Direct Link Networks

1
Direct Link Networks
Textbook Computer Networks A Systems Approach,
L. Peterson, B. Davie, Morgan Kaufmann Chapter
2.
2
Direct Links Outline

• Physical Layer
• Link technologies
• Encoding
• Link Layer
• Framing
• Error Detection
• Reliable Transmission (ARQ protocols)
• Medium Access Control
• Existing protocols Ethernet, Token Rings,
  Wireless

3
Link Technologies

• Cables
• Cat 5 twisted pair, 10-100Mbps, 100m
• Thin-net coax, 10-100Mbps, 200m
• Thick-net coax, 10-100Mbps, 500m
• Fiber, 100Mbps-2.4Gbps, 2-40km
• Leased Lines
• Copper based T1 (1.544Mbps), T3 (44.736Mbps)
• Optical fiber STS-1 (51.84Mbps), STS-N
  (N51.84Mbps)
• Last-Mile Links
• POTS (56Kbps), ISDN (264Kbps)
• xDSL ADSL (16-640Kbps, 1.554-8.448Mbps), VDSL
  (12.96Mbps-55.2Mbps)
• CATV 40Mbps downstream, 20Mbps upstream
• Wireless Links Cellular, Satellite, Wireless
  Local Loop

4
Encoding

• Signals propagate over a physical medium
• modulate electromagnetic waves
• e.g., vary voltage
• Encode binary data onto signals
• e.g., 0 as low signal and 1 as high signal
• known as Non-Return to zero (NRZ)

5
Problem Consecutive 1s or 0s

• Low signal (0) may be interpreted as no signal
• High signal (1) leads to baseline wander
• Receiver compares rx_signal to avg_signal
• Clock drift Unable to recover clock

6
Alternative Encodings

• Non-return to Zero Inverted (NRZI)
• make a transition from current signal to encode a
  one stay at current signal to encode a zero
• solves the problem of consecutive ones
• Manchester
• transmit XOR of the NRZ encoded data and the
  clock
• only 50 efficient.

7
Encodings (cont)
8
Encodings (cont)

• 4B/5B
• every 4 bits of data encoded in a 5-bit code
• 5-bit codes selected to have no more than one
  leading 0 and no more than two trailing 0s
• thus, never get more than three consecutive 0s
• resulting 5-bit codes are transmitted using NRZI
• achieves 80 efficiency

9
Framing

• The physical layer provides a mean to transmit a
  sequence of bits
• How can one determine the beginning/end of a
  frame?
• Solutions
• Character-based framing (use special control
  characters)
• Bit-oriented framing with flags
• Length counts
• Clock based

10
Character Based Framing

• BISYNC BInary SYNchronous Communication
• SYN Synchronous idle, SOH Start of Header, STX
  Start of text, ETX End of text
• Problem 1 if control characters appear within
  the header, or CRC.
• These are known locations, one can skip control
  characters in these fields
• Problem 2 if CTRL characters appear in the
  packet.
• Use a Data Link Escape (DLE) character before ETX
  when it appears within the packet
• If DLE appears within the packet replace it with
  DLE DLE.

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Bit Based Framing

• Sentinel-based (Bit oriented)
• delineate frame with special pattern 01111110
• e.g., HDLC, SDLC
• problem special pattern appears in the payload
• solution bit stuffing
• sender insert 0 after five consecutive 1s
• receiver delete 0 that follows five consecutive
  1s
• Disadvantage potentially increases the length by
  20

12
Counter Based Framing

• include payload length in header
• e.g., DDCMP
• problem count field corrupted
• solution catch when CRC fails

13
Clock Based Framing

• SONET Synchronous Optical Network
• Each frame is 125 micro-seconds long 810 bytes
  for STS-1
• STS-n (STS-1 51.84 Mbps)

14
Direct Links Outline

• Physical Layer
• Link technologies
• Encoding
• Link Layer
• Framing
• Error Detection
• Reliable Transmission (ARQ protocols)
• Medium Access Control (MAC)
• Existing protocols Ethernet, Token Rings,
  Wireless

15
Error Detection

• Bits can get corrupted on transmissions
• How to detect, and if possible, correct them
• Error detection techniques
• Parity bit
• Internet checksum
• Cyclic redundancy check (CRC)

16
Parity Bit

• Simplest of all techniques
• Single parity bit
• Add a bit that takes the XOR of all the data bits
• What kind of errors can it detect?
• Two dimensional parity
• Arrange the data in a 2D matrix
• Take the parity along each row and row column
• Send the data with the parity bits
• What kind of errors can it detect?

