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Week 3 Virtual LANs, Wireless LANs, PPP, ATM 1 Week 3 Virtual LANs It is the territory over which a broadcast or multicast packet is delievered (also known as a broadcast domain) The difference in a VLAN and a LAN, if there is any, is in packaging Virtual LANs allow you to have separate LANs among ports on the same switch For example, a switch might be told that ports 1-32 are in VLAN A and ports 33-64 are in VLAN B 2 Week 3 Virtual and Physical LANs VLAN A A Q V A Q V J J R M B D K VLAN B M B D K 3 Week 3 Why VLANs IP requires that all nodes on a LAN share the same IP address prefix; therefore a node that moves to a different LAN must change its address Changing IP addresses manually is annoying IP broadcasts traffic within a LAN, something that can cause congestion in a large LAN Routing IP (rather than bridging) was slow It might be tempting to bridge everything making your whole topology one giant LAN from the perspective of IP and use layer 2 switches 4 Week 3 Disadvantages of one single LAN Broadcast traffic (such as ARP) grows in proportion to the number of stations Users can snoop on the traffic of other users on the same LAN, so it might be safer to isolate groups of users onto different LANs Some protocols are overly chatty or they get into modes such as broadcast storms. So it seems desirable for users that need to talk to each other a lot to be in the same LAN but keep other groups of users in separate LANs A VLAN makes us broadcast domain as large as we want it 5 Week 3 Mapping ports to VLANs The switch has ports 1 to k in one VLAN and has ports k+1 to 2k in another LAN The switch can be configured with a port/VLAN mapping The switch can be configured with a table of VLAN/MAC address mappings. It then dynamically determines the VLAN/port mapping based on the learned MAC address of the station attached to the port. The switch can be configured with a table of VLAN/IP prefix mappings. It then dynamically determines the VLAN/port mapping based on the source IP address from the station attached to the port. The switch can be configured with a table of VLAN/protocol mappings. It then dynamically determines the VLAN/port mappings based on the protocol type of the stations attached to the port. 6 Week 3 VLAN forwarding with separate router a.b.c.H f.g.k.Q h d a.b.c.D 2 9 3 7 f q 11 13 j a.b.c.R1 x f.g.k.X f.g.k.R2 Router R Router connects VLANs 7 Week 3 VLAN forwarding with switch as router a..b.c.H 2 VLAN A 3 Switch/ Router R 9 f.g.k.Q VLAN B 12 a..b.c.D f.g.k.X Router does not use up ports The switch must know that R’s mac address on VLAN A is f and on VLAN B is j. 8 Week 3 Dynamic binding of links to VLANs X.B Z.C X.A Q.F c Q.E Z.D a b Q.D Q.F The switch now learns that there are two VLANs on port a If enough stations move around, advantage disappears 9 Week 3 VLAN Tagging VLAN 2 VLAN 1 VLAN 2 VLAN 2 VLAN1 VLAN 1 Interswitch port; Packets can belong in either VLAN1 or VLAN2 VLAN 1 VLAN 2 IEEE standardized a scheme for VLAN tagging VLAN 1 10 Week 3 IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions 11 Week 3 802.11 LAN architecture wireless host communicates Internet AP hub, switch or router BSS 1 AP BSS 2 with base station base station = access point (AP) Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only 12 Week 3 802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP’s subnet 13 Week 3 IEEE 802.11: multiple access avoid collisions: 2+ nodes transmitting at same time 802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by other node 802.11: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can’t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) A C A B B C C’s signal strength A’s signal strength space 14 Week 3 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if INITIALLY sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK sender receiver DIFS data SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) 15 Week 3 Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they’re short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations defer transmissions Avoid data frame collisions completely using small reservation packets! 