Other LAN Technologies

2 LAN Standards u 802 Working Groups –802.3Ethernet LANs –802.5Token-Ring Networks –802.11Radio LANs – VG-AnyLAN

Token-Ring Network Standard u Championed by IBM –Official IEEE and OSI standard, but most vendors follow IBM extensions to the standard u More reliable than Ethernet LANs u More complex and therefore more expensive u Lower market share than Ethernet LANs –Mostly in firms with large IBM mainframe networks –Tightly integrated into SNA u Read a tutorial in token-ring networkstutorial

4 Ring Topology in Token-Ring Networks Station B Station A Station E Station D Station C Frame Ring Station B only receives frames from Station A and only transmits frames to Station C Ring

5 Problem with Rings u If the ring breaks, LAN stops –Signals must go all the way around the ring, back to the sender –This becomes impossible

6 Use a Double Ring u One is unused in normal operation u If there is a break, the ring is wrapped –Still a ring NormalWrapped

7 UTP and STP Wiring Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP) Twisted Pair Twisted Pair Shielding Around Pair Outer Shield Around Bundle Plastic Cover (Non-Shielding) Twisted Pair Twisted Pair

8 STP vs. UTP u STP –Little interference –Thick: difficult to install –Expensive u UTP –Thin: easy to install –Inexpensive –Interference is rarely a practical problem –Does the job at a reasonable price, so dominates

9 Access Units in a Ring Access Unit STP link between Access Units STP link from Station to Access Unit Stations Station UTP Link from Station to Access Unit

10 NIC Within the Access Unit u The ring is retained u Powered-up NICs added automatically u Powered-off NICs bypassed automatically Ring Missing NIC Bypassed Node

11 Token Passing in Token-Ring Networks Token Station B Station B may only transmit when it receives a special frame called a token.

12 Ethernet (802.3) vs Token-Ring (802.5) u Physical Layer –Ethernet primarily uses UTP wiring –Token-Ring Networks primarily use shielded twisted pair (STP) wiring u Topology (Layout) of the Wiring –Ethernet always uses bus (broadcast) topology –Token-Ring always uses a ring topology (connectivity) u Access Control –(Control of When Stations May Transmit) –Ethernet always uses CSMA/CD –Token-Ring always uses token passing

13 Ethernet (802.3) vs Token-Ring (802.5) u Speed –Ethernet primarily 10 Mbps (moving to 100 Mbps and gigabit speeds) –Token-Ring Networks usually at 16 Mbps –TRNs can get closer to full capacity because token passing is more efficient than CSMA/CD at high traffic loads –Priority levels for real-time traffic (video teleconferencing, etc.) u Cost –TRN is more complex, so NICs cost much more –TRN has low market share; low vendor competition adds to high NIC costs –Most firms do not find the benefits of TRNs to outweigh the costs

14 Shared Media LANs u Ethernet (802.3) and Token-Ring Networks (802.5) are Shared Media LANs –Only one station may transmit at any moment. –Every station hears every transmission –Stations must wait their turn to transmit

15 Congestion and Latency in Shared Media LANs Transmission Shared Media LAN Station B is Transmitting But Must Stop Soon Station A Must Wait to Transmit Station C Must Wait to Transmit

16 Congestion and Latency u As the number of stations on a shared media LAN increases... –Traffic increases, so –Stations must wait longer to transmit –Latency (delay) increases –This is called congestion u At stations, a 10 Mbps (4-16 Mbps) shared media LAN becomes saturated

Mbps LANs u Reducing Congestion –One way to decrease congestion is to increase LAN speed from 10 Mbps to 100 Mbps or higher –Each transmission will be briefer, because it can be transmitted faster –Therefore more stations can share the LAN before saturation occurs –Only postpones the problem

18 FDDI Network FDDI Ring

19 FDDI u FDDI –Fiber distributed data interface –Token-ring technology (but incompatible with 802.5) –100 Mbps –Mature (1987) –200 km maximum diameter: popular for connecting LANs to local internets, not to connect desktops. –Priority levels for real-time traffic (voice, video) –Expensive NICs and other equipment –Read a tutorial in FDDItutorial

VG-AnyLAN u 100 Mbps u Demand Priority Access Method –Station sends high- or low-priority requests –All high-priority requests on all repeaters served first –Good for real-time applications u Hubs (repeaters) organized as a Tree –One is the master repeater u Not achieving market acceptance

