Lecture 2: Oct. 25, 2009

Physical Media

   physical link: transmitted data bit propagates across link guided media:  signals propagate in solid media: copper, fiber unguided media:  signals propagate freely, e.g., radio Twisted Pair (TP)  two insulated copper wires   Category 3: traditional phone wires, 10 Mbps ethernet Category 5 TP: 100Mbps ethernet 1

Lecture 2: Oct. 25, 2009

Physical Media: coax, fiber Coaxial cable:

   wire (signal carrier) within a wire (shield)   baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet

Fiber optic cable:

   glass fiber carrying light pulses high-speed operation:   100Mbps Ethernet high-speed point-to-point transmission (eg, 40 Gps) very low error rate 2

Lecture 2: Oct. 25, 2009

Physical media: Wireless

    signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects:    reflection obstruction by objects interference

Wireless link types:

   microwave  e.g. up to 45 Mbps channels LAN  (e.g., 802.11b/g) 11/54 Mbps wide-area   (e.g., cellular) e.g. CDPD, 10’s Kbps 3G ~ 2.4 Mbps  satellite  up to 50Mbps channel • multiple smaller channels   270 Msec end-end delay geosynchronous versus LEOS (low earth orbit) 3

Lecture 2: Oct. 25, 2009

The Data Link Layer

Our goals:   understand principles behind data link layer services:    error detection, correction sharing a broadcast channel: multiple access link layer addressing instantiation and implementation of various link layer technologies Overview:     link layer services error detection, correction multiple access protocols and LANs link layer addressing  specific link layer technologies:  Ethernet 4

Lecture 2: Oct. 25, 2009

Link Layer: setting the context

5

Lecture 2: Oct. 25, 2009 Recap: The Hourglass Architecture of the Internet Telnet Email FTP WWW TCP UDP IP Ethernet Wireless FDDI 6 6

Lecture 2: Oct. 25, 2009

Link Layer: setting the context

  two

physically connected

 devices: host-router, router-router, host-host unit of data:

frame

H l H n H t H t H n H t M M M M application transport network link physical data link protocol phys. link adapter card network link physical H l H n H t frame M 7

Lecture 2: Oct. 25, 2009

Link layer: Context

 Data-link layer has responsibility of transferring datagram from one node to another node over a link  Datagram transferred by different link protocols over different links, e.g.,  Ethernet on first link,   frame relay on intermediate links 802.11 on last link

transportation analogy

 trip from New Haven to San Francisco  taxi: home to union station    train: union station to JFK plane: JFK to San Francisco airport shuttle: airport to hotel 8

Lecture 2: Oct. 25, 2009

Link Layer Services

  Framing, link access:    encapsulate datagram into frame, adding header, trailer implement channel access if shared medium, ‘physical addresses’ used in frame headers to identify source, destination • different from IP address!

Reliable delivery between two physically connected devices:   seldom used on low bit error link • E.g., fiber, twisted pair wireless links: high error rates • Q: why both link-level and end-end reliability?

9

Lecture 2: Oct. 25, 2009

Link Layer Services (more)

   Flow Control:  pacing between sender and receivers Error Detection:   errors caused by signal attenuation, noise. receiver detects presence of errors: • signals sender for retransmission or drops frame Error Correction:  receiver identifies

and corrects

bit error(s) without resorting to retransmission 10

Lecture 2: Oct. 25, 2009

Adaptors Communicating

datagram link layer protocol sending node frame adapter   link layer implemented in “adaptor” (aka NIC)  Ethernet card, modem, 802.11 card adapter is semi autonomous, implementing link & physical layers receiving node frame adapter   sending side:   encapsulates datagram in a frame adds error checking bits, rdt , flow control, etc.

receiving side  looks for errors, rdt , flow control, etc  extracts datagram, passes to receiving node 11

Lecture 2: Oct. 25, 2009

Link Layer: Implementation

 implemented in “adapter”   e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus interface, and link interface H l H n H t H t H n H t M M M M application transport network link physical data link protocol phys. link adapter card network link physical H l H n H t frame M 12

Lecture 2: Oct. 25, 2009

Error Detection

EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! Q: why?

