1 MAC Protocols & High Speed LANs Lesson 8 NETS2150/2850

2 Lesson Outline  Random access MAC protocols  Ethernet Implementations Ethernet (10 Mbps) Fast Ethernet (100 Mbps) Gigabit Ethernet - GbE (1 Gbps) 10 Gb Ethernet – 10 GbE (10 Gbps)  Round robin MAC protocol Token Ring (10 Mbps & 100 Mbps)

3 Random Access Protocols  When node has frame to send transmit at full channel data rate R no a priori coordination among nodes  two or more transmitting nodes  collision  random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions)  Examples of random access MAC protocols: ALOHA slotted ALOHA CSMA, CSMA/CD

4 ALOHA  Built for packet radio net across Hawaiian islands  When station has frame, it sends immediately  Wait for round trip time (RTT) RTT is time between send of frame and receive of ACK  If receive ACK, fine. If not, retransmit If no ACK after repeated transmissions, give up  Frame may be damaged by noise or by another station transmitting at the same time (collision)  Max utilisation 18%

5 Slotted ALOHA  Time in uniform slots equal to frame transmission time All frames are same fixed size  Need central clock (or other sync mechanism)  Transmission begins at slot boundary  Frames either miss or overlap totally  Max utilisation 37%

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7 Carrier Sense Multiple Access (CSMA)  First listen for clear medium (i.e. carrier sense)  If medium idle, transmit If two stations start at the same instant, collision  Wait reasonable time (RTT plus ACK contention)  No ACK then retransmit  CSMA utilisation >> ALOHA schemes  Three types: nonpersistent, 1-persistent and p- persistent CSMA

8 Nonpersistent CSMA 1.If medium is idle, transmit; otherwise, go to 2 2.If medium is busy, wait for random time and repeat 1  Random delays reduces probability of collisions  However, capacity is wasted because medium will remain idle following end of transmission Even if stations waiting to access

9 1-persistent CSMA  To avoid idle channel time, 1-persistent protocol used  Station wishing to transmit listens and obeys following: 1.If medium idle, transmit; otherwise, go to step 2 2.If medium busy, listen until idle; then transmit immediately (probability 1)  1-persistent stations are greedy  If two or more stations waiting, collision is guaranteed! Gets sorted out after collision

10 p-persistent CSMA  Compromise that attempts to reduce collisions Like nonpersistent  And reduce idle time Like 1-persistent 1.If medium idle, transmit with probability p, and delay one time unit with probability (1 – p) Time unit is typically maximum propagation delay 2.If medium busy, listen until idle and repeat step 1 3.If transmission is delayed one time unit, repeat step 1  What is an effective value of p?

11 Value of p?  n stations waiting to send  At end of a transmission, expected/average number of stations attempting to transmit is: np  If np > 1, higher chance of a collision  Repeated attempts to transmit almost guaranteeing more collisions as retries compete with new transmissions  Eventually, all stations trying to send Continuous collisions  zero throughput  So np < 1 for expected peaks of n  If heavy load expected, p small  However, as p made smaller, stations wait longer At low loads, this gives very long delays

12 CSMA/CD  With CSMA, collision occupies medium for duration of transmission  With CSMA/CD, stations listen whilst transmitting 1.If medium idle, transmit, otherwise, step 2 2.If busy, listen for idle, then transmit 3.If collision detected, stop frame transmission and send jam signal then cease transmission 4.After jam, backoff random time then start from step 1

13 CSMA/CD Operation

14 Which Persistence Algorithm?  IEEE uses CSMA/CD 1-persistent!  Both nonpersistent and p-persistent have performance problems  1-persistent (p = 1) seems more unstable than p- persistent Greed of the stations But wasted time due to collisions is short (if T frame >> T prop ) With random backoff, unlikely to collide on next tries To ensure backoff maintains stability, IEEE and Ethernet use binary exponential backoff

15 Ethernet uses CSMA/CD  adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense  transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection  Before attempting a retransmission, adapter waits a random time, that is, random access

16 Ethernet CSMA/CD algorithm  If adapter detects another transmission while transmitting aborts and sends jam signal  After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2 m -1}  Adapter waits K*512 bit times and returns to Step 1

17 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Bit time: 0.1  s for 10 Mbps Ethernet ; for K=1023, wait time is about 50 ms Binary Exponential Backoff:  Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer  first collision: choose K from {0,1}; delay is K x 512 bit transmission times  after second collision: choose K from {0,1,2,3}…  after ten collisions, choose K from {0,1,2,3,4,…,1023}

18 Example Suppose stations A and B are on the same 10 Mbps Ethernet segment, and the propagation delay between them is 500 bit times. In the worst case, will A be able to detect a collision involving B? Solution Worst case: Min frame size = 512 bits Time for complete bit emission = Time for collision detection = = 999 Since 576 < 999, collision not detected by A! AB 500 bits

19 IEEE Frame Format Ethernet is similar, but length is replaced by type Both has min frame size = 512 bits (64 octets)

20 IEEE Notation for 10 Mbps Ethernet  10Base510Base210Base-T10Base-F MediumThickThinUTP850nm CoaxialCoaxialfibre SignalingBasebandBasebandBaseband Manchester ManchesterManchester On/Off TopologyBusBusStarStar Nodes

21 100Mbps Fast Ethernet  Use same IEEE MAC protocol and frame format  100BASE-TX uses STP or Cat 5 UTP  100BASE-FX uses optical fiber  100BASE-T4 can use Cat 3 UTP 100 Mbps over lower quality cables Uses 4 twisted-pair lines between nodes Data transmission uses three pairs in one direction at a time  Star-wire physical topology Similar to 10BASE-T

