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Computer Networks

Computer Networks. Chapter 5 – Hubs, Switches, VLANs, Fast Ethernet. Hubs. Ethernet originally a bus Not always convenient Failure can crash whole LAN Star topology Telephone wiring pattern already there Move bus to box, use twisted pair Hub acts as multi-port repeater

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Computer Networks

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  1. Computer Networks Chapter 5 – Hubs, Switches, VLANs, Fast Ethernet CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  2. Hubs • Ethernet originally a bus • Not always convenient • Failure can crash whole LAN • Star topology • Telephone wiring pattern already there • Move bus to box, use twisted pair • Hub acts as multi-port repeater • Security advantages CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  3. Learning Hubs • Dumb hub just repeats • All ports • Collisions • Learning hub • Forward only on port with D on it • Send jamming signal on other ports (why?) • More secure • Learns like a TB (still must b’cast unknown) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  4. Store-and-forward Hubs • When to forward • Don’t have to send jamming signal • Avoid collisions if output is busy • And more… • No maximum distance or minimum frame size • Ports may have different speeds • Aggregate capacity not limited by port speeds • Cut-through forwarding • A.k.a. bridges  CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  5. Switches • Layer 1 Switch • Multi-port repeater • Layer 2 Switch • Bridge • Layer 3 switch • Router • Mixed layer 1 and layer 2 • Collision domains (per card in hub) • Broadcast domains (all ports per hub) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  6. Faster LANs • 10 x speed increase in Ethernet • Make minimum packet length 10x larger • Make maximum LAN diameter 10x smaller (IEEE 802.3 solution for 100 Mbps) • Some combination of the above • Ignore collisions – let a higher layer cope • Use only store-and-forward switches • Gig-E • Packet size extended to 512 bytes CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  7. Gigabit Ethernet • 512 – Byte minimum frame size…. • Smaller frames get carrier extension • Carrier extension allows collision detection • But frame is still the smaller size (if fwded) frm Carrier extension frm Frame padded to 512 B Frame padded to 512 B bridge Gig-E LAN Other LAN • Multiple frame burst • First frame extended to 512 bytes • Other frames follow (avoid duplicates if CD) f1 Carrier extension f2 f3 f4 f5 CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  8. Ethernet Hub Links • Link Negotiations • Speed • Duplexity • Link duplexity • Full duplex (send and receive simultaneously) No need for carrier extension • Half duplex (Rx while Tx = collision) Used when hub is a repeater CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  9. Hub Overload • Congestion may occur • Source port speed > destination port speed • Multiple sources send to same destination • Buffer space may be exceeded • Flow control methods • Full duplex – pause frame (explicit control) • Half duplex – backpressure (simulate collisions) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

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