Local Area Networks

1
Part IV

• Local Area Networks
• (LANs)

2
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
• Now we have PAN (Personal Area Network)

3
The Local Area Network (LAN)

• Engineering classification
• Extremely popular (most networks are LANs)
• Many LAN technologies exist

4
Key Features of a LAN

• High throughput
• Relatively low cost
• Limited to short distance
• Often rely on shared media

5
Scientific Justification ForLocal Area Networks

• A computer is more likely to communicate with
  computers that are nearby than with computers
  that are distant

• Known as the locality principle

6
Topology

• Mathematical term
• Roughly interpreted as geometry for curved
  surfaces

7
Network Topology

• Specifies general shape of a network
• Handful of broad categories
• Often applied to LAN
• Primarily refers to interconnections
• Hides details of actual devices

8
Star Topology

• Central component of network known as hub
• Each computer has separate connection to hub

9
Ring Topology

• No central facility
• Connections go directly from one computer to
  another

10
Bus Topology

• Shared medium forms main interconnect
• Each computer has a connection to the medium

11
Example Bus Network Ethernet

• Most popular LAN
• Widely used
• IEEE standard 802.3
• Several generations
• Same frame format
• Different data rates
• Different wiring schemes

12
IEEE 802 Standard

• IEEE 802.1 High Level Interface
• IEEE 802.1D Local Bridge (Spanning Tree
  Algorithm)
• IEEE 802.1G Remote Bridge
• IEEE 802.2 LLC (Logical Link Control)
• IEEE 802.3 CSMA/CD (Carrier Sense Multiple Access
  with Collision Detection)
• IEEE 802.4 Token-Bus
• IEEE 802.5 Token-Ring
• IEEE 802.6 DQDB (Distributed Queue Dual Bus)
• IEEE 802.7 Broadband Technical Advisory Group
• IEEE 802.8 Fiber Optic Technical Advisory Group
• IEEE 802.9 Integrated Voice and Data LAN Working
  Group
• IEEE 802.10 LAN Security Working Group
• IEEE 802.11 Wireless LAN
• IEEE 802.12 Demand-Priority (100VG-AnyLAN)
• IEEE 802.14 Hybrid Fiber Coaxial Network

13
IEEE 802 Standard

• IEEE 802.15 Wireless Personal Area Network
• IEEE 802.16 Broadband Wireless Access Standard
• IEEE 802.17 Resilient Packet Ring Network
• IEEE 802.18 Radio Regulatory Technical Advisory
  Group ("RR-TAG").
• IEEE 802.19 Coexistence Technical Advisory Group
• IEEE 802.20 Mobile Broadband Wireless Access
  (MBWA).

14
Shared Medium in a LAN

• Shared medium used for all transmissions
• Only one station transmits at any time
• Stations take turns using medium
• Media Access Control (MAC) policy ensures fairness

15
Multiple Access Protocols
ALOHA
Pure ALOHA
the first multiple-access protocol a method for
sharing a transmission channel by enabling the
transmitter to access the channel at random times
ALOHA of U. of Hawaii
Computer Center
413MHz at 9600bps
407MHz at 9600bps
16
Multiple Access Protocols
ALOHA
Pure ALOHA
Frames are transmitted at completely arbitrary
times.
17
Multiple Access Protocols
ALOHA
Pure ALOHA

• protocol
• nodes transmit on a common channel
• transmit frame of fixed length
• when two transmissions overlap, they garble each
  other (collision)
• the central node acknowledges the correct frames
  it receives
• when a node does not get an acknowledgment within
  a specific timeout, it assumes that its frame
  collided
• when a frame collides, the transmitting node
  schedules a
• retransmission after a random delay