16 Week 3 Collision Avoidance: RTS-CTS exchange A AP B reservation collision DATA (A) defer time 17 Week 3 802.11 frame: addressing 2 2 6 6 6 frame address address address duration control 1 2 3 Address 1: MAC address of wireless host or AP to receive this frame 2 6 seq address 4 control 0 - 2312 4 payload CRC Address 4: used only in ad hoc mode Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame 18 Week 3 802.11 frame: addressing R1 router H1 Internet AP R1 MAC addr AP MAC addr dest. address source address 802.3 frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame 19 Week 3 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) 2 2 6 6 6 frame address address address duration control 1 2 3 2 Protocol version 2 4 1 Type Subtype To AP 6 2 1 seq address 4 control 1 From More AP frag 1 Retry 1 0 - 2312 4 payload CRC 1 Power More mgt data 1 1 WEP Rsvd frame type (RTS, CTS, ACK, data) 20 Week 3 802.11: mobility within same subnet H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 router hub or switch BBS 1 AP 1 AP 2 H1 BBS 2 21 Week 3 Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack! 22 Week 3 PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address 23 Week 3 PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers! 24 Week 3 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc) 25 Week 3 PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error detection 26 Week 3 Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> data byte Receiver: 01111101 and 01111110 bytes in a row: discard first byte, continue data reception single 01111110: flag byte 27 Week 3 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 28 Week 3 PPP Data Control Protocol Before exchanging networklayer data, data link peers must configure PPP link (max. frame length, authentication) learn/configure network layer information for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address 29 Week 3 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS 30 Week 3 Virtualization of networks Virtualization of resources: a powerful abstraction in systems engineering: computing examples: virtual memory, virtual devices Virtual machines: e.g., java IBM VM os from 1960’s/70’s layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly 31 Week 3 The Internet: virtualizing networks 1974: multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha) packet radio network ARPAnet "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637-648. … differing in: addressing conventions packet formats error recovery routing satellite net 32 Week 3 The Internet: virtualizing networks Internetwork layer (IP): addressing: internetwork appears as a single, uniform entity, despite underlying local network heterogeneity network of networks Gateway: “embed internetwork packets in local packet format or extract them” route (at internetwork level) to next gateway gateway ARPAnet satellite net 33 Week 3 Cerf & Kahn’s Internetwork Architecture What is virtualized? two layers of addressing: internetwork and local network new layer (IP) makes everything homogeneous at internetwork layer underlying local network technology cable satellite 56K telephone modem today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP! Week 3 34 Generic connection – oriented network For A to talk to B, there must be a special call setup packet that travels from A to B, specifying B as the destination. Each router along the path must make a routing decision based on B’s address This is the identical problem in IP In addition to simply forwarding the call setup packet, the goal is to assign the call a small identifier, which we now call the CI (connection identifier) CIs can be small because they are handed out dynamically and are significant only on a link They only need to be large enough to distinguish between the total number of calls that might simultaneously be routed on the same link 35 Week 3 A wants to talk to B and use CI 57 A 57 c,33 a X b 79c,22 33 a,57 33d,79 79a,33 c R2 R1 c a R4 b 22b,79 c B R5 a R3 Why does the CI have to change hop by hop? The answer is that it would be very difficult to