CEN 5501C - Computer Networks - Spring UF/CISE - Newman1 Computer Networks Chapter 5 – Hubs, Switches, VLANs, Fast Ethernet

CEN 5501C - Computer Networks - Spring UF/CISE - Newman2 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 UF/CISE - Newman3 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 UF/CISE - Newman4 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 UF/CISE - Newman5 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 UF/CISE - Newman6 Faster LANs 10 x speed increase in Ethernet –Make minimum packet length 10x larger –Make maximum LAN diameter 10x smaller (IEEE 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 UF/CISE - Newman7 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) Carrier extension Frame padded to 512 B Multiple frame burst –First frame extended to 512 bytes –Other frames follow (avoid duplicates if CD) frm Frame padded to 512 B frm bridge Gig-E LANOther LAN Carrier extensionf1f2f3f4f5

CEN 5501C - Computer Networks - Spring UF/CISE - Newman8 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 UF/CISE - Newman9 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)