The ALOHA Protocol

1
The ALOHA Protocol

• Free for all whenever station has a frame to
  send, it does so.
• Station listens for maximum RTT for an ACK.
• If no ACK after a specified amount of time (how
  long?), re-sends frame after waiting a random
  amount of time (otherwise?)
• It continues to retry for a number of times and
  then gives up. (What happens then?)
• Receivers check FCS and destination address to
  ACK.
• 0.18 the peak utilization with an average of 0.5
  frame attempted every frame time. (Wait a minute.)

2
Slotted Aloha Performance
How long does it take to send a frame?
one success
k-1 failures
Expected number of transmissions
3
Carrier Sense Multiple Access (CSMA)

• Stations sense the medium before transmitting.
• This can reduce the vulnerable period.

4
CSMA

• 1-persistent
• If medium is idle, then transmit.
• If medium is not idle, then wait until it is and
  then transmit.
• p-persistent (for slotted channels only)
• If medium is idle, then transmit.
• If medium is not idle, then wait until it is idle
• Once idle then transmit with probability p. And
  wait for the next slot with probability 1-p and
  repeat.
• Problem when a collision occurs, it lasts for a
  long time, the whole frame transmission time. We
  shall see that for 10Mbps Ethernet, the max frame
  time is 1.2ms, but the max time to detect
  collision is 50microsec.

5
Carrier Sense Multiple Access with Collision
Detection CSMA/CD
When transmitting, detect if a collision is
occurring. If a collision is occurring, abort as
soon as possible and wait a random amount of time
before retrying.
After beginning transmission, how long until we
know that we have control of the channel? Twice
the propagation delay.
6
CSMA/CD
After beginning transmission, how long until we
know that we have control of the channel? Twice
the propagation delay.
Let T be the propagation delay. At t0, node 1
begins transmission. Suppose that at tT-? node 2
begins to transmit. At tT, node 2 detects the
collision. At t2T-?, node 1 detects the
collision.
Keeping propagation delay small can improve
efficiency. After a collision is detected, often
a jamming signal is applied. This slightly
increases the contention interval.
7
CSMA/CD
What happens if the propagation is too long and
frame too short?
Frame transmission time must be at least twice as
long as the propagation delay.
10Mbps ethernet one bit takes ? Propagation
sec/meter ?
10base max length is 2500 meters (with
repeaters). So minimum frame size is ? But the
the repeaters add some time, so roundtrip
propagation delay lt 50microsec. So minimum frame
? What would be the minimum frame for 100Mbps,
1Gbps?
8
CSMA/CD
9
Ethernet

• ltdata rategtltSignaling methodgtltMax segment lengthgt
• 10Base5 10Base2 10Base-T 10Base-FP
• Medium Coaxial Coaxial UTP 850nm fiber
• Signaling Baseband Baseband Baseband Manchester
• Manchester Manchester Manchester On/Off
• Topology Bus Bus Star Star
• Nodes 100 30 - 33

10
Frame Layout
Preamble 101010.10 for 56 bits allows the
PLL to synchronize Start of Frame 1 byte DA -
destination address MAC address SA - source
address MAC address Length 2 bytes Data so
that total length is at least 64 bytes no more
than 1500. CRC 4 bytes.
Efficiency (1500-26)/1500
11
Ethernet Address

• MAC address
• 6 bytes 7x1013 address
• Each interface has its own address. There are no
  duplicate address.
• Types of destination address
• All bits are one then broadcast, all card on LAN
  read this packet. This is used to learn about
  which other machines are on LAN, DHCP and also
  can be used in a network attack (but not
  anymore).
• Unicast address.
• Multicast address. If the first bit is one, then
  the address corosponds to a multicast group. All
  the members of the group read this packet.

12
CSMA/CD with Exponential Backoff
Time is divided into 51.2 micro sec slots. If a
collision occurs, then each host waits either 0
or 1 slot times before trying again. If this
fails, they wait either 0, 1, 2, or 3 slot
times.. However, the maximum slot times to wait
is 1023. And after 16 total tries, it gives up
and signals upper layer. Performance with
large frames, Ethernet get about 85 efficiency.
13
Hubs and Switches
Hub
To the hosts, the wires appear to be connected.
But there may be so circuitry there.
10base-T
14
Switches
Fewer collisions
No collisions
In the switch there can be much intelligence. The
packets are buffered and the packets only go to
the correct card or set of cards. This stops
packet sniffing.