choose a CI that was unused on all the links along the path Week 3 36 ATM and MPLS ATM, MPLS separate networks in their own right different service models, addressing, routing from Internet viewed by Internet as logical link connecting IP routers just like dialup link is really part of separate network (telephone network) ATM, MPLS: of technical interest in their own right 37 Week 3 Asynchronous Transfer Mode: ATM 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice, video (versus Internet best-effort model) “next generation” telephony: technical roots in telephone world packet-switching (fixed length packets, called “cells”) using virtual circuits 38 Week 3 ATM architecture adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer ATM layer: “network” layer cell switching, routing physical layer 39 Week 3 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” ATM is a network technology Reality: used to connect IP backbone routers “IP over ATM” ATM as switched link layer, connecting IP routers IP network ATM network 40 Week 3 ATM Adaptation Layer (AAL) ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells analogy: TCP segment in many IP packets 41 Week 3 ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video AAL5: for data (eg, IP datagrams) User data AAL PDU ATM cell 42 Week 3 ATM Layer Service: transport cells across ATM network analogous to IP network layer very different services than IP network layer Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no 43 Week 3 ATM Layer: Virtual Circuits VC transport: cells carried on VC from source to dest call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each passing connection link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. Permanent VCs (PVCs) long lasting connections typically: “permanent” route between to IP routers Switched VCs (SVC): dynamically set up on per-call basis 44 Week 3 ATM VCs Advantages of ATM VC approach: QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach: Inefficient support of datagram traffic one PVC between each source/dest pair) does not scale (N*2 connections needed) SVC introduces call setup latency, processing overhead for short lived connections 45 Week 3 ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload Why?: small payload -> short cell-creation delay for digitized voice halfway between 32 and 64 (compromise!) Cell header Cell format 46 Week 3 ATM cell header VCI: virtual channel ID will change from link to link thru net PT: Payload type (e.g. RM cell versus data cell) CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be discarded if congestion HEC: Header Error Checksum cyclic redundancy check 47 Week 3 ATM VCs Advantages of ATM VC approach: QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach: Inefficient support of datagram traffic One PVC between each source/dest pair) does not scale (N*2 connections needed) SVC introduces call setup latency, processing overhead for short lived Week 3 connections 48 Virtual Path Concept The connection identifier in the ATM cell header has two complexities: It’s hierarchical and divided into two subfields VPI (Virtual Path Identifier) and VCI (Virtual Circuit Identifier) VCI is 16 bits VPI is 12 bits What’s a VPI? There might be very high speed backbone carrying many millions of calls The split between VPI and VCI saves the switches in the backbone from requiring that their call mapping database keep track of millions of calls 49 Week 3 Virtual Path Concept The backbone routers only use the VPIU field then if needed Outside the backbone, the switches treat the entire VPI:VCI field as one nonhierarchical unit VP switch looks at only the VPI portion VC switch looks at both 50 Week 3 Example S2 a b S1 D S3 e c S5 S4 S1 is to receive a call setup on port b with CI 17 for destination D • Normal VP switching inside the core with the CI being the 12 bit VPI • Switches outside the core do normal VC switching with the CI being 28 bits • Switches at the border also do VC swiching but the outgoing CI must be chosen so that the VPI portion of the outgoing CI is to the outgoing VPI 51 Week 3 Example S8 a 89c,187.42 S1 c S2 S9 d S5 e S3 S4 