17
Internet Checksum Algorithm

• View message as a sequence of 16-bit integers
  sum using 16-bit ones-complement arithmetic take
  ones-complement of the result.

u_short cksum(u_short buf, int count)
register u_long sum 0 while (count--)
sum buf
if (sum 0xFFFF0000)
/ carry occurred, so wrap around
/ sum 0xFFFF
sum
return (sum 0xFFFF)
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Cyclic Redundancy Check

• Add k bits of redundant data to an n-bit message
• want k ltlt n
• e.g., k 32 and n 12,000 (1500 bytes)
• Represent n-bit message as n-1 degree polynomial
• e.g., MSG10011010 as M(x) x7 x4 x3 x1
• Let k be the degree of some divisor polynomial
• e.g., C(x) x3 x2 1

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CRC (cont)

• Transmit polynomial P(x) that is evenly divisible
  by C(x)
• shift left k bits, i.e., M(x)xk
• subtract remainder of M(x)xk / C(x) from M(x)xk
• Receiver polynomial P(x) E(x)
• E(x) 0 implies no errors
• Divide (P(x) E(x)) by C(x) remainder zero if
• E(x) was zero (no error), or
• E(x) is exactly divisible by C(x)

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Selecting C(x)

• All single-bit errors, as long as the xk and x0
  terms have non-zero coefficients.
• Any odd number of errors, as long as C(x)
  contains the factor (x 1)
• Any burst error (i.e., sequence of consecutive
  error bits) for which the length of the burst is
  less than k bits.
• Most burst errors of larger than k bits can also
  be detected
• See Table 2.6 on page 102 for common C(x)

21
Direct Links Outline

• Physical Layer
• Link technologies
• Encoding
• Link Layer
• Framing
• Error Detection
• Reliable Transmission (ARQ protocols)
• Medium Access Control (MAC)
• Existing protocols Ethernet, Token Rings,
  Wireless

22
Reliable Transmission ARQ

• Automatic Repeat reQuest (ARQ)
• Underlying physical channel
• Each transmitted frame may be delayed by an
  arbitrary and variable time
• Some frames might be lost and may never arrive
• Assume that error detection works correctly
• Frames that arrive are assumed to do so in order,
  with or without errors (this assumption is not
  always necessary, true for direct links)
• Correctness of ARQ protocol each packet is
  released to the network layer once and only once,
  without error
• Efficiency Use of link bandwidth (effective
  throughput)

23
Acknowledgements Timeouts
24
Stop-and-Wait
Sender
Receiver

• Sender ensures that each frame is received
  correctly before sending the next frame.
• How to distinguish between successive packets?
• Use sequence numbers
• But sequence numbers may grow out of bound?
• A 1-bit sequence number suffices for
  Stop-and-Wait!

25
Problems with Stop-and-Wait

• Keeping the pipe full
• Example
• 1.5Mbps link x 45ms RTT 67.5Kb (8KB)
• 1KB frames implies 1/8th link utilization
• How to keep the pipe full?
• Send more frames without waiting for acks

26
Sliding Window

• Allow multiple outstanding (un-ACKed) frames
• Upper bound on un-ACKed frames, called window

27
Sliding Window Sender

• Assign sequence number to each frame (SeqNum)
• Maintain three state variables
• send window size (SWS)
• last acknowledgment received (LAR)
• last frame sent (LFS)
• Maintain invariant LFS - LAR lt SWS
• Advance LAR when ACK arrives
• Buffer up to SWS frames

28
Sliding Window Receiver

• Maintain three state variables
• receive window size (RWS)
• largest frame acceptable (LFA)
• last frame received in order (LFR)
• Maintain invariant LFA - LFR lt RWS
• Frame SeqNum arrives
• if LFR lt SeqNum lt LFA accept update LFR,
  if necessary
• if SeqNum lt LFR or SeqNum gt LFA
  discarded
• Send cumulative ACKs
• Buffer up to RWS packets

29
Buffer Sizes

• SWS set to bandwidth-delay product estimate
• For RWS, two common settings
• RWS 1 receiver will not buffer any frames that
  arrive out of order
• RWS SWS
• Does not make sense to set RWS gt SWS

