CSE524: Lecture 15

1
CSE524 Lecture 15

• Data-link layer
• Functions

2
Administrative

• No office hours next Wednesday
• Demos 12/9
• See web site for available slots
• Sign up for a slot after class or via e-mail
• Read e-mail message for instructions on what to
  submit before demo

3
Where were at

• Internet architecture and history
• Internet protocols in practice
• Application layer
• Transport layer
• Network layer
• Data-link layer
• Functions
• Specific link layer examples and devices
• Physical layer

4
Data-link layer
5
Data-link layer

• Two physically connected devices
• host-router, router-router, host-host,
  host-switch, host-hub
• Implemented on network adapter card
• typically includes RAM, DSP chips, host bus
  interface, and link interface

network link physical
data link protocol
frame
phys. link
adapter card
6
Data-link layer functions

• Moving datagrams between adjacent nodes
• Digital to analog conversion
• Framing
• Physical addressing
• Demux to upper protocol
• Error detection and correction
• Flow control
• Reliable delivery
• Security
• Media access and quality of service

7
DL Digital to analog conversion

• Bits sent as analog signals
• Photonic pulses of a given wavelength over
  optical fiber
• Electronic signals of a given voltage

8
DL Digital to analog conversion

• Will cover electronic transmission (optical
  transmission left for you to research)
• Biggest issue
• When to sample voltage?
• Detecting sequences involves clocking with the
  same clock
• How to synchronize sender and receiver clocks?
• Need easily detectible event at both ends
• Signal transitions help resync sender and
  receiver
• Need frequent transitions to prevent clock skew
• http//www.mouse.demon.nl/ckp/telco/encode.htm

9
DL RZ

• Return to Zero (RZ)
• 1pulse to high, dropping back to low
• 0no transition

10
DL NRZ-L

• Non-Return to Zero Level (NRZ-L)
• 1high signal, 0lower signal
• Long sequence of same bit causes difficulty
• DC bias hard to detect low and high detected by
  difference from average voltage
• Clock recovery difficult
• Used by Synchronous Optical Network (SONET)
• SONET XORs bit sequence to ensure frequent
  transitions
• Used in early magnetic tape storage

11
DL NRZ-L
12
DL NRZ-M

• Non-Return to Zero Mark
• Less power to transmit versus NRZ
• 1signal transition at start of bit, 0no change
• No problem with string of 1s
• NRZ-like problem with string of 0s
• Used in SDLC (Synchronous Data Link Control)
• Used in modern magnetic tape storage

13
DL NRZ-S

• Non-Return to Zero Space
• 1no change, 0signal transition at start of bit
• No problem with string of 0s
• NRZ-like problem with string of 1s

14
DL Manchester (Bi-Phase-Level) coding

• Used by Ethernet
• 0low to high transition, 1high to low
  transition
• Transition for every bit simplifies clock
  recovery
• Not very efficient
• Doubles the number of transitions
• Circuitry must run twice as fast

15
DL Manchester coding

• Encoding for 110100

Bit stream
1
1
0
1
0
0
Manchester encoding
16
DL Other coding schemes

• Bi-Phase-Mark, Bi-Phase-Space
• Level change at every bit period boundary
• Mid-period transition determines bit
• Bi-Phase-M 0no change, 1signal transition
• Bi-Phase-S 0signal transition, 1no change

17
DL Other coding schemes

• Differential Bi-Phase-Space, Differential
  Bi-Phase-Mark
• Level change at every mid-bit period boundary
• Bit period boundary transition determines bit
• Diff-Bi-Phase-M 0signal transition, 1no change
• Diff-Bi-Phase-S 0no change, 1signal transition

18
DL Framing

• Data encapsulation for transmission over physical
  link
• Data embedded within a link-layer frame before
  transmission
• Data-link header and/or trailer added
• Physical addresses used in frame headers to
  identify source and destination (not IP)

19
DL Fixed length framing

• Length delimited
• Beginning of frame has length
• Single corrupt length can cause problems
• Must have start of frame character to
  resynchronize
• Resynchronization can fail if start of frame
  character is inside packets as well

20
DL Variable length framing

• Byte stuffing
• Special start of frame byte (e.g. 0xFF)
• Special escape byte value (e.g. 0xFE)
• Values actually in text are replaced (e.g. 0xFF
  by 0xFEFF and 0xFE by 0xFEFE)
• Worst case can double the size of frame
• Bit stuffing
• Special bit sequence (0x01111110)
• 0 bit stuffed after any 11111 sequence

21
DL Clock-Based Framing

• Used by SONET
• Fixed size frames (810 bytes)
• Look for start of frame marker that appears every
  810 bytes
• Will eventually sync up

22
DL Physical addressing

• LAN (or MAC or physical) address
• Used to get datagram from one interface to
  another physically-connected interface (same
  network)
• IP address used to route between networks
• 48 bit MAC address (for most LANs) burned in
  adapter ROM
• ifconfig a
• arp -a
• Address space assigned and managed by IEEE
• Manufacturer buys portion of MAC address space to
  ensure uniqueness
• Special LAN broadcast address
• FF-FF-FF-FF-FF-FF

23
DL Physical addressing

• Why have separate IP and hardware addresses?

