Advanced Computer Networks: Lecture 17. This lecture will cover the following: virtual paths with ATM; physical layers for ATM; shared ethernet emulation with LANE; ATM/LANE protocol layers; clients and servers in LANE; multi-input multi-output device; switching fabric;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 17 Virtual Paths with ATM • Two level hierarchy of virtual connection: 8bit VPI and 16bit VCI – Switches in the public network use 8bit VPI – Corporate sites use full 24bit address (VPI + VCI) – Much less connectionstate info in switches – Virtual path: fat pipe with bundle of virtual circuits Public network Network A Network B Physical Layers for ATM • ATM may run over several phy media • ATM was assumed to run over SONET but both are entirely separable entities • ATM frame boundaries to be correctly identified – Successive 53byte ATM frames in payload – SONET overhead byte points to the payload – Another way is to calculate CRC (5th byte of the cell) ATM and LANs • ATM grew out of the telephone community and later used for computer communication • Significant advantage of performance and better scalability of switched over shared media • No distance limitation in ATM making it a good choice for highperformance LAN backbone • Pointtopoint, long distance Gigabit Ethernet is a competing technology with ATM ATM as a LAN Backbone H4 H5 Ethernet links ATM links H6 H3 E2 E1 Ethernet switch ATM-attached host H7 ATM switch E3 H1 H2 • Different from traditional LANs; no native support for broadcast or multicast ATM in a LAN • How to broadcast to all nodes on an ATM LAN ? –Without knowing all the addresses –Without setting up VC to all of them ATM in a LAN • Two solutions – Redesign protocols that consider LAN different from what ATM can provide (e.g. ATMARP) – Make ATM behave like shared media, without loosing performance advantage of switched media (e.g. LANE) • ATM address is different from a unique 48bit MAC address Shared Ethernet Emulation with LANE • All hosts think they are on the same Ethernet ATM Switch ATM Switch LANE / Ethernet LANE / Ethernet Adaptor Card Adaptor Card LANE / Ethernet LANE / Ethernet Adaptor Card Adaptor Card Ethernet Switch Ethernet Switch HH HH HH HH HH HH HH HH HH HH LAN Emulation (LANE) with ATM • Transparent shared media emulation of ATM • Adds (not changes) functionality to ATM switches • Each device needs a global MAC address, as well as an ATM address to establish a VC 10 Ports and Fabric • Buffering is required at ports – Buffer management has profound impact on performance – Internal (in fabric) or output buffering is normally used • Fabric: simply move packets from inputs to outputs 30 Design Goals Throughput • An n x m switch can provide max ideal throughput of S = S1 + S2 + ……… + Sn – Only possible if traffic at inputs is evenly distributed across all outputs – Sustained throughput higher than link speed of output is not possible 31 Design Goals Throughput • Variable size packets affect performance – Some operations have constant overhead per packet – Switch performs differently for different sizes of packets – Packet per second (pps) rate is also important • Most switches are subject to internal contention – Determine performance under diff traffic 32 Design Goals Throughput • Traffic models are important to throughput – Arrival time, output port, packet length – Extremely difficult to achieve accurate models – Trafficmodeling very successful in telephony • Designers now expect high range of throughputs – In order to handle a steady stream of 64byte packets, a 40Gbps switch need a rate of 78M 33 pps !!! Design Goals Scalability • Cost of hardware rises fast with increasing the number of ports n – Adding ports increases hardware & design complexity – Scalability in terms of rate of increase in cost • Design complexity determines maximum switch size – Switch designs run into problems at some maximum number of inputs and outputs 34 Switch Performance • Avoid contention with buffering – Use output buffering when possible – Apply backpressure through fabric – Input buffering with “peeking” (nonFIFO semantics) to reduce headofline blocking problems – Drop packets if input buffer overflows • Good scalability – O(N) ports – Port design complexity O(N) gives O(N2) for switch – Port design complexity O(1) gives O(N) for switch 35 Crossbar (“Perfect”) Switch • Problem: hardware scales as O(N2) 36 Knockout Switch: Pick L from N D 8to4 concentrator D 2x2 random selector D D D D D D D D delay unit Outputs Inputs D D D D D • Problem: what if more than L arrive 37 Shared Memory Switch Outputs Mux Buffer memory Write control Demux … … Inputs Read control 38 SelfRouting Fabrics • Use source routing on “network” within switch • Input port attaches output port number as header • Fabric routes packet based on output port • Types – Banyan network – BatcherBanyan network – Sunshine switch 39 Banyan Network Sends 0 bit up Sends 1 bit down MSB LSB • No contention if inputs are sorted and unique 40 Banyan Network 001 001 011 110 011 111 110 MSB LSB 111 • Sends 0 bit up, 1 bit down 41 Batcher (Merge Sort) Network 3 3 6 6 1 Sort 1 Merge 7 Merge Routing packets through a Batcher network • BatcherBanyan Network – Attach the twobacktoback – Arbitrary unique permutations routed without 42 contention BatcherBanyan Network sends 1 bit up sends 0 bit down sends 0 bit up sends 1 bit down 43 Sunshine Switch k Inputs n k Delay Batcher n+k Trap (marks overflow packets) n+k Selector n n n l banyans n n n Outputs • Like a Knockout switch, except • Recirculates overflow packets i.e., when more than L arrive in one cycle 44 ... Uses pointtomultipoint VC with all clients • Servers physically located in one or more devices LES ATM network BUS Point-to-point VC Point-to-multipoint VC LECS H1 H2 15 LANE Registration Client contacts LECS on predefined VC, and sends ATM address to it... Similar solutions for other? ?networks:? ? VPNs on WANs, VLANs on large, switched Ethernets 11 ATM / LANE Protocol Layers Higher-layer protocols (IP, ARP, ) Signalling + LANE Ethernet-like interface AAL5.. .Lecture? ?No.? ?17 Virtual Paths with ATM • Two level hierarchy of virtual connection: 8bit VPI and 16bit VCI