Local Area Networks/School of Engineering in Computer Science/ LAN basics  Networking basics The Internet TCP/IP  LANs topologies  Media Access Control (MAC) techniques

Local Area Networks/School of Engineering in Computer Science/ LAN basics  Networking basics

Local Area Networks (RALIR) /School of Engineering in Computer Science 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., , IP telephony),  to do transactions (banking, e-commerce, stock trading), and…  … save money: downsizing 3

Local Area Networks (RALIR) /School of Engineering in Computer Science 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, … 4 local ISP company network regional ISP router workstation server mobile

Local Area Networks (RALIR) /School of Engineering in Computer Science A closer look at the network structure 1.The network edge: applications and hosts 2.The network core: routers network of networks 3.The access networks and physical media: communication links 5

Local Area Networks (RALIR) /School of Engineering in Computer Science 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; client/server peer-peer model:  host interaction symmetric  e.g.: Gnutella, KaZaA 6

Local Area Networks (RALIR) /School of Engineering in Computer Science 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” 7

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 8

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 9

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 10

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 11

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 12

Local Area Networks (RALIR) /School of Engineering in Computer Science Residential access networks: cable modems 13 Diagram:

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 14

Local Area Networks (RALIR) /School of Engineering in Computer Science 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. 15 base station mobile hosts router

Local Area Networks/School of Engineering in Computer Science/ LAN basics  Networking basics The Internet

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 17 NBP A NBP B NAP regional ISP local ISP local ISP

Local Area Networks (RALIR) /School of Engineering in Computer Science Network Access Points (NAPs) 18 Source: Boardwatch.com Note: Peers in this context are commercial backbones.

Local Area Networks (RALIR) /School of Engineering in Computer Science MCI/WorldCom/UUNET Global Backbone 19 Source: Boardwatch.com

Local Area Networks (RALIR) /School of Engineering in Computer Science The situation in Europe 20 See: Also: More about technolgies:

Local Area Networks/School of Engineering in Computer Science/ LAN basics  Networking basics TCP/IP

Local Area Networks (RALIR) /School of Engineering in Computer Science A simple TCP/IP Example  A user on host argon.tcpip-lab.edu (“Argon”) makes a web access to URL  What actually happens in the network? 22

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 23

Local Area Networks (RALIR) /School of Engineering in Computer Science HTTP Request 24 GET /index.html HTTP/1.1 Accept: image/gif, */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 Host: neon.tcpip-lab.edu Connection: Keep-Alive

Local Area Networks (RALIR) /School of Engineering in Computer Science HTTP Response 25 HTTP/ OK Date: Sat, 25 May :10:32 GMT Server: Apache/ (Unix) Last-Modified: Sat, 25 May :51:33 GMT ETag: " ceff955" Accept-Ranges: bytes Content-Length: 81 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html Internet Lab Click here for the Internet Lab webpage. How does the HTTP request get from Argon to Neon ?

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 26

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 “neon.tcpip-lab.edu” must be translated into a 32-bit IP address. 2. The HTTP server at Neon must be identified by a 16-bit port number. 27

Local Area Networks (RALIR) /School of Engineering in Computer Science Translating a hostname into an IP address  The translation of the hostname neon.tcpip-lab.edu 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 address: argon.tcpip-lab.edu neon.tcpip-lab.edu

Local Area Networks (RALIR) /School of Engineering in Computer Science Finding the port number  Note: Most services on the Internet are reachable via well- 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: ftp21finger79 telnet23http80 smtp 25nntp

Local Area Networks (RALIR) /School of Engineering in Computer Science Requesting a TCP Connection  The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish a connection to port 80 of the machine with address

Local Area Networks (RALIR) /School of Engineering in Computer Science 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) 31

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 Argon know this?)  So, Argon sends the IP datagram to its default gateway  The default gateway is an IP router  The default gateway for Argon is Router137.tcpip-lab.edu ( ). 32

Local Area Networks (RALIR) /School of Engineering in Computer Science The route from Argon to Neon  Note that the gateway has a different name for each of its interfaces. 33

Local Area Networks (RALIR) /School of Engineering in Computer Science 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) 34

Local Area Networks (RALIR) /School of Engineering in Computer Science Address resolution with ARP 35

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 36

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 37

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 38

Local Area Networks (RALIR) /School of Engineering in Computer Science 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). 39

Local Area Networks (RALIR) /School of Engineering in Computer Science Invoking the device driver at the router 40  The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28

Local Area Networks (RALIR) /School of Engineering in Computer Science 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. 41

Local Area Networks (RALIR) /School of Engineering in Computer Science 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. 42

Local Area Networks (RALIR) /School of Engineering in Computer Science 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) …. 43

Local Area Networks/School of Engineering in Computer Science/ LAN basics  LANs topologies

Local Area Networks (RALIR) /School of Engineering in Computer Science 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. … 45

Local Area Networks (RALIR) /School of Engineering in Computer Science 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. 46