30
Sequence Number Space

• SeqNum field is finite sequence numbers wrap
  around
• Sequence number space must be larger then number
  of outstanding frames
• If RWSSWS, SWS lt MaxSeqNum-1 is not sufficient
• suppose 3-bit SeqNum field (0..7)
• SWSRWS7
• sender transmit frames 0..6
• arrive successfully, but ACKs lost
• sender retransmits 0..6
• receiver expecting 7, 0..5, but receives second
  incarnation of 0..5
• SWS lt (MaxSeqNum1)/2 is correct rule
• Intuitively, SeqNum slides between two halves
  of sequence number space

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Concurrent Logical Channels

• Multiplex 8 logical channels over a single link
• Run stop-and-wait on each logical channel
• Maintain three state bits per channel
• channel busy
• current sequence number out
• next sequence number in
• Header 3-bit channel num, 1-bit sequence num
• 4-bits total
• Separates reliability from order
• Data link layer protocol used in ARPANET

32
Direct Links Outline

• Physical Layer
• Link technologies
• Encoding
• Link Layer
• Framing
• Error Detection
• Reliable Transmission (ARQ protocols)
• Medium Access Control (MAC)
• Existing protocols Ethernet, Token Rings,
  Wireless

33
Medium Access Control Protocols
Multiple Access Protocols
Contention
Conflict-free
Dynamic Resolution
Static Resolution
Dynamic Allocation
Static Allocation
Token Passing
Time of arrival
Reservation
ID
Probabilistic
Probabilistic
34
MAC Protocols Evaluation

• Throughput
• Delay
• Buffering
• Stability
• We also generally assume that
• channel is errorless
• feedback is available

35
Carrier Sense Protocols

• Use the fact that in some networks you can sense
  the medium to check whether it is currently free
• 1-persistent CSMA
• non-persistent CSMA
• p-persistent protocol
• CSMA with collision Detection (CSMA/CD)
• 1-persistent CSMA
• when a station has a packet
• it waits until the medium is free to transmit the
  packet
• if a collision occurs, the station waits a random
  amount of time
• first transmission results in a collision if
  several stations are waiting for the channel

36
Carrier Sense Protocols (Contd)

• non-persistent CSMA
• when a station has a packet
• if the medium is free, transmit the packet
• otherwise wait for a random period of time and
  repeat the algorithm
• higher delays, but better performance than pure
  ALOHA
• p-persistent protocol
• when a station has a packet wait until the medium
  is free
• transmit the packet with probability p
• wait for next slot with probability 1-p
• better throughput than other schemes but higher
  delay
• CSMA with Collision Detection (CSMA/CD)
• stations abort their transmission when they
  detect a collision
• e.g., Ethernet, IEEE802.3

37
Ethernet

• History evolution from Aloha, CSMA, CSMA/CD (by
  Xerox PARC) gt Ethernet, gt IEEE802.3 (Digital,
  Intel, Xerox)
• There are slight differences between Ethernet and
  802.3 (e.g., 802.3 length field is used for
  packet type in Ethernet, various transmission
  speeds for 802.3 from 1 to 10Mbps)
• Physical layer (10Mbps Ethernet)
• Manchester encoding (bit synchronous, no-dc
  component)
• Cabling maximum 500 meters with up to 4
  repeaters (max 2500m)

10Base5 Thick coax 500 m 100 nodes Good for backbones
10Base2 Thin coax 200 m 30 Cheapest system
10Base-T Twisted pair 100 m 1024 Easy maintenance
10Base-F Fiber optics 2000 m 1024 Between buildings
38
Frame Format (IEEE802.3)

• Preamble 7x10101010 (allows the receivers
  clock to synchronize)
• SF 10101011
• 10Mbps has only 6 bytes addresses
• Unicast unique per adaptor (ranges are allocated
  to manufacturers)
• Broadcast FFFFFFFFFFFF
• Multicast first address bit 1
• Internet Multicast 01005e000000 -to-
  01005e7fffff
• Pad minimum frame length of 64 bytes

