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CS 6250 – Class 7 – 9.10.2002 Ethernet (IEEE 802.3) News Get project description Form groups (no later than today!) Design report is due on Oct 1 We will be discussing the project in each class (ask many questions!) Homework #3: chapter-2: 18, 25, 29 Ethernet evolution

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slide2
News
  • Get project description
  • Form groups (no later than today!)
  • Design report is due on Oct 1
  • We will be discussing the project in each class (ask many questions!)
  • Homework #3: chapter-2: 18, 25, 29
ethernet evolution
Ethernet evolution
  • Developed in mid-70s at XEROX PARC
  • Standardized by IEEE (802.3 10Base2/5/T standards)
  • Initially, a multiple-access shared-bus local area network technology running at 10Mbps
  • Well known for its “Carrier Sense Multiple Access with Collission Detect” (CSMA-CD) algorithm
  • Recent extensions to 100Mbps (“Fast Ethernet” 100BaseT/F), 1Gbps (“Gig-E” 1000BaseSX/LX/CX/T), and 10Gbps (“10Gig-E”)
    • Normally used as a framing protocol for full-duplex point-to-point links, and as L2 protocol in switched networks
    • Rest of this class focuses on 10Mbps Ethernet (and 802.3 standard)
physical properties

Ethernet segment: coaxial cable up to 500m (10Base5)

    • 200m for 10Base2, or Cat-5 twisted pair up to 100m for 10BaseT
    • For 10Base5, at least 2.5m between host-taps
  • Transceiver and network adaptor:
    • Carrier sensing
    • Collision detection
Physical properties
segments and repeaters
Segments and repeaters
  • No more than 4 repeaters between two hosts
  • Maximum distance from host to host: 2500m
  • Maximum round-trip delay in 10Base5 Ethernet network: 51.2 microseconds
  • Up to 1024 hosts in network

Repeater

Host

collision domain
Collision domain
  • Ethernet hubs vs Ethernet switches
  • The concept of a “collision domain”
  • Only one host can transmit a frame at a time in the entire Ethernet collision domain
  • Half-duplex vs full-duplex Ethernet networks
ethernet frame format
Ethernet frame format
  • Preample + 14B header + CRC (Manchester encoding)
  • Max frame-body size: 1500B
  • Min frame-body size: 46B
  • Header field for frame size?
    • IEEE 802.3 standard: replaces Type with Size
  • Ethernet address format: 8:0:2b:e4:b1:2
    • Each Ethernet card has a different address (e.g., all 00-00-0C-xyz addresses are allocated to Cisco NICs)
    • Address types: unicast, multicast, broadbast
    • Promiscuous receiving mode

64

48

48

16

32

Src

Dest

Preamble

Type

Body

CRC

addr

addr

media access control mac protocol
Media Access Control (MAC) protocol
  • If line is idle, transmit frame; else wait for line to become idle
  • If transceiver detects frame collision, transmit 32b “jamming” sequence, and abort rest of frame
  • After first collision, wait for k*51.2microseconds (k=0,1,2,3 – selected randomly) before trying again
  • If line is still busy after third collision, choose randomly k between 0 and 2n-1 (n: number of collisions) and wait for k*51.2 microseconds
    • Exponential backoff
    • Adapters typically abort after 16 collisions
minimum frame size 64 bytes
Minimum frame size: 64 bytes

A

B

(a)

  • Frame size is at least 512b (14B for header, 46B for data, and 4B for CRC): Why?
  • We must guarantee that a collision will be detected while sender is still transmitting the frame
  • RTT at 2500m 10Base5 network: 51.2 microseconds (or, 512bits at 10Mbps)
  • How can you maintain the same min frame size requirement in 100Mbps Ethernet?

A

B

(b)

A

B

(c)

A

B

(d)