1、Computer Networking Lecture 2Access networks and packet-switch networksSept 4nd, 2013Dr. Chunhua CSome slides are from lectures of Prof. J.F Kurose and K.W. RossReview: the network edge Why we need networks? End systems exchange messages Protocols: regulate the exchange of messages Format and order

2、of messages, and actions token by end systems Computing model Client/server Peer/peer Internet Network of networks, form a loosely hierarchical architecture We have covered the network edgeAccess networks and physical mediaQ: How to connect end systems to edge router? residential access nets institu

3、tional access networks (school, company) mobile access networksKeep in mind: bandwidth (bits per second) of access network shared or dedicatedResidential access: point to point access Dialup via modem (撥號調制解調撥號調制解調器）器） up to 56Kbps direct access to router (often less) Cant surf and phone at same tim

4、e: cant be “always on” DSL: digital subscriber line (數字用戶線，電纜調制解調數字用戶線，電纜調制解調器）器） deployment: telephone company (typically) up to 1 Mbps upstream (today typically 256 kbps) up to 8 Mbps downstream (today typically 10 active at same time is less than .0004Packet switching allows more users to use net

5、work!N users1 Mbps linkQ: how did we get value 0.0004?Packet switching versus circuit switching great for bursty data resource sharing simpler, no call setup excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like beh

6、avior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7)Is packet switching a “slam dunk winner?”Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?“Best effort” in packet-switch networks “Best effort” mean

7、s “No Guarantees” Variable Delay (jitter) Variable rate Packet loss Duplicates Reordering Real challenges for building a reliable network!Review Internet Network of networks Access networks Connect end systems to Internet Circuit-switch and Packet-switch networks Best effort modelInternet structure:

8、 network of networks ISP: Internet Service Provider Architecture: roughly hierarchical At center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage treat each other as equalsTier 1 ISPTier 1 ISPTier 1 ISPTier-1 providers interconnect (peer) privatel

9、yInternet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPsTier 1 ISPTier 1 ISPTier 1 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISP pays tier-1 ISP for connectivity to rest of Internetq tier-2

10、ISP is customer oftier-1 providerTier-2 ISPs also peer privately with each other.Internet structure: network of networks “Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)Tier 1 ISPTier 1 ISPTier 1 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlo

11、calISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPLocal and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of InternetTier-1 ISP: e.g., Sprintto/from customerspeering to/from backbone.POP: point-of-presencePOP: Point of presenceInternet structure: network of networksA packet passes t