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CS 6250 – Class 7 – 9.10.2002 PowerPoint Presentation
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CS 6250 – Class 7 – 9.10.2002

CS 6250 – Class 7 – 9.10.2002

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CS 6250 – Class 7 – 9.10.2002

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  1. CS 6250 – Class 7 – 9.10.2002 Ethernet (IEEE 802.3)

  2. 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

  3. 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)

  4. 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

  5. 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 … … …

  6. 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

  7. 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

  8. 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

  9. 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)