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1.- LAN basics

1.- LAN basics

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1.- LAN basics

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  1. 1.- LAN basics Networking basics The Internet TCP/IP LANs topologies Media Access Control (MAC) techniques

  2. 1.- LAN basics Networking basics

  3. Goals of computer networks • to provide ubiquitous access to shared resources (e.g., printers, databases, file systems...), • to allow remote users to communicate (e.g., email, IP telephony), • to do transactions (banking, e-commerce, stock trading), and… • … save money: downsizing

  4. router workstation server mobile local ISP regional ISP company network A “nuts and bolts” view of a network • Millions of connected computing devices: hosts, end-systems • pc’s workstations, servers • PDA’s phones, toasters running network apps • communication links • fiber, copper, radio, satellite • routers: forward packets (chunks) of data thru network • protocols: control sending, receiving of msgs • TCP, IP, and HTTP, FTP, PPP, …

  5. A closer look at the network structure • The network edge: applications and hosts • The network core: • routers • network of networks • The access networks and physical media: communication links

  6. The network edge • End systems (hosts): • run application programs at the “edge of network” • client/server model • client host requests, receives service from server • e.g., WWW client (browser)/ server; email client/server • peer-peer model: • host interaction symmetric • e.g.: Gnutella, KaZaA

  7. 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 through the network in discrete “chunks”

  8. The network core: Circuit switching • End-end resources reserved for “call” • Characterizing parameters: link bandwidth, switch capacity • dedicated resources: no sharing • circuit-like (guaranteed) performance • call setup required

  9. The network core: Packet switching • Data traffic divided into packets • Each packet contains a header (with address) • Packets travel separately through network • Packet forwarding based on the header • Network nodes may store packets temporarily • Destination reconstructs the message

  10. The network core: Packet switching (routing) • Goal: move packets among routers from source to destination • datagram network: • destination address determines next hop • routes may change during session • analogy: driving, asking directions • virtual circuit network: • each packet carries tag (virtual circuit ID), tag determines next hop • fixed path determined at call setup time, remains fixed thru call • routers maintain per-call state

  11. The access networks and physical media • How to connect end systems to edge router? • Residential access networks • Institutional access networks (school, company) • Wireless access networks

  12. Residential access networks: point to point access • Dialup via modem • up to 56Kbps direct access to router (conceptually) • ISDN: integrated services digital network: 128Kbps all-digital connect to router • ADSL: asymmetric digital subscriber line • up to 1 Mbps home-to-router • up to 8 Mbps router-to-home • ADSL deployment: happening • HFC: hybrid fiber coax • asymmetric: up to 10Mbps upstream, 1 Mbps downstream • network of cable and fiber attaches homes to ISP router • shared access to router among home • issues: congestion, dimensioning

  13. Residential access networks: cable modems Diagram:

  14. Institutional access networks: local area networks • company/univ local area network (LAN) connects end system to edge router • Ethernet: • shared or dedicated cable connects end system and router • 10 Mbs, 100Mbps, Gigabit Ethernet • deployment: institutions, home LANs happening now

  15. router base station mobile hosts Wireless access networks • Shared wireless access network connects end system to router • Wireless LANs: • radio spectrum replaces wire • e.g., WiFi, Bluetooth, WiMAX • Wireless WANs: • GPRS/EDGE over GSM • High-Speed Downlink Packet Access (HSDPA) a 3G (third generation) mobile telephony communications based on Universal Mobile Telecommunications System (UMTS) networks.

  16. 1.- LAN basics Networking basics The Internet

  17. local ISP local ISP NAP NAP Internet structure: network of networks • Roughly hierarchical • National/international backbone providers (NBPs) • e.g. BBN/GTE, Sprint, AT&T, IBM, UUNet • interconnect (peer) with each other privately, or at public Network Access Point (NAPs) • A point of presence (POP) is a machine that is connected to the Internet. • Internet Service Providers (ISPs) provide dial-up or direct access to POPs. • regional ISPs • connect into NBPs • local ISP, company • connect into regional ISPs regional ISP NBP B NBP A regional ISP

  18. Network Access Points (NAPs) Note: Peers in this context are commercial backbones. Source:

  19. MCI/WorldCom/UUNET Global Backbone Source:

  20. The situation in Europe See: Also: More about technolgies:

  21. 1.- LAN basics Networking basics TCP/IP

  22. A simple TCP/IP Example • A user on host (“Argon”) makes a web access to URL • What actually happens in the network?

  23. HTTP Request and HTTP response • Web browser runs an HTTP client program • Web server runs an HTTP server program • HTTP client sends an HTTP request to HTTP server • HTTP server responds with HTTP response

  24. HTTP Request GET /index.html HTTP/1.1 Accept: image/gif, */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 Host: Connection: Keep-Alive

  25. HTTP Response HTTP/1.1 200 OK Date: Sat, 25 May 2002 21:10:32 GMT Server: Apache/1.3.19 (Unix) Last-Modified: Sat, 25 May 2002 20:51:33 GMT ETag: "56497-51-3ceff955" Accept-Ranges: bytes Content-Length: 81 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html <HTML> <BODY> <H1>Internet Lab</H1> Click <a href="">here</a> for the Internet Lab webpage. </BODY> </HTML> • How does the HTTP request get from Argon to Neon ?

  26. From HTTP to TCP • To send a request, the HTTP client program establishes an TCP connection to the HTTP server at Neon. • The HTTP server at Neon has a TCP server running

  27. Resolving hostnames and port numbers • Since TCP does not work with hostnames and also does not know how to find the HTTP server program at Neon, two things must happen: 1. The name “” must be translated into a 32-bit IP address. 2. The HTTP server at Neon must be identified by a 16-bit port number.