• Assign adapters an IP address
• Hardware only works for IP (no IPX, DECNET)
• Must be reconfigured when moved
• Use hardware address as network address
• Need standardized fixed length hardware address
• No route aggregation

24
DL Physical addressing

• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address
• MAC flat address gt portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on network to which one attaches

25
DL ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP
  module, table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• lt .. gt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

26
DL ARP protocol

• A knows B's IP address, wants to learn physical
  address of B
• A broadcasts ARP query pkt, containing B's IP
  address
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) physical layer address
• A caches (saves) IP-to-physical address pairs
  until information becomes old (times out)
• soft state information that times out (goes
  away) unless refreshed

27
DL Routing to another LAN

• walkthrough routing from A to B via R
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
28

• A creates IP packet with source A, destination B
• A uses ARP to get Rs physical layer address for
  111.111.111.110
• A creates Ethernet frame with R's physical
  address as dest, Ethernet frame contains A-to-B
  IP datagram
• As data link layer sends Ethernet frame
• Rs data link layer receives Ethernet frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs physical layer address
• R creates frame containing A-to-B IP datagram
  sends to B

A
R
B
29
DL RARP, BOOTP, DHCP

• ARP Given an IP address, return a hardware
  address
• RARP Given a hardware address, give me the IP
  address
• DHCP, BOOTP Similar to RARP
• Hosts (host portion)
• hard-coded by system admin in a file
• DHCP Dynamic Host Configuration Protocol
  dynamically get address plug-and-play
• host broadcasts DHCP discover msg
• DHCP server responds with DHCP offer msg
• host requests IP address DHCP request msg
• DHCP server sends address DHCP ack msg

30
DL Demux to upper protocol

• Protocol type specification interfaces to network
  layer
• Data-link layer can support any number of network
  layers
• Type field in data-link header specifies network
  layer of packet
• IP is one of many network layers
• Each data-link layer defines its own protocol
  type numbering for network layer

31
DL Demux to upper protocol

• http//www.cavebear.com/CaveBear/Ethernet/type.htm
  l
• Some Ethernet protocol types
• 0800 DOD Internet Protocol (IP)
• 0806 Address Resolution Protocol (ARP)
• 8037 IPX (Novell Netware)
• 80D5 IBM SNA Services
• 809B EtherTalk (AppleTalk over Ethernet)

32
DL Error detection/correction

• Errors caused by signal attenuation, noise.
• Receiver detects presence of errors
• Possible actions
• Signal sender for retransmission
• Drops frame
• Correct bit errors if possible and continue

33
DL Error detection/correction

• EDC Error Detection and Correction bits
  (redundancy)
• D Data protected by error checking, may
  include header fields
• Error detection not 100 reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and
  correction

34
DL Parity checking
Two Dimensional Bit Parity Detect and correct
single bit errors
Single Bit Parity Detect single bit errors
0
0
35
DL Checksums

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• simple to implement, weak detection (easily
  tricked by common bit error patterns)
• used by TCP, UDP, IP..
• sender puts checksum value into header

• Receiver
• compute checksum of received segment
• check if computed checksum equals checksum field
  value
• NO - error detected
• YES - no error detected. But maybe errors
  nonethless? More later .

36
DL Cyclic Redundancy Check (CRC)

• Polynomial code
• Treat packet bits a coefficients of n-bit
  polynomial
• Choose r1 bit generator polynomial (well known
  chosen in advance)
• Add r bits to packet such that message is
  divisible by generator polynomial
• Better loss detection properties than checksums
• All single bit errors, all double bit errors, all
  odd-numbered errors, burst errors less than r

37
DL Cyclic Redundancy Check (CRC)

• Calculate code using modulo 2 division of data
  by generator polynomial
• Subtraction equivalent to XOR
• Weak definition of magnitude
• X gt Y iff position of highest 1 bit of X is the
  same or greater than the highest 1 bit of Y
• Record remainder after division and send after
  data
• Result divisible by generator polynomial

