cs398 Lecture Notes
Spring 2000
Week 4, Tuesday

For today you schould have read the rest of the Chapter
and write answers for Questions 19, 20 and 26(a)

You should also have thought about questions 22 and 23.

We will have an exam on Thursday!

Things that go on exams:

1) basic exercises like the things you've been doing for
   homework (I will probably take some from the book!)

2) short answer questions (like the ones in parentheses in
   the lecture notes)

3) design evaluation questions: what if?

4) perhaps a reading comprehension question: read the
   following text and answer the questions about it

Open book, open notes!


Today:

1) MAC part I

2) review questions


Overview of MAC
---------------

1) Ethernet: shared medium

   collision detection and probablistic, exponential backoff

2) Token ring: not physically a shared medium, but logically it is

   pass the conch

3) Wireless: definitely shared

   similar to Ethernet except collision detection is harder!

   hidden node problem


Ethernet
--------

CSMA/CD =

carrier sense... nodes can distinguish between idle and busy link

multiple-access... like a bus

collision detect... if two nodes start transmitting at the same
	            time, they can tell


Great Ethernet mysteries

1) how does collision detection work?

   (if we could encode an extra bit on the network all the
   time, we would use it for data -- nothing is free)

2) why Manchester encoding?


Arbitration
-----------

Alternatives...

If you can't detect collisions, then you need some form of
bus arbitration.

What wrong with the kind of bus arbitration they do on
internal buses?

It's centralized!

Why are centralized solutions unacceptable in networks?

The need for graceful degradation:

   if one of the devices in your machine breaks, it often
   doesn't matter if the rest of the machine keeps working

   if one node on a network goes down, it would be unacceptable
   for the whole network to come down

   (graceful degradation is the opposite of catastrophic failure)


Wiring
------

Each node on an Ethernet has a transceiver, which connects
the adaptor to the cable.

"The wire" is actually a virtual wire that might be made
up of

   thin coax, thick coax, twisted pair

and

   repeaters, and hubs.

The different kinds of wire all run at the same speed (10 Mbps
for basic Ethernet), but have different maximum distances.

1) internal resistance dissipates the signal as heat

2) impedence dissipates the signal as E/M radiation
   (coax reduces this, as does twisting)

3) all the transceivers divert some of the signal

But most importantly,

The maximum distance determines the maximum delay, which
determines the minimum message size.


Collision domain
----------------

Every signal put on the wire goes to all the nodes,
which means that if any two nodes put a signal on
the wire at the same time, it will cause a collision.

All the nodes that can collide with each other comprise
a collision domain.

Available bandwidth seen by any host depends on the
number of hosts in the collision domain and the amount
of traffic they generate.

In practice, Ethernets seem to work well only up to
loads of 30% (idle 70% of the time).  Above that, the
number of collisions significantly degrades performance.

Look at the figure on page 118.  What is the obvious
improvement we might make in this design?

1) replace the repeaters with gateways

   (what's the difference?)

2) hope for spatial locality in traffic patterns


Encoding and framing
--------------------

As I mentioned, Ethernet uses Manchester encoding, which
is infuriatingly inefficient (the signal is twice as
big as the message)!

Why?

Framing issues dominated by need for collision detection

1) 64 bit preamble

2) minimum 46 bytes data for minimum frame length 64 bytes

3) bit oriented framing, with a twist...

   (how do we detect the end?)

   (what if a preamble appears in the message?)

4) header contains src and dest addresses

   Ethernet addresses: unique 6-byte hardware ID

   (how many Ethernet cards can they make?)

Adaptors are responsible for identifying frames
addressed to them.

   (What do you call adapator that snags _all_ the frames?)

   (What are the implications for security within an Ethernet domain?)


Arbitration
-----------

No negotiation, just blast away.

If detect interference, transmit jamming sequence and stop.

In order to guarantee collision detection, A must transmit
continuously for one (maximal) RTT

  (I don't quite see why this is true, but ok)

and that's why they impose a maximum distance.

    51.2 us at 10 Mbps = 512 bits.