• protocol may miss some errors, but rarely • larger EDC field yields better detection and correction 13

Lecture 2: Oct. 25, 2009

Parity Checking

Single Bit Parity:

Detect single bit errors

Two Dimensional Bit Parity:

Detect and correct single bit errors Parity bit=1 iff Number of 1’s even

0 0 14

Lecture 2: Oct. 25, 2009

Internet checksum

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only) Sender:    treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver:   compute checksum of received segment check if computed checksum equals checksum field value:   NO - error detected YES - no error detected.

But maybe errors nonetheless?

15

Lecture 2: Oct. 25, 2009

Checksumming: Cyclic Redundancy Check

    view data bits, D , as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R , such that  exactly divisible by G (modulo 2)   receiver knows G, divides by G. If non-zero remainder: error detected!

can detect all burst errors less than r+1 bits widely used in practice (ATM, HDCL) 16

CRC Example

Want: D

equivalently:

D .

.

2 2 r r XOR R = nG = nG XOR R

equivalently:

if we divide D .

2 r by G, want reminder R R D .

2 r G Lecture 2: Oct. 25, 2009 17

Lecture 2: Oct. 25, 2009

Example G(x)

 16 bits CRC:   CRC-16: x 16 +x 15 +x 2 +1, CRC-CCITT: x 16 +x 12 +x 5 +1 both can catch • all single or double bit errors • all odd number of bit errors • all burst errors of length 16 or less • >99.99% of the 17 or 18 bits burst errors CRC-CCITT hardware implementation Using shift and XOR registers http://en.wikipedia.org/wiki/CRC-32#Implementation 18

Lecture 2: Oct. 25, 2009

Multiple Access Links and Protocols

Three types of “links”:   point-to-point (single wire, e.g. PPP, SLIP) broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.)  switched (e.g., switched Ethernet, ATM etc) 19

Multiple Access protocols

Lecture 2: Oct. 25, 2009    single shared communication channel two or more simultaneous transmissions by nodes: interference  only one node can send successfully at a time

multiple access protocol:

   distributed algorithm that determines how stations share channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols: • synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance 20

Multiple Access protocols

Lecture 2: Oct. 25, 2009   claim: humans use multiple access protocols all the time class can "guess" multiple access protocols     multiaccess protocol 1: multiaccess protocol 2: multiaccess protocol 3: multiaccess protocol 4: 21

Lecture 2: Oct. 25, 2009

MAC Protocols: a taxonomy

Three broad classes:  Channel Partitioning   divide channel into smaller “pieces” (time slots, frequency) allocate piece to node for exclusive use   Random Access  allow collisions  “recover” from collisions

“Taking turns”

 tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized 22

MAC Protocols: Measures

     Channel Rate = R bps

Efficient:

 Single user: Throughput R

Fairness

 N users  Min. user throughput R/N

Decentralized

 Fault tolerance

Simple

Lecture 2: Oct. 25, 2009 23

Lecture 2: Oct. 25, 2009

Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access

    access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 24

Lecture 2: Oct. 25, 2009

Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access

    channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle 25

Lecture 2: Oct. 25, 2009

TDMA & FDMA: Performance

     Channel Rate = R bps

Single user

 Throughput R/N

Fairness

 Each user gets the same allocation  Depends on maximum number of users

Decentralized

 Requires resource division

Simple

26

Lecture 2: Oct. 25, 2009

Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access)       unique “code” assigned to each user; ie, code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data

encoded signal decoding:

sequence = (original data) X (chipping sequence) inner-product of encoded signal and chipping allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are almost “orthogonal”) 27

Lecture 2: Oct. 25, 2009

CDMA - Basics

    Orthonormal codes:   =0 i≠j =1 Encoding at user i:   Bit 1 send +c i Bit 0 send -c i Decoding (at user i):     Receive a vector r i Compute t= If t=1 THEN bit=1 If t=-1 THEN bit=0 Q: is there a benefit with orthogonal codes?

In practice use “almost orthogonal”

Correctness of decoding

  Single user Multiple users • Assume additive channel.

