ATM, Infrastructure - PolyLAB

hellhollowreadingNetworking and Communications

Oct 26, 2013 (3 years and 7 months ago)


ATM, Infrastructure

Getting around the world if you’re a
network packet

How those layers work

Networks have up to 7 (8!) Layers:

Application: software

Presentation: translation

Session: start and stop comms sessions

Transport: make connection good enough

Network: routing and switching

Data Link (2): Link control and access control
(IEEE 802.2 LLC and MAC)

Physical: the wire, radio, etc.

Where they happen

In the main computer software

Application, Presentation, Transport, some

In the computer’s hardware

Data Link (LLC and MAC), Physical

In the LAN or WAN

Routing, Transport, Session

In the global network

Routing, switching, big time!

From Physical to Network

Data link controlled by network card,
accesses physical network.

Sharing (“multiplexing”)

Collision detection

Hardware address (eg Ethernet/MAC) has to
be mapped onto IP number!

ARP server

software on computers that
announces and communicates IP numbers on

MAC/ARP is important, can go

Computers only occasionally announce
their IP address

Switches need MAC to know it to send to
right place


Hubs can make it hard to detect

Wireless systems need to switch and act
like hubs, often.

Going Local, Going Internet

If it’s a local machine, ARP tells me the MAC
address: send it there (can tell from the IP
number and subnet mask)

If it’s not a local machine:

Work out which gateway/router to send it to (IP

Work out the MAC address of the gateway

Send it THERE!


Each computer can route a little: sending to
the main routers on the LAN.

When a main router receives a packet, it
does exactly the same thing the other
computers do:

Work out which router to send it to

Send it via the MAC address (not always

More Routing (IMPORTANT!)

Packets bounce from one network to
another: Internetworking

Often don’t have enough bandwidth to
transmit all the packets on:

Send those packets first that are important

Throw away less important packets when you
have too much to do!

Quality of Service: making things work

Even More Routing

Need to communicate which route to go,
and make sure things don’t go in circles
(RIP, BGP, and more)

The routing rules determine how the
internet behaves. Very iffy in IPv4.

Video needs to get through fast

Audio musn’t vary too much in speed

Important stuff MUST get through!

How do you decide

they’re all alike!

Quality of Service (QoS)

Bandwidth: how fast

Latency: how long (not the same as how

Packet Loss: how often you drop the

Bandwidth, Packet Loss, Latency variation:
how constant is all this?

The Big Problem with IP

Packets all look alike

really need to
analyze ‘em to work out what you should do
with them. Often just let them all go
through, or all die!

Packets all different sizes

hard to predict
how to fit them all together in the right
order to make things efficient.

The Telco Solution: ATM

Asynchronous Transfer Mode

Chop up all the information into the same
little box: 53 bytes CELLS!

Forget source and destination: label by what
path the cell takes

Set up path depending on what the user wants:

Simple decisions to keep it moving: Only THREE


IP: I want to go from 1131 Barclay St to 105
W. Broadway. Route Route Route.

ATM: I’m going on path 10943

10943 = Take a left out of my door, turn right
onto Thurlow, Turn left on Nelson, Right on
Burrard, take the bridge, Turn right on

“Fabric” remembers the path from the path

QoS and ATM

I ask for how fast I want to go, where I want
to go, how long I want to take going there,
how much I’m willing to put up with

Fabric finds the path that matches my

ATM switches make sure the right cells,
just going on the path number, get
switched/dumped in the right way.

IP over ATM

Big IP pipes are carried by defining virtual
IP networks over ATM

Other protocols can also be carried.

A lot of the “broadcast” mode of IP is
missing though. IP lets me send to more
than one place at once.

ATM can be awfully expensive, but uses
links way more efficiently.

Other QoS stuff

Older systems like Frame: “synchronous” version of
ATM. TDMA is one of these! Big for older satellite
comms, links to remote parts of Canada, and used for
communication with Mars by JPL and others. QoS big
for space stuff!

RSVP: Resource reSerVation Protocol: trying to tell
the routers how to treat packets

TOS: oldest and dumbest: simple way of labelling
certain packets as more important than others.

Why should I care?

Each jitter in video, or weirdness in sound
for IP Telephony or Voice mode in
Messenger, etc, is due to the QoS problems
of IPv4.

Mail loops, denial of service attacks, etc:
cases where we can’t make sure the
important information gets through

Bad QoS wastes links, time, money: how
much am I willing to pay?

Big Pipes

Network QoS problems often solved by
making pipes bigger: if everything gets
through fast, everything is fine

Bit like saying we can turn the entire country
into big highways. REALLY big highways.

What happens if you can’t afford big?

What happens if you can’t afford ATM?

What happens if you DIE without QoS?