Lec 4 and_5


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Circuit switching and packet switching

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Lec 4 and_5

  1. 1. Network Core Packet Switching and Circuit Switching Engr. Abbas Abbasi
  2. 2. Important Terms Review  DSL  LAN  DSLAM  3G  HFC  LTE  CMTS  WiFi  FTTH  Physical Medium Types  AON  UTP  PON  Shared Medium  ONT  OC (Optical Carrier)  OLT  Geostationary Satellite  LEO Satellite
  3. 3. The Network Core  mesh of interconnected routers  the fundamental question: how is data transferred through net?  circuit switching: dedicated circuit per call: telephone net  packet-switching: data sent thru net in discrete “chunks”  Routers Vs Switches  Message length L, Transmission rate R, and transmission delay
  4. 4. Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces”    dividing link bandwidth into “pieces”  frequency division pieces allocated to calls  time division (TDM) resource piece idle if not used by  code division (CDM) owning call (no sharing)
  5. 5. Circuit Switching: FDM and TDM Example: FDM 4 users frequency time TDM frequency time
  6. 6. Numerical example  How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?  All links are 1.536 Mbps  Each link uses TDM with 24 slots/sec  500 msec to establish end-to-end circuit Let’s work it out! t = (0.500 s)+ (640,000 bits)/((1,536,000 bits/s)/ (24 channels)) = 0.5 + 10.0 = 10.5 seconds until last bit leaves Host A but how long until last bit arrives at Host B?
  7. 7. Network Core: Packet Switching each end-end data stream divided into packets  user A, B packets share network resources  each packet uses full link bandwidth (for short time)  resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation resource contention:  aggregate resource demand can exceed amount available  congestion: packets queue, wait for link use  store and forward: packets move one hop at a time  Node receives complete packet before forwarding
  8. 8. Packet Switching: Statistical Multiplexing 100 Mb/s Ethernet A B C statistical multiplexing 1.5 Mb/s queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame.
  9. 9. Packet-switching: store-and-forward L R R  Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps  Entire packet must arrive at router before it can be transmitted on next link: store and forward  delay = 3L/R (assuming zero propagation delay) R Example:  L = 7.5 Mbits  R = 1.5 Mbps  delay = 15 sec Note: local connection so that (a) propagation time is negligible, and (b) no delay due to congestion. more on delay shortly …
  10. 10. Packet switching versus circuit switching Packet switching allows more users to use network!  1 Mb/s link  each user:  100 kb/s when “active”  active 10% of time 1 Mbps link  circuit-switching:  10 users  packet switching:  with 35 users, probability > 10 active less than .0004 N users Q: how did we get value 0.0004? P(n,N) =(N!) ( p^n) (1-p)^(N-n) (n!) (N-n)! P(11,35) =(35!) ( 0.1^11) (0.9^24) (11!) (24!) = 0.0003 P(12,35) = 0.0001
  11. 11. Packet switching versus circuit switching Is packet switching a “slam dunk winner?”  Great for bursty data resource sharing simpler, no call setup  If excessive congestion: packet delay and loss. protocols needed for reliable data transfer, congestion control (TCP does a good job)  Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7)
  12. 12. Internet structure: network of networks  roughly hierarchical  at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T, Cable and Wireless), national/international coverage  treat each other as equals Tier-1 providers interconnect (peer) privately Tier 1 ISP Tier 1 ISP NAP Tier 1 ISP Tier-1 providers also interconnect at public network access points (NAPs)
  13. 13. Tier-1 ISP: e.g., Sprint Sprint US backbone network Seattle Tacoma DS3 (45 Mbps) OC3 (155 Mbps) OC12 (622 Mbps) OC48 (2.4 Gbps) POP: point-of-presence to/from backbone Stockton peering … … . Kansas City … … Anaheim … San Jose Cheyenne New York Pennsauken Relay Wash. DC Chicago Roachdale Atlanta to/from customers Fort Worth Introduction Orlando
  14. 14. Internet structure: network of networks  “Tier-2” ISPs: smaller (often regional) ISPs  Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet  tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP NAP Tier 1 ISP Tier-2 ISP * NAP - Network Access Point Tier-2 ISPs also peer privately with each other, interconnect at NAP* Tier-2 ISP
  15. 15. Internet structure: network of networks  “Tier-3” ISPs and local ISPs  last hop (“access”) network (closest to end systems) local ISP Local and tier3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP NAP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP
  16. 16. Internet structure: network of networks  a packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP NAP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP
  17. 17. IXPs and Content Provider Networks IXP: Internet Exchange Point
  18. 18. Important Terms  Store and Forward        Transmission Messages and Packets Packet Switch Transmission Delay Queuing Buffer or Output Buffer Packet Loss Forwarding Table Routing Protocols  Circuit  Circuit Switching  End to End Connection  FDM  TDM  Silent Period  Tier 1,2,3 ISPs  Access ISP  IXP