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Transcript
Switch Table Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? A: each switch has a switch table, each entry: C’ B 6 Q: how are entries created, maintained in switch table? something like a routing protocol? 1 5 (MAC address of host, interface to reach host, time stamp) looks like a routing table! A 2 3 4 C B’ A’ switch with six interfaces (1,2,3,4,5,6) 5: DataLink Layer 5-1 Switch: self-learning switch learns which hosts can be reached through which interfaces Source: A Dest: A’ A A A’ C’ when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch table B 1 6 5 2 3 4 C B’ A’ MAC addr interface TTL A 1 60 Switch table (initially empty) 5: DataLink Layer 5-2 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 5: DataLink Layer 5-3 Self-learning, forwarding: example Source: A Dest: A’ A A A’ C’ B frame destination unknown: flood A6A’ 1 2 4 5 destination A location known: selective send C A’ A B’ 3 A’ MAC addr interface TTL A A’ 1 4 60 60 Switch table (initially empty) 5: DataLink Layer 5-4 Interconnecting switches switches can be connected together S4 S1 S2 A B S3 C F D E I G H Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3? A: self learning! (works exactly the same as in single-switch case!) 5: DataLink Layer 5-5 Self-learning multi-switch example Suppose C sends frame to I, I responds to C S4 1 S1 S2 A B C 2 S3 F D E I G H Q: show switch tables and packet forwarding in S1, S2, S3, S4 5: DataLink Layer 5-6 Institutional network to external network mail server router web server IP subnet 5: DataLink Layer 5-7 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 5: DataLink Layer 5-8 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 5: DataLink Layer 5-9 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 5: DataLink Layer 5-10 Elements of a wireless network network infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile wireless does not always mean mobility 5: DataLink Layer 5-11 Elements of a wireless network network infrastructure base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell towers, 802.11 access points 5: DataLink Layer 5-12 Elements of a wireless network network infrastructure wireless link typically used to connect mobile(s) to base station also used as backbone link multiple access protocol coordinates link access various data rates, transmission distance 5: DataLink Layer 5-13 Characteristics of selected wireless link standards Data rate (Mbps) 200 54 5-11 802.11n 802.11a,g 802.11b 4 1 802.11a,g point-to-point data 802.16 (WiMAX) 3G cellular enhanced UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 802.15 .384 UMTS/WCDMA, CDMA2000 .056 3G 2G IS-95, CDMA, GSM Indoor Outdoor 10-30m 50-200m Mid-range outdoor Long-range outdoor 200m – 4 Km 5Km – 20 Km 5: DataLink Layer 5-14 Elements of a wireless network network infrastructure infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network 5: DataLink Layer 5-15 Elements of a wireless network ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves 5: DataLink Layer 5-16 Wireless network taxonomy single hop infrastructure (e.g., APs) no infrastructure host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) multiple hops host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET 5: DataLink Layer 5-17 802.16: WiMAX like 802.11 & cellular: point-to-point base station model transmissions to/from base station by hosts with omnidirectional antenna base station-to-base station backhaul with point-to-point antenna point-to-multipoint unlike 802.11: range ~ 6 miles (“city rather than coffee shop”) ~14 Mbps 5: DataLink Layer 5-18 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4 Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 5: DataLink Layer 5-19 Components of cellular network architecture MSC cell connects cells to wide area net manages call setup (more later!) handles mobility (more later!) covers geographical region base station (BS) analogous to 802.11 AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS Mobile Switching Center Public telephone network, and Internet Mobile Switching Center wired network 5: DataLink Layer 5-20 Cellular standards: brief survey 2G systems: voice channels IS-136 TDMA: combined FDMA/TDMA (north america) GSM (global system for mobile communications): combined FDMA/TDMA most widely deployed IS-95 CDMA: code division multiple access GSM Don’t drown in a bowl of alphabet soup: use this for reference only 5: DataLink Layer 5-21 Cellular standards: brief survey 2.5 G systems: voice and data channels for those who can’t wait for 3G service: 2G extensions general packet radio service (GPRS) evolved from GSM data sent on multiple channels (if available) enhanced data rates for global evolution (EDGE) also evolved from GSM, using enhanced modulation data rates up to 384K CDMA-2000 (phase 1) data rates up to 144K evolved from IS-95 5: DataLink Layer 5-22 Cellular standards: brief survey 3G systems: voice/data Universal Mobile Telecommunications Service (UMTS) data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 Mbps CDMA-2000: CDMA in TDMA slots data service: 1xEvlution Data Optimized (1xEVDO) up to 14 Mbps ….. more (and more interesting) cellular topics due to mobility (stay tuned for details) 5: DataLink Layer 5-23