187. d,13 13. e,57 S6 d 57.42 d,83 64000 VCs can be carried within a single VP dramatically reducing the switch table sizes 52 Week 3 Virtual Path and Virtual Channels Virtual Channels (VC) ATM Physical Link Virtual Channel Connection (VCC) Virtual Path (VP) E3 OC–12 Virtual Path (VP) Virtual Channels (VC) Virtual Channel Connection (VCC) Contains Multiple VPs Virtual Path (VP) Contains Multiple VCs Virtual Channel (VC) Logical Path Between ATM End Points Connection Identifier = VPI/VCI 53 Week 3 ATM Switches Input Output Port VPI/VCI Port 45 VPI/VCI 1 29 2 45 2 45 1 29 1 64 3 29 3 29 1 64 29 64 2 1 3 29 ATM switches translate VPI/VCI values VPI/VCI value unique only per interface— eg: locally significant and may be re-used elsewhere in network 54 Week 3 VP and VC Switching VC Switch VCI 1 VCI 2 VPI 1 VP Switch VCI 3 VPI 3 VCI 4 Port 2 VPI 2 VPI 2 Port 1 VCI 1 VCI 2 VCI 1 VCI 2 VPI 1 VPI 3 VPI 4 VPI 5 Port 3 Week 3 55 Virtual Channels and Virtual Paths Virtual Channel Connection (VCC) Virtual Path Connection (VPC) UNI UNI NNI VC Switch VPI = 1 VCI = 1 NNI VP Switch VPI = 2 VCI = 44 VC Switch VPI = 26 VCI = 44 VPI = 20 VCI = 30 This hop-by-hop forwarding is known as cell relay 56 Week 3 Virtual Path and Virtual Channels Virtual Channels (VC) ATM Physical Link Virtual Channel Connection (VCC) Virtual Path (VP) E3 OC–12 Virtual Path (VP) Virtual Channels (VC) Virtual Channel Connection (VCC) Contains Multiple VPs Virtual Path (VP) Contains Multiple VCs Virtual Channel (VC) Logical Path Between ATM End Points Connection Identifier = VPI/VCI 57 Week 3 ATM Switches Input Output Port VPI/VCI Port 45 VPI/VCI 1 29 2 45 2 45 1 29 1 64 3 29 3 29 1 64 29 64 2 1 3 29 ATM switches translate VPI/VCI values VPI/VCI value unique only per interface— eg: locally significant and may be re-used elsewhere in network 58 Week 3 VP and VC Switching VC Switch VCI 1 VCI 2 VPI 1 VP Switch VCI 3 VPI 3 VCI 4 Port 2 VPI 2 VPI 2 Port 1 VCI 1 VCI 2 VCI 1 VCI 2 VPI 1 VPI 3 VPI 4 VPI 5 Port 3 Week 3 59 Virtual Channels and Virtual Paths Virtual Channel Connection (VCC) Virtual Path Connection (VPC) UNI UNI NNI VC Switch VPI = 1 VCI = 1 NNI VP Switch VPI = 2 VCI = 44 VC Switch VPI = 26 VCI = 44 VPI = 20 VCI = 30 This hop-by-hop forwarding is known as cell relay 60 Week 3 Example 61 Week 3 ATM Physical Layer (more) Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below Physical Medium Dependent: depends on physical medium being used TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send 62 Week 3 ATM Physical Layer Physical Medium Dependent (PMD) sublayer SONET/SDH: transmission frame structure (like a container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps unstructured: just cells (busy/idle) 63 Week 3 IP-Over-ATM Classic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses IP over ATM replace “network” (e.g., LAN segment) with ATM network ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs 64 Week 3 IP-Over-ATM app transport IP Eth phy IP AAL Eth ATM phy phy ATM phy ATM phy app transport IP AAL ATM phy 65 Week 3 Datagram Journey in IP-over-ATM Network at Source Host: IP layer maps between IP, ATM dest address (using ARP) passes datagram to AAL5 AAL5 encapsulates data, segments cells, passes to ATM layer ATM network: moves cell along VC to destination at Destination Host: AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP 66 Week 3 IP-Over-ATM Issues: IP datagrams into ATM AAL5 PDUs from IP addresses to ATM addresses just like IP addresses to 802.3 MAC addresses! ATM network Ethernet LANs 67 Week 3 ATM Layer Service: transport cells across ATM network analogous to IP network layer very different services than IP network Guarantees ? layer Service Congestion Network Architecture Internet Model best effort ATM CBR ATM VBR ATM ABR ATM UBR Bandwidth Loss Order Timing feedback none no no no constant rate guaranteed rate guaranteed minimum none yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no 68 Week 3 Traffic Management Why traffic management? Traffic control techniques AAL5/ABR congestion feedback Buffers are your friend 69 Week 3 Why Traffic Management? Proactively combat congestion Provision