as not fitting the layer model very well)
SONET, Frame Relay
older technologies, including FDDI, Token Ring, ...

broadcast medium: many/all can hear each sender
collision: two senders sending at the same time, receiver can't tell what the message was (detected by receiver with checksum/CRC)
send and receive power are very different, so sender cannot detect collision
no central clock, often limited power

transmitters on outer islands send on frequency f₀, receive on frequency f₁ (could collide)
transmitter on Oahu sends on frequency f₁, receives on frequency f₀ (no collision)
transmitter on Oahu rebroadcasts everything it receives, so all stations know whether transmission was successful
if transmission failed, must retransmit
now used with ground stations and satellites

when nobody sends, network utilization is low
when everybody sends, lots of collision, so network utilization is low
optimum is when the network is about 18% utilized
improvement: have everyone start transmission at a given time (base station transmits clock signal), to decrease chances of collision -- slotted Aloha
improvement: use Aloha to decide who gets to transmit (on a bit-by-bit basis), then transmit -- why is this an improvement?

can the Internet protocol be used over wireless links?
hidden sender problem: A sends to B, C does not know A is sending and might send to B at the same time
wirelessness is important for mobility, but how to do addressing and routing?
cellular: each system has a "home", identifies itself to the local base station (strongest radio signal), which tells the home how to reach me
cellular: as I move, my connection must be handed off to another base station
cellular: not peer-to-peer networking

"Access Point" (AP) is in range of all the end stations, forwards data onto the internet (bridging)
ideally, APs can communicate with each other over a wired network, creating a cellular wireless network, but no protocols defined so far (as far as I know)
the above is Managed mode, the alternative is Ad-Hoc mode, in which all nodes are equivalent
in an Ad-Hoc Network, everyone agrees to forward everyone else's packets
who do we forward to? How do we locate the default router? Is this "routing" on the data-link layer? What happens when wireless units move?
Mobile Ad-hoc NETwork, MANET

every node is a router, every node is a host
IP assumes that:
- if A can talk to B on B's interface i, and
- C can talk to B on B's same interface i, then
- A should be able to directly communicate with C
this is not the case in wireless ad-hoc networks
two solutions: modify IP (hard), or run the entire ad-hoc network as a single IP (sub)network (easier)
the second solution requires an underlying, data-link-layer routing protocol

assume a number of nodes in an ad-hoc wireless network
the nodes are deployed in a straight line, each node in range of only the node before it and the node after
the link bandwidth is B bits/second
how many bits per second can the first node communicate to the last?
- if node i transmits to node i+1, node i+1 cannot be transmitting to node i+2
- if node i transmits to node i+1, node i-1 cannot be transmitting to node i
- if node i transmits to node i+1, node i-2 cannot be transmitting to node i-1
- so with one-directional transmission, at most 1/3 of the links can be active at any given time.
- so the maximum data rate in such a configuration is B/3
this is the best possible case, and it is hard to achieve in practice

nodes in a defined space move around at random speeds and directions
new random speed and direction selected when a boundary is reached
high mobility requires very agile routing protocols: it is normal for routes to go down
broadcast always works, but can we do better?
DSR (Dynamic Source Routing): sender builds a route, includes in in every packet. Routing tables are small, but packets are big or routes are short
AODV (Ad-Hoc On-Demand Distance Vector): similar to distance vector, but routes are built on demand only, by broadcasting. Distance vector is very efficient with information exchange, but does not do well when links fail, though the shortage of links in AODV may help prevent cycles

low mobility (though sometimes units disappear), low power, low data rates, often many nodes (mesh of nodes), high reliability needed
restricted application-dependent communication, e.g. merge all the data towards a base station
diffusion: requests (queries) and data messages (events) move at random through the network until their paths intersect each other
some applications may require general communication, e.g. communication in the local area, transmit images and sound, debugging and reconfiguration
geographic routing: move the packet in the direction of the destination. Often fails. Many workarounds, including Geometric Routing, which keeps track of overall network topology.
gradient routing: source or destination sends a broadcast, everyone keeps track of distance. Grad, Lusus.
sender can send a broadcast, which builds a reverse route back to the sender. Receiver uses this route to reply, building a route back to itself. Multipath On-Demand Routing (MOR).
keeping multiple paths (MOR) lets you dynamically load balance (each node may have multiple next hops) and try to avoid congestion (if one transmission fails, try transmitting to a different next hop).

wanted: long range and low power
directional focus: lasers, infrared, phased array antennas, directional antennas. All require aiming, but some could be automatic
omnidirectional: no focus needed, but more power needed for same range
if antenna is high off the ground, power needed increases with r²
if antenna is near the ground, power needed increases with r⁴
Industrial, Scientific, Medical (ISM) bands are generally free from licensing restrictions
ISM bands are available in the 800-900MHz range (but different frequencies e.g. for US and Europe) and 2.4GHz range (worldwide, same frequency as microwave ovens, resonant frequency of water) and other ranges
selection of radios (systems, circuits, chips), maximum power level usually determined by regulation, e.g. FCC in the US
serial radios, 802.11, Bluetooth, Zigbee, and many other up-and-coming standards