18
Multiple Access Protocols
ALOHA
Pure ALOHA
nodes
S
collision?
new frame
channel
G
No
S
old frame
Yes
S the mean number of new frames generated by the
infinite population G the mean number of
transmission attempts (new and old combined)
where P0 is the probability that a frame does not
suffer a collision
19
pure ALOHA and slotted ALOHA
pure ALOHA
time
Nodes can starting transmitting at any time.
slotted ALOHA
slot
time
Nodes must start their transmissions at the
beginning of a time slot.
20
pure ALOHA and slotted ALOHA
vulnerability period
pure ALOHA
slotted ALOHA
packet
packet
Other nodes that are ready at this period will
result in collision.
21
pure ALOHA and slotted ALOHA
The probability that k frames are generated
during a given frame time is given by the Poisson
distribution
So the probability of zero frames in a slot is
just e-G.
In an interval two time slots long, the mean
number of frames generated is 2G. Therefore, the
distribution is
The probability of zero frames is e-2G.
22
pure ALOHA and slotted ALOHA
Using SGP0, we get
For pure ALOHA SGe-2G For slotted ALOHA SGe-G
To find the maximum value
23
pure ALOHA and slotted ALOHA
24
pure ALOHA and slotted ALOHA
In slotted ALOHA, the best we can hope for is 37
success, 37 slots empty, and 26 collisions.
Operating at higher values of G reduces the
number of empties but increases the number of
collisions exponentially.
Consider the transmission of a test frame
success e-G, failure 1-e-G, success for k
attempts
Expected number of transmissions
As a result of the exponential dependence of E
upon G, small increases in the channel load can
drastically reduce its performance.
25
Carrier Sense Multiple Access Protocols
With slotted ALOHA the best channel utilization
that can be achieved is 1/e. This is hardly
surprising, since with stations transmitting at
will, without paying attention to what other
stations are doing, there are bound to be many
collisions.
In local area networks, however, it is possible
for stations to detect what other stations are
doing, and adapt their behavior accordingly.
Protocols in which stations listen for a carrier
(i.e. a transmission) and act accordingly are
called carrier sense protocols.
26
Carrier Sense Multiple Access Protocols
1-persistent CSMA the station transmits with a
probability of 1 whenever it finds the channel
idle, if the channel is busy, it waits until it
becomes idle
non-persistent CSMA the station transmits if the
channel is idle, if the channel is busy, it waits
a random time and tries again
p-persistent CSMA (slotted) the station
transmits with a probability of p whenever it
finds the channel idle, with a probability of
1-p, it waits until the next slot. If another
station has begun transmitting, it acts as if
there had been a collision. It waits a random
time and starts again.
27
Carrier Sense Multiple Access Protocols
28
Illustration of Ethernet Transmission

• Only one station transmits at any time
• Signal propagates across entire cable
• All stations receive transmission
• CSMA/CD media access scheme

29
Ethernet Encoding
Manchester Encoding
Implication A 10 Mbps Ethernet needs a signal
speed of 20M.
30
Minimum Ethernet Frame Length
Minimum frame length 64 bytes
31
Minimum Ethernet Frame Length
As the network speed goes up, the minimum frame
length must go up or the maximum cable length
must come down proportionally. For a 2500-meter
LAN operating at 1 Gbps, the minimum frame size
would have to be 6400 bytes. Alternatively, the
minimum frame size could be 64 bytes and the
maximum distance between any two stations 250
meters.
32
CSMA/CD Paradigm

• Multiple Access (MA)
• Multiple computers attach to shared media
• Each uses same access algorithm
• Carrier Sense (CS)
• Wait until medium idle
• Begin to transmit frame
• Simultaneous transmission possible

33
CSMA/CD Paradigm(continued)

• Two simultaneous transmissions
• Interfere with one another
• Called collision
• CSMA plus Collision Detection (CD)
• Listen to medium during transmission
• Detect whether another stations signal
  interferes
• Back off from interference and try again

34
Backoff After Collision

• When collision occurs
• Wait random time t1, 0 ltt1lt d
• Use CSMA and try again
• If second collision occurs
• Wait random time t2, 0 ltt2lt 2d
• Double range for each successive collision
• Called binary exponential backoff

35
Media Access on a Wireless Net

• Limited range
• Not all stations receive all transmissions
• Cannot use CSMA/CD
• Example in diagram
• Maximum transmission distance is d
• Stations 1 and 3 do not receive each others
  transmissions

36
Media Access on a Wireless Net
The hidden terminal problem
When A transmits to B and C also transmits to B
simultaneously, the frames will be collided at B.
Since A and C can not see each other.
37
Media Access on a Wireless Net
The exposed terminal problem
When C hears Bs transmission intended for A, it
may falsely conclude that it can not send to D
now.
38
CSMA/CA