Local Area Networks (RALIR) /School of Engineering in Computer Science 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. 47

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 48

Local Area Networks (RALIR) /School of Engineering in Computer Science 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 49

Local Area Networks (RALIR) /School of Engineering in Computer Science Simple LAN Topologies  Star topology Every node on the network is connected through a central device 50

Local Area Networks (RALIR) /School of Engineering in Computer Science Hybrid LAN Topologies  Hybrid topology Complex combination of the simple physical topologies  Star-wired ring Star-wired topologies use physical layout of a star in conjunction with token ring-passing data transmission method 51

Local Area Networks (RALIR) /School of Engineering in Computer Science Hybrid LAN Topologies  Star-wired bus In a star-wired bus topology, groups of workstations are star-connected to hubs and then networked via a single bus 52

Local Area Networks (RALIR) /School of Engineering in Computer Science Hybrid LAN Topologies  Daisy-Chained Daisy chain is linked series of devices 53

Local Area Networks (RALIR) /School of Engineering in Computer Science Hybrid LAN Topologies  Hierarchical Uses layers to separate devices by their priority or function 54

Local Area Networks (RALIR) /School of Engineering in Computer Science The UPV extended LAN 55

Local Area Networks/School of Engineering in Computer Science/ LAN basics  Media Access Control (MAC) techniques

Local Area Networks (RALIR) /School of Engineering in Computer Science Media Access Control (MAC)  single shared communication channel  two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time  Media Access Control: distributed algorithm that determines how stations share channel, i.e., determine when a station can transmit communication about channel sharing must use channel itself! Takes also care of:  Assembly of data into frame with address and error detection fields  Disassembly of frame  Address recognition  Error detection 57

Local Area Networks (RALIR) /School of Engineering in Computer Science Media Access Control (MAC)  For the same LLC, several MAC options may be available 58

Local Area Networks (RALIR) /School of Engineering in Computer Science MAC Protocols: a taxonomy  Three broad classes: Channel Partitioning  divide channel into smaller “pieces” (time slots, frequency)  allocate piece to node for exclusive use Random Access  allow collisions  “recover” from collisions “Taking turns”  tightly coordinate shared access to avoid collisions  Goal: efficient, fair, simple, decentralized 59

Local Area Networks (RALIR) /School of Engineering in Computer Science Channel Partitioning MAC protocols TDMA TDMA: time division multiple access  access to channel in "rounds"  each station gets fixed length slot (length = pkt trans time) in each round  unused slots go idle  example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle  inefficient with low duty cycle users and at light load. 60

Local Area Networks (RALIR) /School of Engineering in Computer Science Channel Partitioning MAC protocols FDMA FDMA: frequency division multiple access  channel spectrum divided into frequency bands  each station assigned fixed frequency band  unused transmission time in frequency bands go idle  example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle 61 frequency bands time

Local Area Networks (RALIR) /School of Engineering in Computer Science Random Access MAC protocols  When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes  two or more transmitting nodes -> “collision”,  random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions)  Examples of random access MAC protocols: pure ALOHA slotted ALOHA CSMA and CSMA/CD 62

Local Area Networks (RALIR) /School of Engineering in Computer Science Random Access MAC protocols Pure (unslotted) ALOHA  unslotted Aloha: simpler, no synchronization  pkt needs transmission: send without awaiting for beginning of slot  collision probability increases: pkt sent at t 0 collide with other pkts sent in [t 0 -1, t 0 +1] 63

Local Area Networks (RALIR) /School of Engineering in Computer Science Random Access MAC protocols Slotted Aloha  time is divided into equal size slots (= pkt trans. time)  node with new arriving pkt: transmit at beginning of next slot  if collision: retransmit pkt in future slots with probability p, until successful. 64 Success (S), Collision (C), Empty (E) slots

Local Area Networks (RALIR) /School of Engineering in Computer Science Random Access MAC protocols CSMA: Carrier Sense Multiple Access CSMA: listen before transmit:  If channel sensed idle: transmit entire pkt  If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) Non-persistent CSMA: retry after random interval  human analogy: don’t interrupt others! 65

Local Area Networks (RALIR) /School of Engineering in Computer Science Random Access MAC protocols CSMA collisions 66 collisions can occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes along ethernet

Local Area Networks (RALIR) /School of Engineering in Computer Science “Taking Turns” MAC protocols  “taking turns” protocols look for best of both worlds, because: Channel partitioning MAC protocols:  share channel efficiently at high load  inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols  efficient at low load: single node can fully utilize channel  high load: collision overhead 67

Local Area Networks (RALIR) /School of Engineering in Computer Science “Taking Turns” MAC protocols Polling:  master node “invites” slave nodes to transmit in turn  Request to Send, Clear to Send msgs  concerns: polling overhead latency single point of failure (master) Token passing:  control token passed from one node to next sequentially.  token message  concerns: token overhead latency single point of failure (token) 68