for priority control Maintain well-behaved traffic 70 Week 3 Why Traffic Management? Cell Loss—Data’s Critical Enemy Ethernet (1500 Bytes) = 32 Cells FDDI (4470 Bytes) = 96 Cells IP over ATM–1577 (9180 Bytes) = 192 Cells TCP/IP Packet X Lose one cell and the rest are useless Need to re-transmit 32+ cells for one cell lost Congestion collapse is the result PPD (Partial Packet Discard) EPD (Early Packet Discard) 71 Week 3 Traffic Control Techniques Connection management—Acceptance Traffic management—Policing Traffic smoothing—Shaping 72 Week 3 Traffic Control Techniques Connection Management Contract Contract Contract ATM Network • Traffic Parameters Peak cell rate Sustainable cell rate Burst tolerance Etc. • Quality of Service Delay Cell loss 73 Week 3 Traffic Descriptors Peak Cell Rate(PCR) = 1/T in units of cells/second, where T is the minimum intercell spacing in seconds(i.e., the time interval from the first bit of one cell to the first bit of the next cell) Sustainable Cell Rate(SCR) is the maximum average rate that a bursty, on-off traffic source can be sent at the peak rate Maximum Burst Size(MBS) is the maximum number of cells that can be sent at the peak rate 74 Week 3 QoS Expectations Applications have service requirements on: Throughput Maximum Delay Variance of Delays(Delay Jitter) Loss Probability Network has to guarantee the required Quality of Service(Traffic Contract) Major Problem: Bursty Traffic, i.e., Peak Traffic Rate >> Average Traffic Rate 75 Week 3 Traffic Control Techniques Connection Management Connection Admission Control (CAC) I want a VC: X Mbps Y Delay Z Cell Loss CAC Can I Support this Reliably without Jeopardizing Other Contracts Guaranteed QoS Request No or Yes, Agree to a Traffic Contract Contract ATM Network 76 Week 3 Connection Admission Control The primary function of the CAC is to accept a new connection request only if its stated QoS can be maintained without influencing the QoS of the already accepted connections. It is very likely that certain calls will require more than one connection (e.g., teleconferencing) CAC procedure must be performed for each requested VCC or VPC. CAC must Decide whether connections can be accepted or not. Provide parameters required by the UPC. Perform routing and resource allocation. Week 3 77 Bandwidth Allocation Peak Allocation – Suppose a source has an average BW of 20 Mbps and a peak BW of 45 Mbps. Peak BW allocation requires that 45 Mbps be reserved at the output port for the specific source independent of whether or not the source transmits continuously at 45 Mbps. – Peak BW allocation is used for CBR services. The advantage of peak BW allocation is that it is easy to decide whether to accept a new connection or not. – The new connection is accepted, if the sum of the peak rates of all the existing connections plus the peak rate of the new connection is less than the capacity of the output link. – The disadvantage of the Peak BW allocation is that the output port link will be underutilized if the sources do not transmit at their peak rates. Week 3 78 Bandwidth Allocation Statistical Allocation The allocated BW is less than the peak rate of the source. The sum of all peak rates may be greater than the capacity of the output link. An equivalent capacity is allocated between the peak rate and the mean rate Call admission: if the sum of the equivalent capacities is less than the capacity, reject the incoming call 79 Week 3 Source BehaviorCell Interarrival Time CBR time VBR Burst Duration Cell Interarrival Time Burst to Burst Interval time Call Duration Call Tear-Down Call Set-up VBR Source Description: ON OFF or Burst Length Distribution Interarrival Distribution During Burst Idle (silent) Length Distribution Peak Arrival Rate Average Arrival Rate Mean Burst Length < Bp, Bm, T > 80 Week 3 End-to-end Model CAC is based on an abstract performance model of the network. Multiplexing Entering Entering Entering Cross Traffic Cross Traffic Cross Traffic Demultiplexing Departing Departing Departing Cross Traffic Cross Traffic Cross Traffic - FINITE BUFFERS - DETERMINISTIC SERVICE TIMES Modeling Problems Challenges - Arrival