Computer s Principles LN - thernet Prof. ndrzej Duda duda@imag.fr Interconnection structure - layer subnetwork interconnection layer interconnection layer subnet subnet router switch (bridge) VLN http://duda.imag.fr Interconnection structure - layer Interconnection at layer interconnection layer switch (bridge) VLN Switches (bridges) interconnect s logically separate groups of s (VLNs) managed by one entity ype of the network broadcast Forwarding based on address flat address space forwarding tables: one entry per works if no loops careful management Spanning ree protocol not scalable Protocol architecture I 80. - thernet pplication ransport L PDU ( Frame) L PDU ( Frame) transceiver switch (bridge) Switches are layer intermediate systems ransparent forwarding Management protocols (Spanning ree, VLN) repeater terminator 6
Random ccess protocols When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes -> collision, random access protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) xamples of random access protocols: LOH, slotted LOH CSM, CSM/CD (thernet), CSM/C (80.) CSM/CD (Collision Detection) CSM/CD (Carrier Sense Multiple ccess/ Collision Detection) carrier sensing, deferral if ongoing transmission collisions detected within short time colliding transmissions aborted, reducing channel wastage persistent transmission collision detection: easy in wired LNs: measure signal strengths, compare transmitted, received signals difficult in wireless LNs: receiver shut off while transmitting 7 8 CSM/CD algorithm CSM / CD Collision i = while (i <= maxttempts) do listen until channel is idle transmit and listen wait until (end of transmission) or (collision detected) if collision detected then stop transmitting, send jam bits ( bits) else wait for interframe delay (9.6!s) leave wait random time increment i end do 9 senses idle channel, starts transmitting shortly before, senses idle channel, starts transmitting 0 0 CSM / CD Jam Signal Random retransmission interval senses collision, continues to transmit the jam signal (-bit) senses collision, continues to transmit the jam signal 0 t r = random (0, k -) k = min (0, ttemptnb) slot time =. µs st collision, r = 0, nd collision, r = 0,,, 0 th, r = 0,,, 0 th, stop
CSM / CD Retransmission Retransmission interval waits random time t waits random time t=slottime < t =*slottime senses channel idle and transmits senses channel busy and defers to now waits until channel is idle 0 t t Round trip time Slot limits the interval during which collisions may occur µs +. µs <. µs ( bits) channel is acquired after. µs non-valid frames (results of collisions) < bits minimal frame size (data field 6 bytes) unit of the retransmission interval Frame format (thernet v.) ddressing preamble dest source type data CRC 8 bytes 6 bytes 6 bytes bytes 6-00 bytes bytes Preamble synchronization : 0000.000 ddresses unique, unicast and multicast (starts with the first bit ) broadcast: ype upper layer protocol (IP, IPX, RP, etc.) address: 8 bits = adapter identifier sender puts destination address in the frame all stations read all frames; keep only if destination address matches all address (FF:FF:FF:FF:FF:FF) = broadcast address 08:00:0:7:0d:d C D 00:00:c0:f:6c:a 0:00:e:0:a6:cf (group address) 6 ddressing Data on thernet is transmitted least significant bit of first byte first (a bug dictated by Intel processors) Canonical representation thus inverts the order of bits inside a byte (the first bit of the address is the least significant bit of the first byte) examples of addresses: 0:00:e:0:a6:cf (a group address) 08:00:0:7:0d:d (a SUN machine) 00:00:c0:f:6c:a (a PC ) 00:00:0c:0:78:6 (a CISCO router) FF:FF:FF:FF:FF:FF the broadcast address s Function of a simple, port repeater: repeat bits received on one port to other port if collision sensed on one port, repeat random bits on other port One network with repeaters = one collision domain s perform only physical layer functions (bit repeaters) 7 8
From s to Hubs Multiport repeater (n ports), logically equivalent to: n simple repeater connected to one internal thernet segment Multi-port repeaters make it possible to use point-to-point segments (thernet in the box) ease of management fault isolation Multiport S S S thernet Hub UP segment Multiport 0 S Hubs ree topology (star) (répéteur multiport) max. s to other 9 0 ridges port ridge D port port ridges are intermediate systems, or switches, that forward frames to destinations based on addresses ransparent bridges: learn the Forwarding able C Forwarding able Dest Port Nb addr C D ransparent ridging () ridges are intermediate systems that forward frames to destinations based on addresses Interconnect systems beyond one LN segment, keeping main characteristics of LN without additional addresses addresses used to identify end systems nd systems ignore that there are transparent bridges bridge is transparent frames not changed by bridges frames not sent to bridge, but rather: bridge is promiscuous listens to all frames and retransmits if needed Collision domains s and ridges in OSI Model bridge to 7 pplication Presentation Session pplication Presentation Session to 7 ridges separate collision domains a bridged LN maybe much larger than a repeated LN there may be several frames transmitted in parallel in a bridged LN ransport L PDU ( Frame) L PDU ( Frame) ridges are layer intermediate systems ransport nd System /Hub ridge nd System s are in layer intermediate systems Routers are layer intermediate systems (IP routers)
Protocol architecture Switched thernet Switched thernet = bridge in the box pplication ransport L PDU (IP packet) pplication ransport otal bandwidth is not shared parallel frame transmission Half and full-duplex operation L PDU ( Frame) switch (bridge) L PDU ( Frame) router ridge ridge Routers are layer intermediate systems xplicit forwarding has to know the address of the first router Management protocols (control, routing, configuration) C D U V W X 6 Switching Store and forward receive full frame, check if valid, retransmit 0 µs delay for a 6 bytes frame Cut through address read, retransmit at line speed bytes read and decision made on output link if link busy, defaults to store and forward 0 µs delay for a 6 bytes frame transmission of non-valid frames CRC check at end of forwarding Flow control back pressure using false collision notification on full duplex links, send PUS to s VLN - Virtual LN Keep the advantages of Layer interconnection auto-configuration (addresses, topology - Spanning ree) performance of switching nhance with functionalities of Layer extensibility spanning large distances traffic filtering Limit broadcast domains Security separate subnetworks C D VLN ridge/switch VLN 7 8 Virtual LNs 80.Q No traffic between different VLNs VLNs build on bridges or switches VLN trunk D S data D S t tag data ridge/switch trunk ridge/switch 80. frame 80./P frame Frame encapsulation extension for assigning frame priority and VLN tag C D VLN VLN X Y Z U VLN VLN t - bytes of PI (ag Protocol Identifier): 0x800 tag - bytes of CI (ag Control Information): priority ( bits), VLN Id ( bits) (VID 0x00 often reserved for management) max length = bytes 9 0
agged VLNs VLN table VLN tag ports untagged,, tagged untagged, tagged C VLN VLN SWICH 6 trame non taguée interface physique IP em0 D trame taguée VLN em0 em0. interface IP em0. virtuelle Summary Original thernet is a shared medium: one collision domain per LN ridges are connectionless intermediate systems that interconnect LNs Using bridging, we can have several collision domains per LN thernet switches use bridging State of the art switched 00 Mb/s thernet to the Gb thernet between switches Wireless LNs become increasingly popular WiFi, luetooth