39
Ethernet Algorithm

• Receiver accepts frames with a correct CRC
• Sender CSMA/CD 1-persistent algorithm
• If the adaptor has a frame and the line is idle
  transmit, otherwise wait until idle line then
  transmit
• If a collision occurs
• When detected a 32-bit jamming sequence is sent
• Binary exponential backoff select a random
  number ? 0, 2i-1 and waits for that many slots
  before transmitting
• After ten collisions the randomization interval
  is frozen to max 1023
• After 16 collisions the controller throws away
  the frame
• What is the reason for having a minimal frame
  length? (Hint RTT 51.2ms)

40
Ethernet Performance

• Assume that retransmissions occur with
  probability p, k stations ready to transmit
• Probability that a station acquires the channel
  Akp(1-p)k-1
• Maximum when p1/k, k-gt? A-gt1/e
• Probability that a contention interval has
  exactly j slots is A(1-A)j-1
• Mean number of slots per contention is 1/A -gt e
  2.718
• Slot duration 2t 51.2ms
• Channel efficiency P/(P2te), where P is
  transmission time for a packet

41
Ethernet Performance
42
Ethernet Capture Effect

• A and B have a large queue of packets
• There exists a situation where B will keep
  increasing its backoff interval (and finally
  dropping its packet) while A is transmitting its
  packets
• One of the reasons why frame is dropped after 16
  collisions

43
Token Passing MAC

• Token Bus (IEEE802.4)
• broadcast bus
• logical ring
• token special control frame
• only the token holder station can transmit frames
• 0, 2, 4, 6 traffic priority classes
• Token Ring (initiated by IBM gt IEEE802.5 gt
  FDDI)
• token regenerated/modified at each node
• stations have two modes
• listen (forwards bits with delay 1)
• transmit (seizes the first token by transforming
  into the start of frame)

44
Token Ring
45
Token Ring (cont)

• Idea
• Frames flow in one direction upstream to
  downstream
• special bit pattern (token) rotates around ring
• must capture token before transmitting
• release token after done transmitting
• immediate release
• delayed release
• remove your frame when it comes back around
• stations get round-robin service
• Frame Format

46
Fiber Distributed Data Interface

• Evolution of IEEE802.5
• Designed for fiber (100Mbps) but also supports
  coax and twisted pair
• Architecture dual ring
• Tolerates one broken link or one station failure
• Stations buffer at least 9 bits and at most 80
  bits
• Uses 4B/5B encoding
• Specific Timed-Token Algorithm

47
Token Times

• Token Holding Time (THT)
• upper limit on how long a station can hold the
  token
• Token Rotation Time (TRT)
• how long it takes the token to traverse the ring.
• TRT lt ActiveNodes x THT RingLatency
• Target Token Rotation Time (TTRT)
• agreed-upon upper bound on TRT

48
Token Maintenance

• Lost Token
• no token when initializing ring
• bit error corrupts token pattern
• node holding token crashes
• Generating a Token (and agreeing on TTRT)
• execute when join ring or suspect a failure
• send a claim frame that includes the nodes TTRT
  bid
• when receive claim frame, update the bid and
  forward
• if your claim frame makes it all the way around
  the ring
• your bid was the lowest
• everyone knows TTRT
• you insert new token

49
Maintenance (cont)

• Monitoring for a Valid Token
• should periodically see valid transmission (frame
  or token)
• maximum gap ring latency max frame lt 2.5ms
• set timer at 2.5ms and send claim frame if it
  fires

50
Ad Hoc Wireless Networks

• Physical transmission spread-spectrum radio and
  diffused infrared
• Key issue collision avoidance, as for the
  Ethernet
• More complex than the Ethernet because the nodes
  are not directly connected with one another
• Multi-hop network
• Potential problems
• Hidden terminal problem
• Exposed terminal problem

51
802.11 Multiple Access with Collision Avoidance
(MACA 1990)

• MACA is designed for ad-hoc wireless networks
• When a station S1 has a packet to transmit to
  station S2
• S1 senses the channel. If the channel is busy
  defers the transmission until idle
• if channel is idle S1 sends a special packet
  called Request-To-Send (RTS) to S2
• (if the RTS is correctly received by S2) S2 sends
  a Clear-To-Send (CTS), CTS includes the frame
  length
• (if the CTS is correctly received by S1) S1
  starts the data transmission
• Stations which sense
• RTS defer transmission until after CTS
• CTS defer transmission until the transmission of
  data completes
• If a station does not receive CTS in response to
  its RTS, it invokes an exponential backoff