04-02-2013, 11:29 AM
Advanced Medium Access Control Protocols
1Advanced Medium.pdf (Size: 124.58 KB / Downloads: 21)
What is a LAN?
Definition
A LAN (Local Area Network) is a network for the bit-serial transmission of
data between independent peer stations.
It is usually under the legal control of a single user (single enterprise) and is
usually limited to the user pemises.
Characteristics of a Local Area Network
High transmission speed (10 - 1000 MBit/s)
Easy, inexpensive connection of stations
No need to take Telekom rules and regulations into account
Different types of devices can be connected easily:
personal computers
department file-, mail or printer servers
mobile phones (via WLAN) and IP-phones
embedded system (e.g., the ARM-based Raspberry PI
board)
home-automation systems
Ethernet-enabled multiple plugs
bridges to sensor networks (mostly serial-line emulation
over IP)
Interconnection to Wide-Area Networks (WANs) is possible via
routers (layer 3).
Early MAC: Aloha
Collision detection approach:
Each station can send at any time
A collision of two or more packets can be detected
After a collision, affected stations must repeat their packets. In order
not to create another collision immediately, both wait a random time,
hoping not to harm one another again.
CSMA/CD Basic Assumptions and Principles
Assumptions
All stations can hear each other on the medium.
The frame transmission time is much longer than the maximum
propagation delay between the stations. In other words, the
sender is still sending when the most distant station in the LAN
segment begins to hear the transmission.
Principles
Carrier Sensing, Multiple Access (CSMA)
(also called "listen before talk")
A station that wants to send senses the medium:
If the medium is occupied, sending is postponed.
If the medium is free, sending begins immediately.
CSMA/CD Retry Strategies
What will a station do when it finds the medium busy?
non-persistent
The station waits a random exponential backoff time interval and then starts a
new transmission attempt.
1-persistent: The station waits for the end of the transmisson and then starts its
own transmission immediately after the end of the other transmission (transmission
probability = 1)
p-persistent (0 < p < 1): The station waits for the end of the transmisson and
then sends its data immediately with a probability p or waits a random
exponential backoff time interval with probability 1-p.