streams are non-Poisson - Finite buffers at the multiplexers and switches - Correlated cell arrivals - Large state-space of the resulting system - Simulations of such systems take very long to converge 81 Week 3 Traffic Control Techniques Traffic Management Usage Parameter Control (UPC) aka Policing Contract REBEL APPLICATION You are Not in Conformance with the Contract. What Should the Penalty Be?? ?DECISION? • PASS ATM Network • MARK CLP BIT • DROP 82 Week 3 Traffic Control Techniques Traffic Management UPC Marked 0 0 0 D r o p 0 1 0 ?DECISION? • PASS • MARK CLP BIT • DROP CLP Control—When congested drop marked cells Public UNI—Generic Cell Rate Algorithm (GCRA) 83 Week 3 Policing The operation of the CAC and the correct allocation of resources depend heavily on the guarantee that the traffic source will behave as expected, i.e., as described by the traffic descriptor. Thus a monitoring/policing function is needed to force the traffic to comply to the traffic descriptor. This monitoring/policing function is performed by the UPC (policer). The UPC is in the form of preventive congestion control. It enforces a certain cell arrival rate or “shape”, such that it does not exceed certain values that would cause network elements to overload and lead to congestion. A UPC usually consists of a counter-based mechanism that drops or marks data units when they are found in violation of a certain agreement between end-user and the communication system. It does not use information from remote network elements. 84 Week 3 Generic Cell Rate Algorithm (I, L) The GCRA is reference algorithm for a cell rate which determines if a cell is conforming. Arrival of a cell k at time ta (k) TAT ta(k) Non Conforming Cel YES YES X’=X-(ta(k)-LCT) TAT = ta(k) X’< 0 TAT < ta(k) + L NO Non Conforming Cell X’>= L X’=0 X=X’+I LCT = ta(k) Conforming Cell TAT = TAT + I Conforming Cell VIRTUAL SCHEULING ALGORITHM TAT Theoretical Arrival Time ta(k) Time of arrival of a cell YES YES I : Increment L : Limit Virtual Scheduling Algorithm TAT:= ta(1) initially CONTINUOUS-STATE LEAKY BUCKET ALGORITHM X : Value of the Leaky Bucket counter X’ : auxiliary variable LCT Last Compliance Time Leaky Bucket Algorithm X := 0 LCT := ta(1) initially 85 Week 3 Traffic Contact and Performance Definitions CBR GCRA(T0+1 , CDVT) in relation to the PCR0+1 T0+1 is the inverse of PCR0+1 Nonconformant cells are dropped VBR (one of the standardized definitions) GCRA(T0+1 , CDVT) in relation to the PCR0+1 GCRA(Ts0 , BT0 + CDVT) in relation to the SCR of the CLP = 0 cell stream • BT = (MBS – 1) (1/SCR - 1/PCR) If CLP = 0 cell conforms to (1) and (2), that cell is conformant If CLP = 0 cell is not conforming to (2) but is conforming to (1) then it will be remarked as CLP = 1 Week 3 86 Example • Consider a Video-on-Demand service where the negotiated PCR = 50kcells/s and the CDV Tolerance ( ) =50sec. • The cells arrive at times as indicated by t(k). Note: GCRA(I,L) where I = T = 1/PCR = 20sec/cell and L = t = 50 sec. GCRA(T,) Figure: Example of the GCRA LCT(k) 0 T(k) 0s X(k) X'(k) 0s 0s 0s Yes 1 20s 0s 20s 0s Yes 2 25s 20s 20s 15s Yes 3 30s 25s 35s 30s Yes 4 35s 30s 50s 45s Yes k Co nforming 40s 35s 65s 60s No 5 6 45s 35s 65s 50s No 7 50s 35s 65s 50s No 8 55s 50s 70s 65s No 9 80s 50s 70s 40s Yes 10 100s 80s 60s 40s Yes 22 87 Week 3 Traffic Control Techniques Traffic Management UPC Marked 0 0 0 D r o p 0 3 1 0 2 Intelligent Packet Discard—IPD Discard cells from same ‘bad’ packet Tail packet discard Maximize “Goodput” 88 Week 3 Traffic Control Techniques Traffic Smoothing I Want to Comply With My Contract. So, I Will Smooth/Shape My Traffic Shaper Go Ahead, Make My Day Actual Data Shaped Data Private ATM Network Public ATM Network Traffic shaper at customer site Changes traffic characteristics Leaky bucket algorithm 89 Week 3 Traffic Control Techniques Buffers Are Your Friend Absorb traffic bursts from simultaneous connections Switches schedule traffic based on priority of traffic according to QoS Switch must reallocate buffers as the traffic mix changes Effective buffering maximizes throughput of usable cells as opposed to raw cells (aka goodput) 90 Week 3