1. Ethernet

2. Overview Ethernet is now the dominant LAN technology in the world.
Ethernet is not one technology but a family of LAN technologies.
Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations.
However, the basic frame format and addressing scheme is the same for all varieties of Ethernet.

3. A system called Alohanet was developed.
The original concept for Ethernet grew out of the problem of collisions.
A system called Alohanet was developed.
Allowed structured access to the shared radio frequency band.
This work later formed the basis for the Ethernet access method known as CSMA/CD.
Ethernet developed in by Xerox.

4. Ethernet Generations Original Ethernet: Coaxial cable (10Base5)
Thicknet.
Next Generation:
Thin coax cable (10Base2)
Thinnet.
Modern Ethernet:
Twisted pair ethernet (10BaseT)
Uses hub: physical star but logical bus.

6. Why Ethernet The success of Ethernet is due to the following factors:
Simplicity and ease of maintenance
Ability to incorporate new technologies
Reliability
Low cost of installation and upgrade
Scalable

7. Operation
Ethernet operates in two areas of the OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer.

8. Multiple Access Methods
CS 640
Multiple Access Methods
Controlled
Partition channel so each node gets a slice of the bandwidth
Essentially circuit switching – thus inefficient
Examples: TDMA, FDMA, CDMA (all used in wireless/cellular environments)
Contention-based
Nodes contends equally for bandwidth and recover from collisions
Examples: Aloha, Ethernet
Token-based or reservation-based
Take turns using the channel
Examples: Token ring

9. Ethernet Fundamentals
Gigabit Ethernet Started as a LAN technology
Now extends out to distances that make Ethernet a metropolitan-area network (MAN) and wide- area network (WAN) standard.
Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps.

10. Experiences with Ethernet
CS 640
Ethernets work best under light loads
Utilization over 30% is considered heavy
Network capacity is wasted by collisions
Most networks are limited to about 200 hosts
Specification allows for up to 1024
Most networks are much shorter
5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive, fast and easy to administer!

11. Ethernet Problems
Ethernet’s peak utilization is pretty low (like Aloha)
Peak throughput worst with
More hosts
More collisions needed to identify single sender
Smaller packet sizes
Longer links
Collisions take longer to observe, more wasted bandwidth
Efficiency is improved by avoiding these conditions

12. Switched Ethernet
CS 640
Switches forward and filter frames based on LAN addresses
It’s not a bus or a router (although simple forwarding tables are maintained)
Very scalable
Options for many interfaces
Full duplex operation (send/receive frames simultaneously)
Connect two or more “segments” by copying data frames between them
Switches only copy data when needed
key difference from repeaters
Higher link bandwidth
Collisions are completely avoided
Much greater aggregate bandwidth
Separate segments can send at once

13. Ethernet Frames
Preamble is a sequence of 7 bytes, each set to “ ”
Used to synchronize receiver before actual data is sent
Addresses
unique, 48-bit unicast address assigned to each adapter
example: 8:0:e4:b1:2
Each manufacturer gets their own address range
broadcast: all 1s
multicast: first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data

14. Fast and Gigabit Ethernet
CS 640
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet
Uses different physical layer encoding (4B5B)
Many NIC’s are 10/100 capable
Can be used at either speed
Gigabit Ethernet (1,000Mbps)
Compatible with lower speeds
Uses standard framing and CSMA/CD algorithm
Distances are severely limited
Typically used for backbones and inter-router connectivity
Becoming cost competitive
How much of this bandwidth is realizable?

15. Physical Layer Configurations for 802.3
CS 640
Physical layer configurations are specified in three parts
Data rate (10, 100, 1,000)
10, 100, 1,000Mbps
Signaling method (base, broad)
Baseband
Digital signaling
Broadband
Analog signaling
Cabling (2, 5, T, F, S, L)
5 - Thick coax (original Ethernet cabling)
F – Optical fiber
S – Short wave laser over multimode fiber
L – Long wave laser over single mode fiber

16. Ethernet Family Tree

21. Various Line Coding Methods
Gigabit Ethernet uses NRZ.
Manchester
consumes
too much
Bandwidth.
Gigabit
Ethernet
cannot afford
to use it.

23. MAC Addresses
Ethernet uses MAC addresses that are 48 bits in length and expressed as twelve hexadecimal digits.
MAC addresses are referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes.
FF:FF:FF:FF:FF:FF (Broadcast)

24. Interframe Spacing
The minimum spacing between two non- colliding frames is also called the interframe spacing.
This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame.

25. Interframe Spacing Cont.
Regardless of the speed/version of Ethernet, the spacing remains the same.
This interval is referred to as the spacing gap. The gap allows slow stations time to process the previous frame and prepare for the next frame.

26. Backoff Timing
After a collision occurs all stations allow the cable to become idle (each waits the full interframe spacing)
Stations that collided must wait an additional and potentially progressively longer period of time before attempting to retransmit the collided frame.

29. Traditional Ethernet is half duplex
Either transmit or receive but not both simultaneously
With full-duplex, station can transmit and receive data simultaneously
With full duplex, Throughput (actual transmission rate) is doubled.
10-Mbps Ethernet in full-duplex mode, theoretical transfer rate becomes 20 Mbps
100-Mbps Ethernet in full-duplex mode, theoretical transfer rate becomes 200 Mbps

30. Changes that should be made with any computer in order to operate in Full-Duplex Mode
Attached stations must have full-duplex NIC cards
Must use two pairs of wire one pair for transmitting from host to switch (inbound) and the other pair for transmitting from switch to host (outbound)
Must use a switch as a central device not a hub
Devices must be connected point-to-point (dedicated) to the switch
Each station constitutes separate collision domain
CSMA/CD algorithm no longer needed (no collision)
No limit on the segment length
Same MAC frame format used

31. Ethernet Errors Collision or runt
Simultaneous transmission occurring before slot time has elapsed
Late collision
Simultaneous transmission occurring after slot time has elapsed
Jabber, long frame and range errors
Excessively or illegally long transmission 
Short frame, collision fragment or runt
Illegally short transmission
FCS error
Corrupted transmission
Alignment error
Insufficient or excessive number of bits transmitted
Range error
Actual and reported number of octets in frame do not match
Ghost or jabber
Unusually long Preamble or Jam event