5: DataLink Layer5a-1 Chapter 5 Data Link Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

5: DataLink Layer5a-2 5a-1 5: DataLink Layer Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

5: DataLink Layer5a-3 5a-2 5: DataLink Layer Hubs Physical Layer device  Simplest way to interconnect LANs  Operates on bits rather than frames  When a bit comes into a hub interface, the hub broadcasts the bit on all the other interfaces.

5: DataLink Layer5a-4 5a-3 5: DataLink Layer Interconnecting with hubs r Multi-tier hub design (hierarchy) r Backbone hub interconnects three academic depts or “LAN segments” r Depts have a 10BaseT Ethernet that provides network access r Hosts have point-to-point connections to departmental hub

5: DataLink Layer5a-5 5a-4 5: DataLink Layer Hubs: Benefits r Provides interdepartmental communication r Extends the max distance betw any pair of nodes r Multi-tier design provides a degree of graceful degradation m If one dept malfunctions, the backbone hub can detect the prob and disconnect the dept hub

5: DataLink Layer5a-6 5a-5 5: DataLink Layer Hubs: Limitations r When departmental LANs are interconnected by a hub, then the independent collision domains become one collision domain. r If depts use different Ethernet technologies, may not be able to interconnect them r Each Ethernet technology has restrictions on m max # of nodes in collision domain m max distance between two hosts m max # of tiers m Constrains # of hosts as well as geographical reach of the multi-tier LAN.

5: DataLink Layer5a-7 5a-6 5: DataLink Layer Bridges Link layer device r Operate on Ethernet frames unlike hubs r are full-fledged packet switches that forward and filter Ethernet frames using the LAN destination addresses r examines frame header and selectively forwards frame based on MAC dest address r when frame is to be forwarded on segment, uses CSMA/CD to access segment

5: DataLink Layer5a-8 5a-7 5: DataLink Layer Bridges: traffic isolation r Bridge installation breaks LAN into LAN segments r bridges filter packets: m same-LAN-segment frames not usually forwarded onto other LAN segments m segments become separate collision domains bridge collision domain collision domain = hub = host LAN (IP network) LAN segment

5: DataLink Layer5a-9 5a-8 5: DataLink Layer Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...

5: DataLink Layer5a-10 5a-9 5: DataLink Layer Self learning r A bridge has a bridge table r entry in bridge table: m (Node LAN Address, Bridge Interface, Time Stamp) m stale entries in table dropped (TTL can be 60 min) r bridges learn which hosts can be reached through which interfaces m when frame received, bridge “learns” location of sender: incoming LAN segment m records sender/location pair in bridge table

5: DataLink Layer5a-11 5a-10 5: DataLink Layer Filtering/Forwarding When bridge receives a frame: index bridge table using MAC dest address 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 to all but the interface on which the frame arrived

5: DataLink Layer5a-12 5a-11 5: DataLink Layer Bridge example Suppose C sends frame to D and D replies back with frame to C.  Bridge receives frame from from C  notes in bridge table that C is on interface 1  because D is not in table, bridge sends frame into interfaces 2 and 3  frame received by D

5: DataLink Layer5a-13 5a-12 5: DataLink Layer Bridge Learning: example  D generates frame for C, sends  bridge receives frame  notes in bridge table that D is on interface 2  bridge knows C is on interface 1, so selectively forwards frame to interface 1 D C

5: DataLink Layer5a-14 5a-15 5: DataLink Layer Bridges Spanning Tree r for increased reliability, desirable to have redundant, alternative paths from source to dest r with multiple paths, cycles result - bridges may multiply and forward frame forever r solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

5: DataLink Layer5a-15 5a-16 5: DataLink Layer Some bridge features rIsolates collision domains resulting in higher total max throughput rCan have limitless number of nodes and geographical coverage rCan be used to combine Ethernet segments using different Ethernet technologies with Ethernet bridges m10Base2, 100BaseT, 10Base2 = can use a Gigabit bridge rTransparent (“plug-and-play”): no configuration necessary

5: DataLink Layer5a-16 Interconnection without backbone - all traffic between EE and SE must path over CS segment r Not recommended for two reasons: - single point of failure at Computer Science hub

5: DataLink Layer5a-17 Backbone configuration Recommended ! Backbone: a network that has direct connections to all the LAN segments

5: DataLink Layer5a-18 Bridges vs. Routers r bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms r both store-and-forward devices m routers: network layer devices (IP Address) m bridges are link layer devices (LAN Address) r routers maintain routing tables, implement routing algorithms

5: DataLink Layer5a-19 Routers vs. Bridges - Bridges do not offer protection from broadcast storms Bridges Pros + Bridge operation is simpler resulting in high packet filtering and forwarding rates. + Bridge tables are self learning + “plug-and-play” Bridges Cons - All traffic confined to spanning tree, even when more direct (but disconnected) path.