  28. Translating a hostname into an IP address • The translation of the hostname into an IP address is done via a database lookup • The distributed database used is called the Domain Name System (DNS) • All machines on the Internet have an IP

  29. Finding the port number • Note: Most services on the Internet are reachable viawell-known ports. E.g. All HTTP servers on the Internet can be reached at port number “80”. • So: Argon simply knows the port number of the HTTP server at a remote machine. • On most Unix systems, the well-known ports are listed in a file with name /etc/services. The well-known port numbers of some of the most popular services are: ftp 21 finger 79 telnet 23 http 80 smtp 25 nntp 119

  30. Requesting a TCP Connection • The HTTP client at requests the TCP client to establish a connection to port 80 of the machine with address

  31. Invoking the IP Protocol • The TCP client at Argon sends a request to establish a connection to port 80 at Neon • This is done by asking its local IP module to send an IP datagram to • (The data portion of the IP datagram contains the request to open a connection)

  32. Sending the IP datagram to an IP router • Argon ( can deliver the IP datagram directly to Neon (, only if it is on the same IP network (sometimes called “subnet”). • But Argon and Neon are not on the same IP network (Q: How does Argonknow this?) • So, Argon sends the IP datagram to its defaultgateway • The default gateway is an IP router • The default gateway for

  33. The route from Argon to Neon • Note that the gateway has a different name for each of its interfaces.

  34. Finding the MAC address of the gateway • To send an IP datagram to Router137, Argon puts the IP datagram in an Ethernet frame, and transmits the frame. • However, Ethernet uses different addresses, so-called Media Access Control (MAC) addresses (also called: physical address, hardware address) • Therefore, Argon must first translate the IP address into a MAC address. • The translation of addressed is performed via the Address Resolution Protocol (ARP)

  35. Address resolution with ARP

  36. Invoking the device driver • The IP module at Argon, tells its Ethernet device driver to send an Ethernet frame to address 00:e0:f9:23:a8:20

  37. Sending an Ethernet frame • The Ethernet device driver of Argon sends the Ethernet frame to the Ethernet network interface card (NIC) • The NIC sends the frame onto the wire

  38. Forwarding the IP datagram • The IP router receives the Ethernet frame at interface, recovers the IP datagram and determines that the IP datagram should be forwarded to the interface with name • The IP router determines that it can deliver the IP datagram directly

  39. Another lookup of a MAC address • The router needs to find the MAC address of Neon. • Again, ARP is invoked, to translate the IP address of Neon ( into the MAC address of neon (00:20:af:03:98:28).

  40. The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28 Invoking the device driver at the router

  41. Sending another Ethernet frame • The Ethernet device driver of Router71 sends the Ethernet frame to the Ethernet adapter, which transmits the frame onto the wire.

  42. Data has arrived at Neon • Neon receives the Ethernet frame • The payload of the Ethernet frame is an IP datagram which is passed to the IP protocol. • The payload of the IP datagram is a TCP segment, which is passed to the TCP server • Note: Since the TCP segment is a connection request (SYN), the TCP protocol does not pass data to the HTTP program for this packet. Instead, the TCP protocol at neon will respond with a SYN segment to Argon.

  43. Wrapping-up the example • So far, Neon has only obtained a single packet • Much more work is required to establish an actual TCP connection and the transfer of the HTTP Request • The example was simplified in several ways: • No transmission errors • The route between Argon and Neon is short (only one IP router) • Argon knew how to contact the DNS server (without routing or address resolution) • ….

  44. 1.- LAN basics LANs topologies

  45. LAN basics • A local area network is a communication network that interconnects a variety of data devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates. • They are typically private • Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments. • Functions of a LAN: a few examples • File server - A large storage disk drive that acts as a central storage repository. • Print server - Provides the authorization to access a particular printer, accept and queue print jobs, and provides a user access to the print queue to perform administrative duties. • Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks • Manufacturing support - LANs can support manufacturing and industrial environments. • Distributed processing - LANs can support network operating systems which perform the operations of distributed processing. • …

  46. LAN Selection Criteria • Cost • For most of us, cost is an overriding constraint, and you must choose the best solution within your budget. Usually, cost is the most inflexible constraint under which you must operate, and in the final analysis the LAN must be a cost-effective solution to your problem. • Number of Workstations • Each LAN is physically capable of supporting some maximum number of workstations. If you exceed that maximum number, you must make some provision for extending the maximum number. • Number of Concurrent Users / type of use • As the number of concurrent users goes up, so does the LAN workload. As the LAN workload increases, you have two basic choices: You can allow system responsiveness to decrease, or you can increase the work potential of the system. • Many concurrent users may increase the LAN workload.

  47. LAN Selection Criteria (cont.) • Distance and Medium • Attaining high speed over long distances can be very expensive. Thus, each LAN has a maximum distance it can cover. • Speed • It is important to you select a LAN capable of meeting your performance goals. Available LAN speeds are 10, 100, and 1,000 Mbps, and the trend is for increasing speeds. • Device connectivity • Some organizations need to attach special devices to the LAN, for example, a plotter or scanner. LAN interfaces for such devices may not be available on some LANs or on some LAN file servers. • Connectivity to Other Networks • A variety of connection capabilities exist, but a given LAN may not support all of them. • Adherence to Established Standards • There are several standards for LAN implementation. Some LANs conform to these standards whereas others do not.

  48. Simple LAN Topologies • Physical topology: Physical layout of a network • Bus topology consists of a single cable—called a bus— connecting all nodes on a network without intervening connectivity devices

  49. Simple LAN Topologies • Ring topology • Each node is connected to the two nearest nodes so the entire network forms a circle • Active topology • Each workstation transmits data • Each workstation functions as a repeater

  50. Simple LAN Topologies • Star topology • Every node on the network is connected through a central device