Ethernet

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.

• 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 1973 by Xerox.

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.

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

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.

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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

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.

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Experiences with Ethernet

• 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!

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

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Switched Ethernet

• 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

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Ethernet Frames

• Preamble is a sequence of 7 bytes, each set to “10101010” ▫ 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

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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?

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Physical Layer Configurations for 802.3

• 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

Ethernet Family Tree

Various Line Coding Methods

Gigabit Ethernet uses NRZ.

Manchesterconsumes too much Bandwidth.GigabitEthernetcannot afford to use it.

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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)

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.

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.

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.

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

Changes that should be made with any computer in order to operate in Full-Duplex Mode1) Attached stations must have full-duplex NIC cards2) 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)

3) Must use a switch as a central device not a hub4) 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 802.3 MAC frame format used

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