• Used on wireless networks
• Both sides send small message followed by data
  transmission
• X is about to send to Y
• Y is about to receive from X
• Data sent from X to Y
• Purpose inform all stations in range of X or Y
  before transmission
• Known as Collision Avoidance (CA)

39
CSMA/CA
CSMA/CA basis for IEEE802.11 wireless LAN
standard
The basic idea behind it is for the sender to
stimulate the receiver into outputting a short
frame, so stations nearby can detect this
transmission and avoid transmitting themselves
for the during of upcoming (large) data frame.
40
CSMA/CA
RTS (30 bytes) and CTS contains the data length
that will eventually follow.
41
CSMA/CA
Any station hearing the RTS is clearly close to A
and must remain silent long enough for the CTS to
be transmitted back to A without conflict. Any
station hearing the CTS is clearly close to B and
must remain silent during the upcoming data
transmission, whose length it can tell by
examining the CTS frame.
42
CSMA/CA
Despite these precautions, collisions can still
occur. For example, B and C could both send RTS
frames to A at the same time. In the event of a
collision, an unsuccessful transmitter (i.e., one
that does not hear a CTS within the expected time
interval) waits a random amount of time and tries
again later. The algorithm used is binary
exponential backoff.
43
Identifying a Destination

• All stations on shared-media LAN receive all
  transmissions
• To allow sender to specify destination
• Each station assigned unique number
• Known as stations address
• Each frame contains address of intended recipient

44
Ethernet Addressing

• Standardized by IEEE
• Each station assigned by unique 48-bit address
• Address assigned when network interface card
  (NIC) manufactured

45
Ethernet Address Recognition

• Each frame contains destination address
• All stations receive a transmission
• Station discards any frame addresses to another
  station
• Important interface hardware, not software,
  checks address

46
Possible Destinations

• Packet can be sent to
• Single destination (unicast)
• All stations on network (broadcast)
• Subset of stations (multicast)
• Address used to distinguish

47
Advantages of Address Alternatives

• Unicast
• Efficient for interaction between two computers
• Broadcast
• Efficient for transmitting to all computers
• Multicast
• Efficient for transmitting to a subset of
  computers

48
Broadcast on Ethernet

• All 1s address specifies broadcast
• Sender
• Places broadcast address in frame
• Transmits one copy on shared network
• All stations receive copy
• Receiver always accepts frame that contains all
  1s as address

49
Multicast

• Half of addresses reserved for multicast
• Network interface card
• Always accepts unicast and broadcast
• Can accept zero or more multicast addresses
• Software
• Determines multicast address to accept
• Informs network interface card

50
Promiscuous Mode

• Designed for testing / debugging
• Allows interface to accept all packets
• Available on most interface hardware

51
Identifying Frame Contents

• Integer type field tells recipient the type of
  data being carried
• Two possibilities
• Self-identifying or explicit type (hardware
  record type)
• Implicit type (application sending data must
  handle type)

52
Conceptual Frame Format

• Header
• Contains address and type information
• Layout fixed
• Payload
• Contains data being sent

53
Ethernet Frame Structure (Ethernet Encapsulation)
7 1 6 6 2
4
Data
preamble SFD DA SA type
CRC
60 to 1514 bytes
synchronize the receiver
Cyclic Redundancy Check
type
0800 IPv4 datagram 0806 ARP request/reply 8035
RARP request/reply 86DD IPv6
start frame delimiter
54
802.3 frame format
single address
0
multicast (all 1's for broadcast)
group address
1
local address
0
No significance outside
one of 246 unique address
global address
1
55
When Network HardwareDoes Not Include Types

• Sending and receiving computers must agree
• To only send one type of data
• To put type information in first few octets of
  payload
• Most systems need type information

56
Illustration of TypeInformation Added to Data

• In practice
• Type information small compared to data carried
• Format of type information standardized

57
A Standard For Type Information

• Defined by IEEE
• Used when hardware does not include type field
• Called LLC / SNAP header

58
Demultiplexing On Type

• Network interface hardware
• Receives copy of each transmitted frame
• Examines address and either discards or accepts
• Passes accepted frame to system software
• Network device software
• Examined frame type
• Passes frame to correct software module

59
Network Analyzer

• Device used for testing and maintenance
• Listens in promiscuous mode
• Produces
• Summaries (e.g., of broadcast frames)
• Specific items (e.g., frames from a given address)