VG-AnyLAN Hub Hierarchy Repeater A Repeater BRepeater C Repeater D Repeater E Master Repeater Station 1 Station 2 High-Priority RequestLow-Priority Request First Level Repeater Second Level Repeaters Third Level Repeaters

22 100Base-X u 100 Mbps u Uses Normal MAC Layer Frame u Family of Standards –100Base-TX uses Cat 5 wiring (most popular to desk) –100Base-T4 uses Cat 3 and Cat 4 wiring –100Base-FX uses optical fiber

23 100Base-TX u Many install 100Base-TX instead of 10Base-T Today u Requires 100 Mbps hubs instead of 10 Mbps u Requires 100 Mbps NICs instead of 10 Mbps –Some hubs can also serve 10Base-T NICs, so not all stations have to be upgraded at once u Uses Category 5 wiring, making upgrading easy

24 Upgrading from 10Base-T to 100Base-T u Need New Hub –All 100Base-TX is expensive –Often many 10Base-T hubs for client PCs –A few 100Base-TX hubs for servers u Need New NICs –Only in stations with 100Base-T NICs u Retain Old Wiring –If Cat 5 –Avoids a major expense

25 Ethernet 100Base-TX Network 100Base-TX Hub Station AStation B Station C 100 m Segment Maximum 100 m Segment Maximum ~50 maximum - 5 UTP wiring - NICs are replaced 100Base-TX Hub

26 Ethernet 100Base-TX Network u The most popular 100Base-X standard, runs over existing 5 UTP wire of 10Base-T u Only two segments, length ~200m u Can mix 10 Base-T and 100Base-T stations/NICs with hubs that take both types u Use the same MAC standard of 10 Base-T u Market has chosen Ethernet 100Base-TX for desktop connection over FDDI and 100VG- AnyLAN100Base-TX u Read classic tutorial on Fast EthernetFast Ethernet

Base-X (Gigabit Ethernet) u 1000 Mbps u Usually used to link 100Base-X hubs 1000Base-X Hub 100Base-T Hubs

Base-X u Family of Standards (802.3z) u 1000Base-LX –Long-wave (lower frequency) laser –550 meters on multimode optical fiber –3 km on single mode fiber u 1000Base-SX –Short-wave ( higher frequency) laser –300 meters on 62.5 micron multimode fiber

29 Full Duplex Ethernet u CSMA/CD is half duplex –Only one station may transmit at a time –Others must wait –Because transmission system is shared u If station or hub connects directly to a hub, –The access line is not shared –Some 100Base-X and 1000Base-X hubs and NICs support full duplex operation –Disable CSMA/CD –802.3x standard

30 Shared media LANs u Limits to Shared Media LANs –FDDI, 100Base-X, 100VG-AnyLAN all shared media LANs v Only one station can transmit at a time, causing latency v Every station hears every message, so as the number of stations grow, the LAN saturates –100 Mbps speed only delays saturation

31 Shared media LANs u Shared Media Networks with Hubs (such as 10Base-T) –Incoming frame arrives through a single port –Hub broadcasts frames out all ports –Congestion on output ports Hub

32 Switched LANs u In a switched network –Incoming frame arrives on a single port –Frame sent out again only on a single port--the one leading to the receiver –No congestion on other ports Switch

33 Switch Station A Station B Station C Station D Connection 1 A-C Connection 1 A-C Connection 2 B-D Connection 2 B-D With a switch, multiple stations may transmit simultaneously: no congestion as traffic grows.

34 Switching in Perspective u Switching is the wave of the future for LANs –Congestion does not increase as the number of stations grows u However, –Today, however, switches are still more expensive than 10Base-T or 100Base-X hubs u Read CISCO white paperwhite paper –discount the sales talk –see 3COM images of switches.images

35 Switch connections u paths called connections must be pre-defined between stations u a fixed logical data link (logical connection) is established between stations before transmission even begins u during the transmission, all traffic between the stations must pass over that data link u unless a data link has been pre-established, two stations may not communicate at all u only OSI Layer 2 (Data Link Layer) protocols are needed

36 Ethernet Switches u Ethernet Hubs are Half Duplex u Most Ethernet Switches are Full Duplex –No collisions are possible –So two stations can both transmit to each other at the same time (full duplex operation) –Requires full duplex switches –Requires full duplex NICs u Lowest-cost LAN switches u Not standardized, so buyers tend to get locked into a single vendor

37 ATM Switches u Asynchronous Transfer Mode u Will allow much higher speeds –155 Mbps to a few Gbps u Can also be used for long-distance networking –A single solution for both needs u Quality of service guaranteed u Far more expensive than Ethernet LAN switches

