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Ethernet
Kenneth Castelino
Network Architecture - Protocols
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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.
Media Access Control - MAC
• 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.
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.