Data Communications and Networking. Chapter 9 High Speed LAN s and Wireless LANs References: Book Chapters 16 and 17 Data and Computer Communications, 8th edition By William Stallings. Outline. Fundamentals of Ethernet ALOHA, slotted ALOHA, CSMA CSMA/CD Ethernet Examples
High Speed LANs and Wireless LANs
Book Chapters 16 and 17
Data and Computer Communications, 8th edition
By William Stallings
C detects a collision!
A detects a collision!
The amount of time that it takes to detect a collision is no greater than twice the end-to-end propagation delay.
Frames should be long enough to allow collision detection prior to the end of transmission. If shorter frames are used, then collision detection does not occur.
For 10 and 100Mbps Ethernet, the frame length is at least 512 bits.
For 1Gbps Ethernet, the frame length is at least 4096 bits, using carrier extension or frame bursting.
Preamble: 7 octets of 10101010
Length: the maximum frame size is 1518 octets, excluding the preamble and SFD.
Pad: octets added to ensure that the frame is long enough for collision detection
FCS: 32-bit CRC, based on all fields except preamble, SFD, and FCS
10-Gbps Ethernet also targets at Metropolitan Area Networks (MANs) and WANs.
CHidden Terminal Problem
A and B can hear each other. B and C can hear each other. But A and C cannot hear each other.
When A is sending data to B, C cannot sense this activity and hence C is allowed to send data to B at the same time. This will cause a collision at B.
Consider the effect of RTS/CTS:
RTS alerts all stations within range of source (i.e., A) that exchange is under way; CTS alerts all stations within range of destination (i.e., B).