5: DataLink Layer5a-20 Routers vs. Bridges r bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts) Routers Cons Routers Pros + arbitrary topologies can be supported, cycling is limited by TTL counters (datagram) + provide protection against broadcast storms - require IP address configuration (not Plug-and-play) - require higher packet processing - Pronunciation

5: DataLink Layer5a-21 Ethernet Switches r often: individual hosts, star-connected into switch m Dedicated Access r Essentially a multi-interface bridge r layer 2 (frame) forwarding, filtering using LAN addresses r Switching: A-to-A’ then B-to-B’ simultaneously No collisions (full duplex mode)

5: DataLink Layer5a-22 Ethernet Switches r combinations of shared/dedicated, 10/100/1000 Mbps interfaces r cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame m slight reduction in latency m ex. Caravan

5: DataLink Layer5a-23 Not an atypical LAN (IP network) Dedicated Shared

5: DataLink Layer5a-24 Summary comparison hubsbridgesroutersswitches traffic isolation noyes plug & playyes noyes optimal routing no yesno cut through yesno yes

5: DataLink Layer5a-25 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

5: DataLink Layer5a-26 Wireless Links r The wave of the future for networking: wireless links r Examples of end systems: m Portable PCs, PDAs, airport hubs, wireless telephony (such as the cellphone pictured) m Future appliances may include cameras, automobiles, pets, security systems, kitchen appliances, and plants. r IEEE b – most popular standard wireless LANs r Bluetooth – new standard that allows devices to communicate with each other r Three classifications: m Power, range, data rate m Bluetooth – low, short, low m – high, medium, high

5: DataLink Layer5a-27 IEEE Wireless LAN r b m Currently most popular form of wireless LAN: wireless Ethernet, Wi-Fi m GHz unlicensed radio spectrum m up to 11 Mbps m physical layer and Media Access Control (MAC) layer for wireless local area network m direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code Not a multi access protocol (does not coordinate channel access from multiple hosts m widely deployed, using base stations

5: DataLink Layer5a-28 IEEE Wireless LAN r Other wireless standards m a – operates on 5-6GHz range and uses OFDM (orthogonal frequency-division multiplexing, not DSSS m Speeds can get up to 54Mbps m g – operates at 2.4GHz m Speeds up to 54Mbps r All use CSMA/CA for multi-access and have base stations and ad-hoc network versions

5: DataLink Layer5a-29 Base station approch r Wireless host communicates with a base station m base station = access point (AP) r Basic Service Set (BSS) (a.k.a. “cell”) contains: m wireless hosts m access point (AP): base station r BSS’s combined to form distribution system (DS)

5: DataLink Layer5a-30 Ad Hoc Network approach r No AP (i.e., base station) r wireless hosts communicate with each other m to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z r Applications: m “laptop” meeting in conference room, car m interconnection of “personal” devices m battlefield r IETF MANET (Mobile Ad hoc Networks) working group

5: DataLink Layer5a-31 IEEE : multiple access r Collision if 2 or more nodes transmit at same time r CSMA makes sense: m get all the bandwidth if you’re the only one transmitting m shouldn’t cause a collision if you sense another transmission r Collision detection doesn’t work: hidden terminal problem

5: DataLink Layer5a-32 IEEE MAC Protocol: CSMA/CA CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff CSMA receiver - if received OK return ACK after SIFS (ACK is needed due to hidden terminal problem)

5: DataLink Layer5a-33 Collision avoidance mechanisms r Problem: m two nodes, hidden from each other, transmit complete frames to base station m wasted bandwidth for long duration ! r Solution: m small reservation packets m nodes track reservation interval with internal “network allocation vector” (NAV)

5: DataLink Layer5a-34 Collision Avoidance: RTS-CTS exchange r sender transmits short RTS (request to send) packet: indicates duration of transmission r receiver replies with short CTS (clear to send) packet m notifying (possibly hidden) nodes r hidden nodes will not transmit for specified duration: NAV

5: DataLink Layer5a-35 Collision Avoidance: RTS-CTS exchange r RTS and CTS short: m collisions less likely, of shorter duration m end result similar to collision detection r IEEE allows: m CSMA m CSMA/CA: reservations m polling from AP

5: DataLink Layer5a-36 Cellular vs Wireless LAN r 3G Cellular mobile m 2Mbps indoor m 384kbps outdoor m Licensed radio freq (1885 – 2025 and 2110 – 2200 MHz) r Cons m 3G is more costly ($2000 for radio freq licenses) m Competition from wireless LAN tech. r IEEE b wireless LAN enjoys more widespread usage m LAN capable cards will be installed in most all prepackaged computers m Continue with other devices m Bulk of traffic local termination m Handles GSM/GPRS

5: DataLink Layer5a-37 A word about Bluetooth r Low-power, small radius, wireless networking technology m meters r omnidirectional m not line-of-sight infared r Interconnects gadgets r GHz unlicensed radio band r up to 721 kbps r Interference from wireless LANs, digital cordless phones, microwave ovens: m frequency hopping helps r MAC protocol supports: m error correction m ARQ r Each node has a 12-bit address