60
Ethernet Wiring

• Three schemes
• Correspond to three generations
• All use same frame format

61
Original Ethernet Wiring
Auxiliary Unit Interface

• Used heavy coaxial cable
• Formal name 10Base5
• Called thicknet

62
Second Generation Ethernet Wiring
Bayonet Neill-Concelman (the inventors of the
BNC connector)

• Used thinner coaxial cable
• Formal name 10Base2
• Called thinnet

63
Modern Ethernet Wiring

• Uses a hub
• Formal name 10Base-T
• Called twisted pair Ethernet

64
Ethernet Wiring In An Office
65
A Note About Ethernet Topology

• Apparently
• Original Ethernet used bus topology
• Modern Ethernet uses star topology
• In fact, modern Ethernet is
• Physical star
• Logical bus
• Called star-shaped bus

66
Higher Speed Ethernets

• Fast Ethernet
• Operates at 100 Mbps
• Formally 100Base-T
• Two wiring standards
• 10/100 Ethernet devices available
• Gigabit Ethernet
• Operates at 1000 Mbps (1 Gbps)
• Slightly more expensive

67
Ring Topology

• Second most popular LAN topology
• Bits flow in single direction
• Several technologies exist

68
Token Passing

• Used with ring topology
• Guarantees fair access
• Token
• Special (reserved) message
• Small (a few bits)

69
Token Passing Paradigm

• Station
• Waits for the token to arrive
• Transmits one packet around ring
• Transmits token around ring
• When no station has data to send
• Token circulates continuously

70
Token Passing Ring Transmission

• Station waits for token before sending
• Signal travels around entire ring
• Sender receives its own transmission

71
Strengths of Token Ring Approach

• Easy detection of
• Broken ring
• Hardware failures
• Interference

72
Weaknesses of Token Ring Approach

• Broken wire disables entire ring
• Point-to-point wiring
• Awkward in office environment
• Difficult to add / move stations

73
Token Passing Ring Technologies

• ProNet-10
• Operated at 10 Mbps
• IBM Token Ring
• Originally operated at 4 Mbps
• Later version operated at 16 Mbps
• Fiber Distributed Data Interconnect (FDDI)
• Operated at 100 Mbps

74
FDDI Terminology

• FDDI
• Uses optical fibers
• High reliability
• Immune to interference
• CDDI
• FDDI over copper
• Same frame format
• Same data rate
• Less noise immunity

75
FDDI Hub Technology

• Part of FDDI standard
• Stations attach to hub
• Same frame format and data rate as FDDI
• Called star-shaped ring

76
FDDI Failure Recovery

• Uses two rings
• Automatic failure recovery
• Terminology
• Dual-attached
• Counter rotating
• Self healing

77
Illustration of FDDIFailure Recovery
78
Another Example of aPhysical Star Topology

• Asynchronous Transfer Mode (ATM)
• Designed by telephone companies
• Intended to accommodate
• Voice
• Video
• Data

79
ATM

• Building block known as ATM switch
• Each station connects to switch
• Switches can be interconnected

80
Details of ATM Connection

• Full-duplex connections
• Two fibers required

81
ATM Characteristics

• High data rates (e.g. 155 Mbps)
• Fixed size packets
• Called cells
• Important for voice
• Cell size is 53 octets
• 48 octets of data
• 5 octets of header

82
Summary

• Local Area Networks
• Designed for short distance
• Use shared media
• Many technologies exist
• Topology refers to general shape
• Bus
• Ring
• Star

83
Summary (continued)

• Address
• Unique number assigned to station
• Put in frame header
• Recognized by hardware
• Address forms
• Unicast
• Broadcast
• Multicast

84
Summary continued)

• Type information
• Describes data in frame
• Set by sender
• Examined by receiver
• Frame format
• Header contains address and type information
• Payload contains data being sent

85
Summary (continued)

• LAN technologies
• Ethernet (bus)
• IBM Token Ring
• FDDI (ring)
• ATM (star)

86
Summary (continued)

• Wiring and topology
• Can distinguish
• Logical topology
• Physical topology (wiring)
• Hub allows
• Star-shaped bus
• Star-shaped ring

87
Homework

• P118 8.7
• P136 9.3, 9.4