• R = c 1 – c 2 • Output = + <-c 2 ,c 1 > = 1 + 0 = 1 28

CDMA Encode/Decode

Lecture 2: Oct. 25, 2009 29

Lecture 2: Oct. 25, 2009

CDMA: two-sender interference

30

Random Access protocols

Lecture 2: Oct. 25, 2009     When node has packet 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:   slotted ALOHA ALOHA  CSMA and CSMA/CD 31

Lecture 2: Oct. 25, 2009

Slotted Aloha [Norm Abramson]

   time is divided into equal size slots (= pkt trans. time) node with new arriving pkt: transmit at beginning of next slot if collision: retransmit pkt in future slots with probability p, until successful.

Success (S), Collision (C), Empty (E) slots 32

Lecture 2: Oct. 25, 2009

Slotted Aloha efficiency Q:

A: what is max fraction slots successful?

Suppose N stations have packets to send  each transmits in slot with probability p  prob. successful transmission S is: by single node: S= p (1-p)

(N-1)

by any of N nodes S = Prob (only one transmits)

= N p (1-p)

(N-1)

… choosing optimum p =1/N as N -> infinity ...

S≈ 1/e = .37 as N -> infinity At best:

of time!

channel use for useful transmissions 37% 33

Goodput vs. Offered Load

Lecture 2: Oct. 25, 2009 Slotted Aloha    0.5

1.0

1.5

2.0

G = offered load = Np when pN < 1, as p (or N) increases  probability of empty slots reduces probability of collision is still low, thus goodput increases  when pN > 1, as p (or N) increases,   probability of empty slots does not reduce much, but probability of collision increases, thus goodput decreases goodput is optimal when pN = 1 34

Maximum Efficiency vs. n

Lecture 2: Oct. 25, 2009 0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0 2 1/e = 0.37 7 12

At best:

of time!

channel use for useful transmissions 37% 17

n

35

Lecture 2: Oct. 25, 2009

Pure (unslotted) ALOHA

   unslotted Aloha: simpler, no synchronization pkt needs transmission:  send without awaiting for beginning of slot collision probability increases:  pkt sent at t 0 collide with other pkts sent in [t 0 -1, t 0 +1] 36

Lecture 2: Oct. 25, 2009

Pure Aloha (cont.)

P(success by given node) = P(node transmits) .

P(no other node transmits in [t 0 -1,t 0 ] .

P(no other node transmits in [t 0 ,t 0 +1] = p . (1-p) N-1 . (1-p) N-1 P(success by any of N nodes) = N p . (1-p) N-1 . (1-p) N-1 … choosing optimum p=1/(2N-1) 0.4

as N -> infty ... S≈ 1/(2e) = .18 0.3

0.2

0.1

Slotted Aloha

protocol

constrains effective channel throughput!

Pure Aloha 0.5

1.0

1.5

2.0

G = offered load = Np 37

Lecture 2: Oct. 25, 2009

Aloha: Performance

     Channel Rate = R bps

Single user

 Throughput R !

Fairness

 Multiple users  Combined throughput only 0.37*R

Decentralized

 Slotted needs slot synchronization

Simple

38

Lecture 2: Oct. 25, 2009

CSMA: Carrier Sense Multiple Access

CSMA:  listen before transmit: If channel sensed idle: transmit entire pkt   If channel sensed busy, defer transmission  Persistent CSMA: retry immediately with probability p when channel becomes idle  Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!

39

Lecture 2: Oct. 25, 2009

CSMA collisions

collisions can occur: propagation delay means two nodes may not yet hear each other’s transmission collision: entire packet transmission time wasted note: role of distance and propagation delay in determining collision prob.

spatial layout of nodes along Ethernet 40

t 0 Lecture 2: Oct. 25, 2009

CSMA/CD: Collision Detection

spatial layout of nodes along Ethernet A B C D spatial layout of nodes along Ethernet A B C D t 0 B detects collision, aborts D detects collision, aborts instead of wasting the whole packet transmission time, abort after detection.

41

Lecture 2: Oct. 25, 2009

CSMA/CD (Collision Detection) CSMA/CD:

 carrier sensing, deferral as in CSMA collisions detected within short time   colliding transmissions aborted, reducing channel wastage persistent or non-persistent retransmission   collision detection:  easy in wired LANs: measure signal strengths, compare transmitted, received signals  difficult in wireless LANs: receiver shut off while transmitting human analogy: the polite conversationalist 42

Lecture 2: Oct. 25, 2009

CSMA/CD collision detection

43

Lecture 2: Oct. 25, 2009

Efficiency of CSMA/CD

 Given collision detection, instead of wasting the whole packet transmission time (a slot), we waste only the time needed to detect collision.

