Comp 410 AOS Packet Switching These slides derived

Comp 410 AOS Packet Switching These slides derived from Computer Networking: A Top Down Approach , 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. Introduction 1 -1

The Network Core q mesh of interconnected routers q the fundamental question: how is data transferred through net? v circuit switching: Resources dedicated circuit per reserved call: telephone net v packet-switching: data sent thru net in Resources allocated discrete “chunks” on demand. Core Resources: buffers, link transmission rate Introduction 1 -2

The Network Core q The internet is packet switched q Telephone network is circuit switched Introduction 1 -3

Network Core: Circuit Switching End-end resources reserved for “call” q link bandwidth, switch q q q capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required State maintained Data transferred at a guaranteed rate Introduction 1 -4

Network Core: Circuit Switching End-end resources reserved for “call” q Links between circuit switches q Each link can support n circuits q There can be n simultaneous connections. q Each circuit thus gets 1/n of the link’s bandwidth. Introduction 1 -5

Network Core: Circuit Switching network resources (e. g. , bandwidth) divided into “pieces” q pieces allocated to calls q dividing link bandwidth into “pieces” v frequency division v time division q resource piece idle if not used by owning call (no sharing) q When two hosts want to communicate network establishes a dedicated end-to-end connection between the hosts. Introduction 1 -6

Circuit switching: analysis q Disadvantages: v Network resources are wasted during “silent” times (when no one is talking but still connected) v Establishing end-to-end circuits and reserving end-to-end bandwidth is complicated and requires complex signaling software. q Advantage: v Guaranteed bandwith and transmission time v Necessary for some applications (streamed music/video) Introduction 1 -7

Network Core: Packet Switching transmission: q Each end-end data stream divided into packets q Each packet travels through communication links q Links connected by packet switches q Routers q Or link-level switches. q Switches use store-and-forward transmission q Switch receives entire packet before it transmits any of it again Introduction 1 -8

Network Core: Packet Switching transmission: Introduction 1 -9

Network Core: Packet Switching each end-end data stream divided into packets q user A, B packets share network resources q each packet uses full link bandwidth q resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation Introduction 1 -10

Packet-switching: store-and-forward L R R This Figure: q takes L/R seconds to transmit (push out) packet of L bits on to link at R bps q store and forward: entire packet must arrive at router before it can be transmitted on next link q delay = 3 L/R (assuming zero propagation delay) R Example: q L = 7. 5 Mbits q R = 1. 5 Mbps q transmission delay = ? ? v 15 sec more on delay shortly … Introduction 1 -11

Network Core: Packet Switching delays: q Each switch has multiple links q Each link has buffer q If packet arrives and another packet is already being transmitted on that link, must wait in queue q Called queuing delay. q Varies depending on network congestion q Packet loss: queue is full when packet arrives. Introduction 1 -12

Packet Switching: Statistical Multiplexing 100 Mb/s Ethernet A B statistical multiplexing C 1. 5 Mb/s queue of packets waiting for output link D On-demand sharing of resources is called statistical multiplexing E Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. Introduction 1 -13

Packet switching versus circuit switching Packet switching allows more users to use network! q 1 Mb/s link q For each user assume: v 100 kb/s when “active” v active 10% of time q circuit-switching: v 10 users q packet switching: v with 35 users, probability > 10 active at same time is less than. 0004 N users 1 Mbps link Q: how did we get value 0. 0004? Introduction 1 -14

Routing q How do packets make their way through packet-switched Networks? Introduction 1 -15

Internet structure: network of networks q roughly hierarchical q at center: “tier-1” ISPs (e. g. , Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage v treat each other as equals Tier-1 providers interconnect (peer) privately Tier 1 ISP Introduction 1 -16

Internet structure: network of networks q “tier-1” ISPs Form their own network v Each tier-1 ISP is connected directly to each of the other tier-1 ISPs v Each tier-1 ISP is connected to many tier-2 ISPs v Tier-1 providers know as the internet backbone Tier 1 ISP No group officially sanctions tier-1 status! Tier 1 ISP Introduction 1 -17

Tier-1 ISP: e. g. , Sprint POP: point-of-presence to/from backbone peering … … … to/from customers Introduction 1 -18

Internet structure: network of networks q “Tier-2” ISPs: smaller (often regional) ISPs v 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 q tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier 1 ISP Tier-2 ISPs also peer privately with each other. Tier-2 ISP Introduction 1 -19

Internet structure: network of networks q “Tier-3” ISPs and local ISPs v last hop (“access”) network (closest to end systems) local ISP Local and tier 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier 3 ISP Tier-2 ISP local ISP Tier-2 ISP Tier 1 ISP Tier-2 ISP local ISP Points of Presence (POP): a link or the group of routers in an ISP where other ISPs or customers connect. Tier-2 ISP local ISP Introduction 1 -20

Internet structure: network of networks q a packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP Tier-2 ISP Tier 1 ISP Tier-2 ISP local ISP Introduction 1 -21