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The Data Link Layer Addressing Last time Our goals: different address scheme in different link layer services understand principles layers error detection, correction behind data link layer services: multiple access

  1. The Data Link Layer Addressing Last time Our goals: � different address scheme in different � link layer services � understand principles layers � error detection, correction behind data link layer services: � multiple access protocols and � application layer: host names LANs � error detection, � (transport layer: port number) correction � link layer addressing, ARP, DHCP � sharing a broadcast � network layer: IP-addresses channel: multiple access � Ethernet � link layer: LAN addresses Today � link layer addressing � translation mechanisms � reliable data transfer, � hubs and switches flow control: done! � PPP � DNS � instantiation and implementation of various � ARP link layer technologies 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU Hubs Switch … physical-layer (“dumb”) repeaters: � link-layer device: smarter than hubs, take � bits coming in one link go out all other links at active role same rate � store, forward Ethernet frames � all nodes connected to hub can collide with one another � examine incoming frame’s MAC address, selectively forward frame to one-or-more � no frame buffering outgoing links when frame is to be forwarded on � no CSMA/CD at hub: host NICs detect segment, uses CSMA/CD to access segment collisions � transparent � hosts are unaware of presence of switches twisted pair � plug-and-play, self-learning hub � switches do not need to be configured 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU Switch: allows multiple simultaneous Switch Table transmissions A A � Q: how does switch know that � hosts have dedicated, C’ C’ B A’ reachable via interface 4, B direct connection to switch B’ reachable via interface 5? � switches buffer packets 1 2 3 1 2 3 � A: each switch has a switch 6 6 � Ethernet protocol used on table, each entry: 4 4 each incoming link, but no 5 5 � MAC address of host, interface collisions; full duplex C C to reach host, time stamp � each link is its own collision � looks like a routing table! domain B’ B’ A’ A’ � switching: A-to-A’ and B- � Q: how are entries created, to-B’ simultaneously, maintained in switch table? switch with six interfaces switch with six interfaces without collisions (1,2,3,4,5,6) (1,2,3,4,5,6) � something like a routing � not possible with dumb hub protocol? 7/10-08 7/10-08 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 1

  2. Switch: frame filtering/forwarding Switch: self-learning Source: A Dest: A’ When frame received: A A A’ � switch learns which hosts C’ can be reached through B 1. record link associated with sending host which interfaces 2. index switch table using MAC dest address 1 2 3 � when frame received, 6 3. if entry found for destination switch “learns” location of 4 then { 5 sender: incoming LAN segment if dest on segment from which frame arrived C � records sender/location then drop the frame pair in switch table B’ else forward the frame on interface indicated A’ MAC addr interface TTL } A 1 60 else flood Switch table forward on all but the interface (initially empty) on which the frame arrived 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU Interconnecting switches Self-learning, Source: A Dest: A’ forwarding: A A’ A � switches can be connected together example C’ B S 4 � frame destination S 1 S 3 1 2 3 unknown: flood S 2 6 A A A’ A A’ A A’ A A’ A A’ F 4 I 5 D � destination A C B H G E location known: C A’ A selective send B’ A’ � Q: sending from A to G - how does S 1 know to MAC addr interface TTL forward frame destined to F via S 4 and S 3 ? A 1 60 Switch table � A: self learning! (works exactly the same as in 60 A’ 4 (initially empty) single-switch case!) 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU Switches vs. Routers Point to Point Data Link Control � both store-and-forward devices � one sender, one receiver, one link: easier than � routers: network layer devices (examine network layer broadcast link: headers) � no Media Access Control � switches are link layer devices � routers maintain routing tables, implement routing � no need for explicit MAC addressing algorithms � e.g., dialup link, ISDN line � switches maintain switch tables, implement � popular point-to-point DLC protocols: filtering, learning algorithms � PPP (point-to-point protocol) � HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!) 7/10-08 7/10-08 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 2

  3. The Internet: virtualizing networks Virtualization of networks 1974: multiple unconnected … differing in: Virtualization of resources: a powerful abstraction in nets � addressing conventions systems engineering: � ARPAnet � packet formats � computing examples: � data-over-cable networks � error recovery � virtual memory � packet satellite network (Aloha) � routing � virtual devices � packet radio network � Virtual machines: e.g., java � layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, 7/10-08 Datakommunikation - Jonny Pettersson, UmU May, 1974, pp. 637 - 6 48. The Internet: virtualizing networks Cerf & Kahn’s Internetwork Architecture Gateway: What is virtualized? Internetwork layer (IP): � “embed internetwork packets in � addressing: internetwork local packet format or extract � two layers of addressing: internetwork and local appears as a single, uniform them” entity, despite underlying local network network heterogeneity � route (at internetwork level) to next gateway � network of networks � new layer (IP) makes everything homogeneous at internetwork layer � underlying local network technology � cable � satellite gateway � 56K telephone modem � ATM, MPLS … “invisible” at internetwork layer. Looks like a link ARPAnet satellite net layer technology to IP! 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU The Data Link Layer Chapter 6: Wireless and Mobile Networks Our goals: Last time Background: � understand principles � link layer services � # wireless (mobile) phone subscribers now behind data link layer � error detection, correction exceeds # wired phone subscribers! services: � multiple access protocols and � error detection, � computer nets: laptops, palmtops, PDAs, LANs correction � link layer addressing, ARP, Internet-enabled phone promise anytime � sharing a broadcast DHCP channel: multiple access untethered Internet access � Ethernet � link layer addressing � two important (but different) challenges Today � reliable data transfer, � wireless: communication over wireless link flow control: done! � hubs and switches � instantiation and � mobility: handling the mobile user who changes point � PPP implementation of various of attachment to network Next link layer technologies � Wireless and Mobile Networks 7/10-08 7/10-08 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 3

  4. Chapter 6 outline Elements of a wireless network 6.1 Introduction Mobility wireless hosts � laptop, PDA, IP phone � 6.5 Principles: � run applications addressing and routing Wireless � may be stationary to mobile users � 6.2 Wireless links, (non-mobile) or mobile � 6.6 Mobile IP characteristics network � wireless does not infrastructure � 6.7 Handling mobility in always mean mobility � CDMA cellular networks � 6.3 IEEE 802.11 wireless LANs (“wi-fi”) � 6.8 Mobility and higher- layer protocols � 6.4 Cellular Internet Access � architecture 6.9 Summary � standards (e.g., GSM) 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU Elements of a wireless network Elements of a wireless network base station wireless link � typically connected to � typically used to wired network connect mobile(s) to base station � relay - responsible for sending packets � also used as backbone between wired link network network and wireless network � multiple access host(s) in its “area” infrastructure infrastructure protocol coordinates � e.g., cell towers, link access 802.11 access � various data rates, points transmission distance 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7/10-08 Datakommunikation - Jonny Pettersson, UmU Characteristics of selected wireless link Elements of a wireless network standards infrastructure mode � base station connects mobiles into wired 200 802.11n network 54 802.11a,g 802.11a,g point-to-point � handoff: mobile data Data rate (Mbps) changes base station 5-11 802.11b 802.16 (WiMAX) providing connection network 4 3G cellular UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO into wired network infrastructure enhanced 1 802.15 .384 UMTS/WCDMA, CDMA2000 3G .056 2G IS-95, CDMA, GSM Indoor Outdoor Mid-range Long-range 10-30m 50-200m outdoor outdoor 200m – 4 Km 5Km – 20 Km 7/10-08 7/10-08 Datakommunikation - Jonny Pettersson, UmU Datakommunikation - Jonny Pettersson, UmU 4

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