P: packet size, e.g. 1000 bits C: link capacity, e.g. 10Mbps P/C   Use a contention slot of 2 T , where T is one-way propagation delay (why 2 T ?) When the transmission probability p is approximately optimal (p = 1/N), we try approximately e times before each successful transmission 44

Lecture 2: Oct. 25, 2009

Efficiency of CSMA/CD

 The efficiency (the percentage of useful time) is approximately

P C P C



e

2

T

 1  1 5

P T C

 1 1  5

a

, where

a



TC P

 The value of

a

plays a fundamental role in the efficiency of CSMA/CD protocols.

 Question: you want to increase the capacity of a link layer technology (e.g., , 10 Mbps Ethernet to 100 Mbps, but still want to maintain the same efficiency, what do you do?

45

CDMA/CD

     Channel Rate = R bps

Single user

 Throughput R

Fairness

  Multiple users Depends on Detection Time

Decentralized

 Completely

Simple

 Needs collision detection hardware Lecture 2: Oct. 25, 2009 46

Lecture 2: Oct. 25, 2009

“Taking Turns” MAC protocols

channel partitioning MAC protocols:  share channel efficiently at high load  inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols  efficient at low load: single node can fully utilize channel  high load: collision overhead “taking turns” protocols look for best of both worlds!

47

Lecture 2: Oct. 25, 2009

“Taking Turns” MAC protocols

Polling:   master node “invites” slave nodes to transmit in turn Request to Send, Clear to Send msgs  concerns:  polling overhead   latency single point of failure (master) Token passing:  control token passed from one node to next sequentially.

  token message concerns:   token overhead latency  single point of failure (token) 48

Lecture 2: Oct. 25, 2009

Reservation-based protocols

Distributed Polling:   time divided into slots begins with N short reservation slots  reservation slot time equal to channel end-end propagation delay    station with message to send posts reservation reservation seen by all stations after reservation slots, message transmissions ordered by known priority 49

Lecture 2: Oct. 25, 2009

Summary of MAC protocols

 What do you do with a shared media?

   Channel Partitioning, by time, frequency or code • Time Division,Code Division, Frequency Division Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technologies (wire), hard in others (wireless) • CSMA/CD used in Ethernet Taking Turns • polling from a central cite, token passing • Popular in cellular 3G/4G networks where base station is the master 50

Lecture 2: Oct. 25, 2009

LAN technologies

Data link layer so far:  services, error detection/correction, multiple access Next: LAN technologies       addressing Ethernet hubs, bridges, switches 802.11

PPP ATM 51

Lecture 2: Oct. 25, 2009

LAN Addresses 32-bit IP address:

  network-layer address used to get datagram to destination network

LAN (or MAC or physical) address:

  used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM 52

LAN Addresses

Each adapter on LAN has unique LAN address Lecture 2: Oct. 25, 2009 53

Lecture 2: Oct. 25, 2009

LAN Address (more)

      MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) Analogy: (a) MAC address: like ID number תוהז תדועת (b) IP address: like postal address םירוגמ תבותכ MAC flat address => portability  can move LAN card from one LAN to another IP hierarchical address NOT portable  depends on network to which one attaches ARP protocol translates IP address to MAC address 54

Lecture 2: Oct. 25, 2009 Comparison of IP address and MAC Address  IP address is hierarchical for routing scalability  IP address needs to be globally unique (if no NAT)  IP address depends on IP network to which an interface is attached  NOT portable  MAC address is flat  MAC address does not need to be globally unique, but the current assignment ensures uniqueness  MAC address is assigned to a device  portable 55

Lecture 2: Oct. 25, 2009

ARP: Address Resolution Protocol

  Each IP node (Host, Router) on LAN has ARP table ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL>  TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) [yry3@cicada yry3]$ /sbin/arp Address HWtype HWaddress Flags Mask Iface zoo-gatew.cs.yale.edu ether AA:00:04:00:20:D4 C eth0 artemis.zoo.cs.yale.edu ether 00:06:5B:3F:6E:21 C eth0 lab.zoo.cs.yale.edu ether 00:B0:D0:F3:C7:A5 C eth0 Try /proc/net/arp 56

