The Data Link Layer Last time Our goals: � link layer services � understand principles � error detection, correction behind data link layer services: � multiple access protocols and LANs � error detection, correction � link layer addressing, ARP, DHCP � sharing a broadcast channel: multiple access � Ethernet Today � link layer addressing � reliable data transfer, � hubs and switches flow control: done! � PPP � instantiation and implementation of various link layer technologies 7/10-08 Datakommunikation - Jonny Pettersson, UmU Addressing � different address scheme in different layers � application layer: host names � (transport layer: port number) � network layer: IP-addresses � link layer: LAN addresses � translation mechanisms � DNS � ARP 7/10-08 Datakommunikation - Jonny Pettersson, UmU Hubs … physical-layer (“dumb”) repeaters: � bits coming in one link go out all other links at same rate � all nodes connected to hub can collide with one another � no frame buffering � no CSMA/CD at hub: host NICs detect collisions twisted pair hub 7/10-08 Datakommunikation - Jonny Pettersson, UmU 1
Switch � link-layer device: smarter than hubs, take active role � store, forward Ethernet frames � examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment � transparent � hosts are unaware of presence of switches � plug-and-play, self-learning � switches do not need to be configured 7/10-08 Datakommunikation - Jonny Pettersson, UmU Switch: allows multiple simultaneous transmissions A � hosts have dedicated, C’ B direct connection to switch � switches buffer packets 1 2 3 6 � Ethernet protocol used on 4 each incoming link, but no 5 collisions; full duplex C � each link is its own collision domain B’ A’ � switching: A-to-A’ and B- to-B’ simultaneously, switch with six interfaces without collisions (1,2,3,4,5,6) � not possible with dumb hub 7/10-08 Datakommunikation - Jonny Pettersson, UmU Switch Table A � Q: how does switch know that C’ A’ reachable via interface 4, B B’ reachable via interface 5? 1 2 3 � A: each switch has a switch 6 table, each entry: 4 5 � MAC address of host, interface C to reach host, time stamp � looks like a routing table! B’ A’ � Q: how are entries created, maintained in switch table? switch with six interfaces (1,2,3,4,5,6) � something like a routing protocol? 7/10-08 Datakommunikation - Jonny Pettersson, UmU 2
Switch: self-learning Source: A Dest: A’ A A A’ � switch learns which hosts C’ can be reached through B which interfaces 1 2 3 � when frame received, 6 switch “learns” location of 4 5 sender: incoming LAN segment C � records sender/location pair in switch table B’ A’ MAC addr interface TTL A 1 60 Switch table (initially empty) 7/10-08 Datakommunikation - Jonny Pettersson, UmU Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived 7/10-08 Datakommunikation - Jonny Pettersson, UmU Self-learning, Source: A Dest: A’ forwarding: A A’ A example C’ B � frame destination 1 2 3 unknown: flood 6 A A’ A A’ A A’ A A’ A A’ 4 5 � destination A location known: C A’ A selective send B’ A’ MAC addr interface TTL A 1 60 Switch table A’ 4 60 (initially empty) 7/10-08 Datakommunikation - Jonny Pettersson, UmU 3
Interconnecting switches � switches can be connected together S 4 S 1 S 3 S 2 A F I D B C H G E � Q: sending from A to G - how does S 1 know to forward frame destined to F via S 4 and S 3 ? � A: self learning! (works exactly the same as in single-switch case!) 7/10-08 Datakommunikation - Jonny Pettersson, UmU Switches vs. Routers � both store-and-forward devices � routers: network layer devices (examine network layer headers) � switches are link layer devices � routers maintain routing tables, implement routing algorithms � switches maintain switch tables, implement filtering, learning algorithms 7/10-08 Datakommunikation - Jonny Pettersson, UmU Point to Point Data Link Control � one sender, one receiver, one link: easier than broadcast link: � no Media Access Control � no need for explicit MAC addressing � e.g., dialup link, ISDN line � popular point-to-point DLC protocols: � 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 Datakommunikation - Jonny Pettersson, UmU 4
Virtualization of networks Virtualization of resources: a powerful abstraction in systems engineering: � computing examples: � virtual memory � virtual devices � Virtual machines: e.g., java � layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly 7/10-08 Datakommunikation - Jonny Pettersson, UmU The Internet: virtualizing networks 1974: multiple unconnected … differing in: nets � addressing conventions � ARPAnet � packet formats � data-over-cable networks � error recovery � packet satellite network (Aloha) � routing � packet radio network ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637 - 6 48. The Internet: virtualizing networks Gateway: Internetwork layer (IP): � “embed internetwork packets in � addressing: internetwork local packet format or extract appears as a single, uniform them” entity, despite underlying local network heterogeneity � route (at internetwork level) to next gateway � network of networks gateway ARPAnet satellite net 7/10-08 Datakommunikation - Jonny Pettersson, UmU 5
Cerf & Kahn’s Internetwork Architecture What is virtualized? � two layers of addressing: internetwork and local network � new layer (IP) makes everything homogeneous at internetwork layer � underlying local network technology � cable � satellite � 56K telephone modem � ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP! 7/10-08 Datakommunikation - Jonny Pettersson, UmU The Data Link Layer Our goals: Last time � understand principles � link layer services behind data link layer � error detection, correction services: � multiple access protocols and � error detection, LANs correction � link layer addressing, ARP, � sharing a broadcast DHCP channel: multiple access � Ethernet � link layer addressing Today � reliable data transfer, flow control: done! � hubs and switches � instantiation and � PPP implementation of various Next link layer technologies � Wireless and Mobile Networks 7/10-08 Datakommunikation - Jonny Pettersson, UmU Chapter 6: Wireless and Mobile Networks Background: � # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! � computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access � two important (but different) challenges � wireless: communication over wireless link � mobility: handling the mobile user who changes point of attachment to network 7/10-08 Datakommunikation - Jonny Pettersson, UmU 6
Chapter 6 outline 6.1 Introduction Mobility � 6.5 Principles: addressing and routing Wireless to mobile users � 6.2 Wireless links, � 6.6 Mobile IP characteristics � 6.7 Handling mobility in � 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 Elements of a wireless network wireless hosts � laptop, PDA, IP phone � run applications � may be stationary (non-mobile) or mobile network � wireless does not infrastructure always mean mobility 7/10-08 Datakommunikation - Jonny Pettersson, UmU Elements of a wireless network base station � typically connected to wired network � relay - responsible for sending packets between wired network and wireless network host(s) in its “area” infrastructure � e.g., cell towers, 802.11 access points 7/10-08 Datakommunikation - Jonny Pettersson, UmU 7
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