38 ATM Switches u standardized (others not yet) u scalable: as low as 1 Mbps to 2.4 Gbps –can start with relative slow speeds (cheaper) –increase the speed as needs arise –without changing protocol

39 ATM and Ethernet u 100Mbps and Gigabit Ethernet are outselling ATM for LAN usage u High-speed Ethernet is less expensive u Staff does not have to learn ATM technology u Sales of NICs - Ethernet, Token Ring and ATM. Sales of NICs

40 Wireless LAN Broadcast Signal Transceiver Transmitting Transceiver Receiving Cluster Transceiver Receiving Antenna Hub Controller Wireless LAN

41 Typical Wireless LAN Operation with Access Points Switch Client PC Server Large Wired LAN Access Point A Access Point B UTPRadio Link Handoff If mobile computer moves to another access point, it switches service to that access point Notebook CSMA/CA+ACK UTP

42 Typical Wireless LAN Operation with Access Points Wireless Notebook NIC Access Point Industry Standard Coffee Cup To Ethernet Switch Antenna (Fan) PC Card Connector

43 Typical Wireless LAN Operation with Access Points D-Link Wireless Access Point Using Two Antennas Reduces Multipath Interference (See Ch. 3)

44 Linksys Switch With Built-In Wireless Access Point Using Two Antennas Reduces Multipath Interference (See Ch. 3) Typical Wireless LAN Operation with Access Points

45 Typical Wireless LAN Operation with Access Points u The Wireless Station sends an frame to a server via the access point u The access point is a bridge that converts the frame into an Ethernet frame and sends the frame to the server Mobile Station Access Point Ethernet Switch Server Frame Frame

46 Typical Wireless LAN Operation with Access Points u The server responds, sending an frame to the access point u The access point converts the frame into an frame and sends the frame to the mobile station. Mobile Station Access Point Ethernet Switch Server Frame Frame

Wireless LAN Speeds u Mbps (rare) 2.4 GHz band (limited in bandwidth) u b11 Mbps, 2.4 GHz 3 channels/access point u a54 Mbps, 5 GHz (> bandwidth than 2.4 GHz) 11 channels/access point u g54 Mbps, 2.4 GHz limited bandwidth

Broadcast Operation u The Wireless Stations and Access Points Broadcast their Signals. –Only one access point or wireless station may transmit at any moment or signals will become scrambled. Collision About to Occur Access Point Wireless Station Wireless Station

49 CSMA/CA + ACK in Wireless LANs u CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) –Station or access point sender listens for traffic v If there is no traffic, can send if there has been no traffic for a specified amount of time v If the specified amount of time has not been met, must wait for the specified amount of time. Can then send if the line is still clear

50 CSMA/CA + ACK in Wireless LANs u CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) –Station or access point sender listens for traffic v If there is traffic, the sender must wait until traffic stops v The sender must then set a random timer and must wait while the timer is running v If there is no traffic when the station or access point finishes the wait, it may send

51 CSMA/CA + ACK in Wireless LANs u ACK (Acknowledgement) –Receiver immediately sends back an acknowledgement; no waiting because ACKs have highest priority –If sender does not receive the acknowledgement, retransmits using CSMA/CA

52 Who Implements CSMA/CA+ACK? u Stations (when they send) u Access Points (when they send) Mobile Station Access Point Frame CSMA/CA+ACK

53 Request to Send (RTS) / Clear to Send (CTS) u There is a widely used option we should cover. –After a station may send, its first message may be a Request-to-Send (RTS) message instead of a data message –Only if the other party sends a Clear-to-Send (CTS) message does the sender begin sending data Mobile Station Access Point RTS CTS

54 Ad Hoc Networks u Ad Hoc Mode –There is no access point. –Stations broadcast to one another directly –Not scalable but can be useful for SOHO use –NICs automatically come up in ad hoc mode

Security u Attackers can lurk outside your premises –In “war driving,” drive around sniffing out unprotected wireless LANs –In “drive by hacking,” eavesdrop on conversations or mount active attacks. Site with WLAN Outside Attacker

Security u By default, security on WLAN NICs and access points is turned off, making external attacks trivial u WLAN vendors offer Wired Equivalent Privacy (WEP), but this is weak and easily broken. u The Working Group is working on a temporary replacement (TKIP) and longer-term security replacement, i u Even if corporate access points can be secured, many departments create unauthorized rogue access points that are seldom secured.