Lecture 2: Oct. 25, 2009

ARP Protocol

 ARP is “plug-and-play”:  nodes create their ARP tables without intervention from net administrator  A

broadcast

 protocol: A broadcasts query frame, containing queried IP address • all machines on LAN receive ARP query  destination D receives ARP frame, replies • frame sent to A’s MAC address (unicast) 57

Lecture 2: Oct. 25, 2009

Ethernet

“dominant” LAN technology:  cheap $5-10 for 10/100/1000 Mbs!

   first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 1, 10, 100, 1000 Mbps Metcalfe’s Etheret sketch 58

Lecture 2: Oct. 25, 2009

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble:  7 bytes with pattern 10101010 followed by one byte with pattern 10101011  used to synchronize receiver, sender clock rates 59

Lecture 2: Oct. 25, 2009

Ethernet Frame Structure (more)

   Addresses: adapters on a LAN and dropped if address does not match 6 bytes, frame is received by all Type:  indicates the higher layer protocol mostly IP but others may be supported (such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped 60

Ethernet: uses CSMA/CD

Lecture 2: Oct. 25, 2009 A: sense channel, if idle

then

{ transmit and monitor the channel;

If

detect another transmission

then

{ abort and send jam signal ;

else

} update # collisions; delay as required by exponential backoff goto A algorithm; {done with the frame; set collisions to zero}

else

} {wait until ongoing transmission is over and goto A } 61

Ethernet’s CSMA/CD (more)

Lecture 2: Oct. 25, 2009 Jam Signal: make sure all other transmitters are aware of collision; 48 bits; 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 n-th collision: choose K from {0,1,…, 2 n -1} after ten or more collisions, choose K from {0,1,2,3,4,…,1023} 62

Lecture 2: Oct. 25, 2009

Exponential Backoff (simplified)

       N users Interval of size 2 n Prob Node/slot is 1/2 n Prob of success N(1/2 n )(1 – 1/2 n ) N-1 Average slot success N(1 – 1/2 n ) N-1 Intervals size: 1, 2, 4, 8, 16 … Fraction (out of N) of success:    2 n 2 n 2 n = N/8 -> 0.03 % 2 n = N/2 -> 15% 2 n = 2N -> 60% = N/4 -> 2% = N -> 37 % 63

Lecture 2: Oct. 25, 2009

Ethernet Technologies: 10Base2

  10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology   repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

64

Lecture 2: Oct. 25, 2009

10BaseT and 100BaseT

    10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus “star topology” (multi-port repeater) CSMA/CD implemented at hub 65

Lecture 2: Oct. 25, 2009

10BaseT and 100BaseT (more)

   Max distance from node to Hub is 100 meters Hub can disconnect “jabbering” adapter Hub can gather monitoring information, statistics for display to LAN administrators 66

Lecture 2: Oct. 25, 2009

Gbit Ethernet

     use standard Ethernet frame format allows for point-to-point links and shared broadcast channels in shared mode, CSMA/CD is used; short distances between nodes to be efficient uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links  Wide area networks 67

Token Rings (IEEE 802.5)

Lecture 2: Oct. 25, 2009  

A ring topology is a single unidirectional loop connecting a series of stations in sequence Each bit is stored and forwarded by each station’s network interface

68

Lecture 2: Oct. 25, 2009

Token Ring: IEEE802.5 standard

  4 Mbps (also 16 Mbps) max token holding time: 10 ms, limiting frame length   SD, ED mark start, end of packet AC:  access control byte: token bit: value 0 means token can be seized, value 1 means data follows FC   priority bits: priority of packet reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free 69

Lecture 2: Oct. 25, 2009

Token Ring: IEEE802.5 standard

     FC: frame control used for monitoring and maintenance source, destination address: address, as in Ethernet 48 bit physical data: packet from network layer checksum: CRC FS:  frame status: set by dest., read by sender set to indicate destination up, frame copied OK from ring  DLC-level ACKing 70