22 100Mbps (Fast Ethernet) 100Base-TX100Base-FX100Base-T4 2 pair, STP2 pair, Cat 5 UTP2 optical fibre4 pair, cat 3,4,5 MLT-3MLT-34B5B, NRZI8B6T,NRZ

23 100BASE-T Options

24 Full Duplex Operation  Traditional Ethernet half duplex Either transmit or receive but not both simultaneously  With full-duplex, station can transmit and receive simultaneously 100-Mbps Ethernet in full-duplex mode, theoretical transfer rate 200 Mbps  Must use switches Each station constitutes separate collision domain! In fact, no collisions

25 Gigabit Ethernet - Differences  Same frame format and MAC protocol as before  Carrier extension is used for short frames At least 4096 bit-times long (cf. 512 for 10/100)  T frame > T prop (legacy compatibility)  Frame bursting – allows multiple short frames transmission  1000BaseT is standardised as IEEE 802.3ab

26 Gigabit Ethernet – Physical  1000Base-SX Short wavelength, multimode fibre  1000Base-LX Long wavelength, Multi or single mode fibre  1000Base-CX Copper jumpers < 25m, shielded twisted pair (STP)  1000Base-T 4 pairs of Cat 5 UTP

27 Gigabit Ethernet Medium Options

28 Cisco® High-end Switches

29 Gigabit Ethernet Configuration

30 10 Gigabit Ethernet - Uses  High-speed, local backbone interconnection between large-capacity switches or server farm  Campus wide connectivity  Allows construction of MANs and WANs Connect geographically dispersed LANs between campuses  Ethernet competes with ATM and other WAN technologies  10GbE provides substantial value over ATM  10GBaseT is standardised as IEEE 802.3ae

31 10GbE - Advantages  No expensive, bandwidth-consuming conversion between Ethernet packets and ATM cells  Network is Ethernet, end to end Optimizing operation and cost for LAN, MAN, or WAN  Variety of standard optical and STP interfaces specified for 10 GbE

32 10 GbE Implementations  Maximum link distances cover 300 m to 40 km  10GBASE-S (short): 850 nm on multimode fiber Up to 300 m  10GBASE-L (long) 1310 nm on single-mode fiber Up to 10 km  10GBASE-E (extended) 1550 nm on single-mode fiber Up to 40 km

33 10GbE Distance Options

34 Cisco® 10GbE module  Supports 10GBase-S/L/E/CX  Up to GbE ports  256 MB buffer per port  Up to 400 million frames per sec (mfps)  Supports jumbo frame size (up to 9216 octets)!

35 “Taking Turns” MAC Protocols  Involve a controlled access  No collision!  A station cannot send unless been “authorised”  There are two main types: Polling Token-passing

36 The Polling Scheme  The master/central node “invites” slave nodes to transmit in turn  Main concerns: polling overhead latency single point of failure (master)

37 Token Ring  Developed from IBM's commercial token ring  Because of IBM's large presence, token ring has gained broad acceptance  But, never achieved popularity of Ethernet!  Currently, large installed base of token ring products  Market share likely to decline

38 Ring Operation  Each repeater connects to two others via unidirectional transmission links Single closed path  Data transferred bit by bit from one repeater to the next  Repeater regenerates and retransmits each bit  Frame removed by transmitter after one trip round ring

39 Ring Repeater States

40 IEEE Frame Format Data Frame Token Frame

41 IEEE MAC Protocol- Token Passing  A special frame (i.e. token) circulates continuously  Station waits for the token Changes one bit in token to make it SOF for data frame Append rest of data frame  Frame makes round trip and is absorbed by transmitting station Inserts new token when transmission has finished How long to hold token – token holding time (THT)  Under light loads, some inefficiency  Under heavy loads, round robin

42 Token Ring Operation

43 LAN Performance Comparison Fig

44 Wireless LAN Overview  A wireless LAN uses wireless medium Saves installation of LAN cabling  Eases relocation and other modifications to network structure  Popularity of wireless LANs has grown rapidly  Role for the wireless LAN Manufacturing plants, stock exchange trading floors, warehouses Historical buildings Small offices where wired LANs not economical  IEEE has specified this technology in standard

45 IEEE Wireless LAN  b GHz unlicensed radio spectrum up to 11 Mbps widely deployed, using base stations  a 5-6 GHz range up to 54 Mbps  g GHz range up to 54 Mbps  All use CSMA/CA for MAC protocol  All have infrastructure and ad-hoc network versions

46 Infrastructure Approach  Wireless host communicates with an access point  Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless stations one access point (AP)  BSSs combined to form a distribution system (DS) McGraw-Hill © The McGraw-Hill Companies, Inc., 2004

47 Ad Hoc Approach  No AP!  Wireless stations communicate with each other  Typical usage: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield  IETF MANET (Mobile Ad hoc Networks) working group looks into this approach Special needs such wireless routing, security

48 IEEE : MAC protocol  Collision if 2 or more nodes transmit at same time as the wireless channel is shared  CSMA makes sense: get all the bandwidth if you’re the only one transmitting shouldn’t cause a collision if you sense another transmission  Thus, it uses CSMA with collision avoidance (CSMA/CA) Not CD because detecting collision is difficult in wireless environment Two-handshaking used

49 Summary  Random access protocol CSMA/CD in (Ethernet)  Round Robin Token passing in (Token Ring)  Wireless LAN  Read Stallings chapter 16  Next: Layer-3  Network layer