Ethernet

Kenneth Castelino

Network Architecture - Protocols

• Physical: Actual signal transmission• Data-Link: Framing / Error Detection• Network: Routing / Addressing• Transport: Congestion / Flow Control• Application: Specific to user needs

Layered Protocols – (HTTP)

Data Link Layer - Ethernet

• Invented in 1973 @ Xerox. (IEEE 802.3)• Originally a LAN technology – extended to MAN / WAN.• Same frame format, different wiring schemes, data rates

across generations.• Most common version (10BaseT) – 1990.

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.

Ethernet Components

• NIC – Network Interface Card

Integrated Tx/Rx – direct interface to medium.• MAU – Media Attachment Unit

Attaches network interface to the medium (integrated into NIC).• AUI – Attachment Unit Interface

Decouple physical layer -reuse MAC design with different media.• MII – Media Independent Interface

Like AUI for gigabit / faster ethernets.

Ethernet Addressing

• 48-bit address• Address assigned when NIC card is manufactured.• Packets can be sent to

Single address – Unicast

All stations on network – Broadcast (address = all 1s.)

Subset of stations – Multicast

• Broadcast (address = all 1s.)

All receivers accepts unicast / broadcats.

• Half addresses reserved for multicast (247)

NIC can accepts zero or more multicasts.

Ethernet Frame

Sender adds:

Senders address is source

Recepients addreess in destination

Type of data in frame type

Error check data (CRC)

Receiver NIC:

Gets transmitted frame.

Examines address and either accepts or rejects.

Passes frame to system software.

• Shared medium – stations take turns at sharing the medium.• Media access control ensures fairness.

CSMA / CD• Carrier Sense: wait till medium is idle before sending frame.

• Multiple Access: multiple computers use the same shared media. Each uses same access algorithm.

• Collision Detection: Listen to medium – detect if another station’s signal interferes – back off and try again later.

Media Access Control - MAC

CSMA / CD

• If collision occurs: wait a random time t1 - 0< t1<d.

D depends on transmission speed – time for frame width or 512 bits.

• If second collision occurs, wait a random time t2 - 0< t2<2d.

Double range for each succesive collision.

Exponential backoff

• No acknowledgement like TCP.

• CSMA/CA used in wireless networks where not all stations receive message.

• Both sides send small message followed by data:X is about to send to Y

Y is about to receive from X

Data frame sent from X to Y.

Recent Developments

• 100Base-FX

LED light source / MMF / 2 km max distance.

Modal dispersion – limited bandwidth

• 100Base-SX (IEEE 802.3z)

Short wavelength laser (850 nm)

Max distance = 5 km.

• 100Base-LX

Long wavelength laser (1310 nm)

Max distance = 5 km.

Beyond Gigabit Ethernet

• 10 Gb/s Ethernet

No CSMS/CD, same frame format.• Applications

Upgrade LANs / Backbone.

MAN applications.

References

[1] I.P. Kaminow, Tingye Li, Ed., “Optical Fiber Telecommunications IV”, Chapter 11, Academic Press, 2002.

[2] John Chuang, SIMS 250 , UC Berkeley, Fall 2001.