1 Fall 2005 LAN Technologies and Network Topology Qutaibah Malluhi Computer Science and Engineering Department Qatar University

2 Agenda  LANs  Topologies  Bit Encoding  Media Access Control  Examples of LANs –Ethernet emphasized

3 Network Classification Terminology  Network technologies classified into three broad categories –Local Area Network (LAN) –Metropolitan Area Network (MAN) –Wide Area Network (WAN)  LAN and WAN most widely deployed

4 Point-to-Point Networks  Computers connected by communication channels that each connect exactly two computers  Point-to-point network –Allows flexibility in communication hardware, packet formats, etc. –Provides security and privacy because communication channel is not shared  Number of wires grows as square of number of computers  For N hosts, No. Connections = (N 2 -N)/2  Adding a new computer requires N - 1 new connections

5 Local Area Network  Developed in the 60’s and 70’s  Interconnect a wide range of devices over short distances, e.g., within the same floor, building or campus (typically up to 10 km diameter).  Key idea - reduce number of connections by sharing connections among many computers –Computers take turns – TDM –Reduce cost but... attached computers compete for use of shared connection –Must include techniques for synchronizing use  In practice –Local communication almost exclusively LAN –Long distance almost exclusively point-to-point »E.g., ATM

6 LAN (Cont.)  Typically, all hosts on a LAN share a common medium. –operate on a broadcast mode  High throughput, low delay  Many LAN technologies and standards exist –E.g., Ethernet and FDDI are popular ones –LAN standards are collectively known as the IEEE 802 standards –Must include techniques for synchronizing hosts

7 Significance of LAN and Locality  LANs are most popular types of networks WHY? –Economical –Principle of locality  Principle of locality of reference helps predict computer communication patterns: –Spatial (or physical) locality of reference »computers likely to communicate with other computers that are located nearby –Temporal locality of reference »computers are likely to communicate with the same computers repeatedly  Thus - LANs are effective because of spatial locality of reference, and temporal locality of reference

8 Different LAN Types  LAN parameters –topology –shared medium (twisted pair, coaxial, fiber) –medium access control technique »governs the access to the LAN transmission medium.  Topology –Specifies general “shape” of a network –Handful of broad categories –Three most popular: »Star »Ring »Bus

9 Star Topology  Central component of network known as hub  Hub: repeats incoming signal to all outgoing links  Each computer has separate connection to hub

10 Star topology in practice  Previous figure is idealized  In practice:

11 Ring Topology  No central facility  Connections go directly from one computer to another  In practice, there is a short connector cable from the computer to the ring  Fault tolerance with two rings

12 Bus Topology  Single cable connects all computers  Each computer has connector to shared cable  Computers must synchronize and allow only one computer to transmit at a time

13 Why Multiple Topologies?  Each has advantages and disadvantages: –Ring ease synchronization; may be disabled if any cable is cut –Star easier to manage and more robust; requires more cables –Bus requires fewer cables; may be disabled if cable is cut  Industry is settling on star topology as most widely used

14 Ethernet  Widely used LAN technology –Invented at Xerox PARC (Palo Alto Research Center) in 1970s –Defined in a standard by Xerox, Intel and Digital - DIX standard –Standard now managed by IEEE - defines formats, voltages, cable lengths,...  Uses bus topology –Single coax cable - the ether –Multiple computers connect to the ether  One Ethernet cable is sometimes called a segment –Limited to 500 meters in length –Minimum separation between connections is 3 meters  Speed –Originally 3Mbps (Obsolete) –Popular standard is 10Mbps (Classic Ethernet) –Fast Ethernet operates at 100Mbps –Now Gigabit and 10 Gigabit

15 Ethernet Operation  One station transmits at a time  Signal propagates across entire cable  All stations receive transmission

16 Manchester Encoding  Ethernet standard uses Manchester encoding  Uses rising and falling edges to encode data –Edge triggered hardware  Falling edge to encode 0, rising edge to encode 1  Use preamble for synchronization  Preamble consists of 64 alternating 1’s and 0’s

17 Characteristics of Good Encoding Scheme  Self synchronization –Digital signal includes information about bit boundaries –Transitions at the beginning, middle or end of the signal are used –A signal with the same voltage level for a long period of time is bad  No DC Component –DC Component: Useless extra energy residing on the line –Signal with only positive voltage (unipolar) has a DC component –Signal with positive and negative voltages (polar) reduce the DC component  Bit rate versus pulse rate (baud rate)  The first two (Self-Synch and No DC Comp) are provided by Manchester encoding

18 Manchester Encoding Efficiency  Classical Ethernet uses Manchester encoding –Up to two signal transitions per-bit –1 Gbps Requires baud rate of 2 G –Waste of bandwidth  Newer Faster networks use a more efficient block encoding schemes (e.g. 4b/5b encoding for fast Ethernet and 8b/10b encoding for Gb Ethernet)

19 Block Coding Schemes

20 Careful Selection of Valid Codes  Consider 4b/5b coding  Select a subset of the 5-bit codes such that no more than three consecutive zeros are sent.  Use a coding scheme that does not waste bandwidth (unlike Manchester Encoding)  Advantages –Synchronization »No same voltage level for long period of time –Error detection »Errors may generate a non- valid 5 bit code –Higher bit rate (than Manchester)