38
DL Cyclic Redundancy Check (CRC)
39
DL CRC example

• Data
• 101110
• Generator Polynomial
• x3 1 (1001)
• Send
• 101110011

40
DL CRC example

• Data
• 10000
• Generator Polynomial
• x2 1 (101)
• Send
• 1000001

G
10101
101 1000000 101 010
000 100 101
010 000 100
101 01
D
R
41
DL Cyclic Redundancy Check (CRC)

• CRC-16 implementation
• Shift register and XOR gates

42
DL CRC polynomials

• CRC-16 x16 x15 x2 1 (used in HDLC)
• CRC-CCITT x16 x12 x5 1
• CRC-32 x32 x26 x23 x22 x16 x12 x11
  x10 x8 x7 x5 x4 x2 x 1 (used in
  Ethernet)

43
DL Forward error correction

• FEC
• Use error correcting codes to repair losses
• Add redundant information which allows receiver
  to correct bit errors
• Suggest looking at information and coding theory
  work.

44
DL Flow control

• Pacing between sender and receiver
• Sender prevented from overrunning receiver
• Ready-To-Send, Clear-To-Send

45
DL Reliable delivery

• Reliability at the link layer
• Handled in a similar manner to transport
  protocols
• When and why should this be used?

• Rarely done over twisted-pair or fiber optic
  links
• Usually done over lossy links for performance
  improvement (versus correctness)

46
DL Security

• Mainly for broadcast data-link layers
• Encrypt payload of higher layers
• Hide IP source/destination from eavesdroppers
• Important for wireless LANs especially
• Parking lot attacks
• 802.11b and WEP
• If time permits, security will be covered at the
  end of the course.

47
DL Media access and quality of service

• How and when can a node transmit?
• Directly controls per-hop quality-of-service
• 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)

48
DL Media access

• Point-to-point link and switched media no problem
• Broadcast links?
• Network arbitration
• Give everyone a fixed time/freq slot?
• Ok for fixed bandwidth (e.g., voice)
• What if traffic is bursty?
• Centralized arbiter
• Ex cell phone base station
• Single point of failure
• Distributed arbitration
• Aloha/Ethernet
• Humans use multiple access protocols all the time

49
DL Media access protocols

• 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 uses 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

50
DL Media access protocols

• 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
51
DL Channel Partitioning MAC protocols

• Synchronous TDMA time division multiple access
• TDM (Time Division Multiplexing) channel divided
  into N time slots, one per user access to
  channel in "rounds"
• each station gets fixed length slot (length pkt
  trans time) in each round
• unused slots go idle
• inefficient with low duty cycle users and at
  light load.
• example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

52
DL Channel Partitioning MAC protocols

• 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

time
frequency bands
53
DL Channel Partitioning MAC protocols

• CDMA (Code Division Multiple Access)
• unique code assigned to each user ie, code set
  partitioning
• used mostly in wireless broadcast channels
  (cellular, satellite,etc)
• each user has own chipping sequence (ie, code)
  to encode data
• encoded signal (original data) X (chipping
  sequence)
• decoding inner-product of encoded signal and
  chipping sequence
• allows multiple users to coexist and transmit
  simultaneously with minimal interference (if
  codes are orthogonal)

54
DL Channel Partitioning MAC protocols

• CDMA Encode/Decode

55
DL Channel Partitioning MAC protocols

• CDMA two sender interference

56
DL Random access protocols

• Asynchronous TDMA
• When node has packet to send
• transmit at full channel data rate R.
• no a priori coordination among nodes
• Two or more transmitting nodes -gt collision
• To avoid deterministic collisions randomize
• Random-access MAC protocols
• specify how to recover from collisions (e.g., via
  delayed retransmissions)
• Examples slotted ALOHA, ALOHA, CSMA, CSMA/CD

57
DL Random access MAC protocols

• Slotted Aloha
• 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
58
DL Random access MAC protocols

• Slotted Aloha efficiency
• Q what is max fraction slots successful?
• A 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 as n -gt infty
  ...
• 1/e .37 as N -gt infty

59
DL Random access MAC protocols

• Pure (unslotted) ALOHA
• Simpler, no synchronization
• Packet needs transmission
• Send without awaiting for beginning of slot
• Collision probability increases
• Packet sent at t0 collides with other packets
  sent in t0-1, t01

60
DL Pure Aloha (cont.)

• P(success by given node) P(node transmits) .
• P(no
  other node transmits in p0-1,p0 .
• P(no
  other node transmits in p0,p01
• 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 as n -gt infty ...
  1/(2e) .18

S throughput goodput (success rate)