21 CSMA/CD  No central control managing when computers transmit on ether  Ethernet employs CSMA to coordinate transmission among multiple attached computers  Carrier Sense with Multiple Access –Multiple access »multiple computers are attached to shared media »each uses same access algorithm –Carrier sense »computer wanting to transmit tests the media for carrier before transmitting  Simultaneous transmission possible  Collision

22 Carrier-Sensing Effects 10 Mbps Propagation delay

23 CSMA/CD (cont’d)  Even with CSMA, two computers may transmit simultaneously –Both check ether at same time, find it idle, and begin transmitting –Window for transmission depends on speed of propagation in ether  Signals from two computers will interfere with each other –Overlapping frames is called a collision –Data from both frames is garbled  Ethernet uses CSMA + Collision Detection (CD) to coordinate transmission –Ethernet interfaces include hardware to detect transmission »Monitor outgoing signal »Garbled signal is interpreted as a collision –Listen to medium during transmission –Detect whether another station’s signal interferes –Back off from interference (wait random amount of time) and try again

24 Recovery from Collision  Computer that detects a collision sends special signal to force all other interfaces to detect collision  Computer then waits for ether to be idle before transmitting –If both computers wait same length of time, frames will collide again –Standard specifies maximum delay, and both computers choose random delay less than maximum  After waiting, computers use carrier sense to avoid subsequent collision

25 Exponential Backoff  Even with random delays, collisions may occur  Especially likely with busy segments  Computers double delay with each subsequent collision  Reduces likelihood of sequence of collisions  Also called binary backoff

26 CSMA/CD Algorithm Wait while Medium is busy to small value Double

27 Wireless LAN  Use radio signals at 900 MHz  Data rate of 2 Mbps  Shared medium - radio instead of coax  In contrast with wired LAN, not all participants may be able to reach each other –Low signal strength –Propagation blocked by walls, etc.  Can't depend on CD; not all participants may hear

28 CSMA/CA  CSMA: Sense before sending. Only send if idle for IFS (Inter-Frame Space). O/W backoff.  Wireless uses collision avoidance rather than collision detection –Transmitting computer sends very short message to receiver –Receiver responds with short message reserving slot for transmitter  Response from receiver is broadcast so all potential transmitters receive reservation

29 Handling Collisions  Receiver may receive simultaneous requests –Results in collision at receiver –Both requests are lost –Neither transmitter receives reservation; both use backoff and retry  Receiver may receive closely spaced requests –Selects one –Selected transmitter sends message –Transmitter not selected uses backoff and retries

30 Physical and Logical Ring Synchronization

31 Token Ring  Many LAN technologies that use ring topology use token passing for synchronized access to the ring  Ring itself is treated as a single, shared communication medium  Bits pass from transmitter, past other computers and are copied by destination  Hardware must be designed to pass token even if attached computer is powered down

32 Token Passing Synchronization  Used with ring topology  Guarantees fair access: IEEE standards  Token: Special small (a few bits) reserved (can not appear in data) message  Only computer holding the token can transmit –Because there is only one token, only one computer will transmit at a time –Hardware must regenerate token if lost  Token gives computer permission to send one frame –If all ready to transmit, enforces ``round-robin'' access –If none ready to transmit, token circulates around ring

33 Token Ring Transmission  Station waits for token before sending  Signal travels around entire ring  Sender receives its own transmission

34 Token Release Mechanisms  Release After Reception (RAR): –Each station reissues the free token only after it receives the transmitted frame. –Used on lower speed token rings ( <= 4Mbps).  Release After Transmission (RAT): –Each station attaches a free token at the end of its frame –Possible multiple frames propagate in a ring –Used on higher speed token rings (>= 16Mbps)

35 Token Passing Ring Technologies  IBM token ring –Very widely used –Originally 4mbps, now 16Mbps  Fiber Distributed Data Interface (FDDI) –Operates at 100 Mbps

36 FDDI  Fiber Distributed Data Interconnect (FDDI) –Uses ring token passing synchronization (RAT token release) –Uses fiber as transmission media –Transmits data at 100Mbps –Also suitable for MAN –Can attach 1000 stations, can be up to 200 km –Uses pairs of fibers to form two concentric rings  FDDI uses counter-rotating rings in which data flows in opposite directions ==> Reliability

37  In case of fiber or station failure, remaining stations loop back and reroute data through the spare ring  All stations automatically configure loop back by monitoring the data ring FDDI Self-healing

38 ATM  Asynchronous Transfer Mode technology consists of electronic packet switches to which computers can connect  ATM switches form hub into which computers connect in a star topology  Computers get point-to-point connections - data from transmitter is routed directly through hub switches to destination  Transmits data at over 100Mbps  Uses fiber optics to connect computer to switch  Each connection includes two fibers

39 ATM Switches

40 IEEE 802.x LAN Standards  10 Mbps CSMA/CD (802.3)  100 Mbps CSMA/CD (802.3u)  1000 Mbps CSMA/CD (802.3z)  Token Bus (802.4)  Token Ring (802.5)  Wireless LAN (IEEE )  And many more……

41 Summary  Local Area Networks –Designed for short distance –Use shared media »Transmitting computer has exclusive use of communication medium »computers must synchronize transmission –Many technologies exist »Ethernet, Wireless, IBM Token Ring, FDDI, ATM  Topology refers to general shape –Bus –Ring –Star  Ethernet –CSMA/CD –Manchester encoding –Exponential Binary backoff  Synchronization by token passing in a ring –IBM token ring and FDDI