1. William Stallings Data and Computer Communications
Chapter 13
Local Area Network
Technology

2. LAN (Local Area Networks)
A LAN consists of
Shared transmission medium
set of hardware and software for the interfacing devices
regulations for orderly access to the medium

3. LAN Applications (1) Personal computer LANs
mostly in office environment
to share data and resources (e.g. laser printer)
Back end networks and storage area networks
Interconnecting large systems (mainframes and large storage devices)
High data rate
Limited distance
Limited number of devices

4. LAN Applications (2) High speed office networks Backbone LANs
Desktop image processing
High capacity local storage
Backbone LANs
Interconnect low speed local LANs with a higher capacity LAN

5. LAN Architecture Layered protocol architecture Physical (topologies)
Media access control
Logical Link Control

6. Protocol Architecture
Lower layers of OSI model
IEEE 802 reference model
Physical
Logical link control (LLC)
Media access control (MAC)

7. IEEE 802 v OSI

8. 802 Layers - Physical Signal encoding/decoding
Preamble generation/removal (for synchronization)
Bit transmission/reception
Transmission medium and topology
below the physical layer

9. 802 Layers - Medium Access Control & Logical Link Control
OSI layer 2 (Data Link) is divided into two in 802
Logical Link Control (LLC) layer
Medium Access Control (MAC) layer
MAC layer
Assembly of data into frame with address and error detection fields (for transmission)
Disassembly of frame (on reception)
Address recognition
Error detection
Govern access to transmission medium
Not found in traditional layer 2 data link control

10. 802 Layers - Medium Access Control & Logical Link Control
LLC layer
Interface to higher levels
flow control

11. LAN Protocols in Context

12. LAN Topologies
Bus
Ring
Star

13. Bus Topology Multipoint medium Heard by all stations
Need to identify target station
Each station has unique address
Full duplex connection between station and tap
Allows for transmission and reception
Terminator absorbs frames at end of medium
Need to regulate transmission
To avoid collisions
To avoid continuous transmission from a single station
Solution: Data in small blocks - frames

14. Frame Transmission - Bus LAN

15. Ring Topology Repeaters joined by point- to-point links in closed loop
Receive data on one link and retransmit on another
Links unidirectional
Stations attach to repeaters
Data transmitted in frames
Circulate past all stations
Destination recognizes address and copies frame
Frame circulates back to source where it is removed
Medium access control determines when station can insert frame

16. Frame Transmission Ring LAN

17. Star Topology Each station connected directly to central node
Usually via two point-to-point links
Central node can broadcast
Physical star, logically bus
Only one station can transmit at a time
Central node can act as frame switch
retransmits only to destination

18. Medium Access Control Why? Synchronous Asynchronous order
efficient use of medium
Synchronous
everyone knows when to transmit
like TDM
Asynchronous
dynamic
Three categories
Round robin
Reservation
Contention

19. Medium Access Control Round robin Reservation
each station has a turn to transmit
declines or transmits (to a certain limit)
overhead of passing the turn in either case
Good if many stations have data to transmit over extended period
Reservation
Stations reserve slots to transmit
Good for stream traffic
centralized/distributed reservations

20. Medium Access Control Contention
No control to determine whose turn is it
All stations contend for time/slot to transmit
Good for bursty traffic
Efficient under light or moderate load
Performance is bad under heavy load

21. MAC Frame Format Actual format differs from protocol to protocol
MAC layer receives data from LLC layer
MAC layer detects errors and discards frames
LLC optionally retransmits unsuccessful frames

22. Bus LANs Signal balancing
Signal must be strong enough to meet receiver’s minimum signal strength requirements
Give adequate signal to noise ratio
Not so strong that it overloads transmitter and creates distortions
Simultaneous satisfaction for all stations on bus
Solution is to divide network into small segments
Segments are connected with repeaters

23. Transmission Media Baseband coaxial cable Broadband coaxial cable
Used by Ethernet and IEEE 802.3
not a preferred option nowadays
Broadband coaxial cable
Included in specification but no longer made
Optical fiber
Expensive
Not used

24. Baseband Coaxial Cable
Uses digital signaling
Manchester or Differential Manchester encoding
Bi-directional
Few kilometer range
due to attenuation
Ethernet (basis for 802.3) at 10Mbps
50 ohm cable

25. 10Base5
Thick coax
Ethernet and originally used 0.4 inch diameter cable at 10Mbps
Max cable length 500m
Distance between taps a multiple of 2.5m
Ensures that reflections from taps do not add in phase
Max 100 taps
Vampire taps, twisted pair transceiver cable
10Base5

26. 10Base2 Thin coax 0. 5 cm cable Cheaper electronics T-connectors
More flexible
Easier to bring to workstation
Cheaper electronics
T-connectors
easier maintanence
Greater attenuation, lower noise resistance
Fewer taps (30)
Shorter distance (200m)

27. Repeaters Transmits in both directions Joins two segments of cable
No buffering
No logical isolation of segments
If two stations on different segments send at the same time, packets will collide
Only one path of segments and repeaters between any two stations

28. Baseband Configuration

29. Star LANs Use unshielded twisted pair wire in star wiring
Attach to a central active hub
Two links
Transmit and receive
Hub repeats incoming signal on all outgoing lines
Link lengths limited to about 100m
Logical bus - with collisions

30. Two Level Star Topology
All stations are logically on the same bus

31. Shared Medium Hub Central hub
Hub retransmits incoming signal to all outgoing lines
Only one station can transmit at a time
With a 10Mbps LAN, total capacity is 10Mbps

32. Switched Hubs Hub acts as switch
Incoming frame switches to appropriate outgoing line
Unused lines can also be used to switch other traffic
Each device has dedicated capacity equal to the LAN capacity

33. Switched Hubs No change to software or hardware of devices
Attachment is not logically different from the attached device point of view
Store and forward switch
Accept input, buffer it briefly, then output
Cut through switch
Take advantage of the destination address being at the start of the frame
Begin repeating incoming frame onto output line as soon as address recognized
May propagate some bad frames

34. Bridges Need to expand beyond single LAN
Interconnection to other LANs/WANs
Use Bridge or router
Bridge is simpler
Connects similar LANs
Identical protocols for physical and link layers
Minimal processing
Router more general purpose
Interconnect various LANs and WANs

35. Functions of a Bridge
Read all frames transmitted on one LAN and accept those address to any station on the other LAN
Using MAC protocol for second LAN, retransmit each frame
Do the same the other way round

36. Bridge Operation

37. Bridge Design Aspects No modification to content or format of frame
No additional header
Exact bitwise copy of frame from one LAN to another
that is why two LANs must be identical
Enough buffering to meet peak demand
Routing and addressing intelligence
Must know the addresses on each LAN to be able to tell which frames to pass
May be more than one bridge to reach the destination
May connect more than two LANs

38. Bridge Protocol Architecture
IEEE 802.1D
operates at MAC level
Station address is at this level
Bridge does not need LLC layer

39. Routing in Bridges Bridge must have routing capability
Bridge must decide whether to forward frame
Bridge must decide which LAN to forward frame on
Alternate routes

40. Fixed Routing
Routing selected for each source-destination pair of LANs
Usually least hop route
Only changed when topology changes
Manual routing data entry needed
Similar to fixed routing in packet switched networks
central routing table shows the first bridge in the route for each source-destination pair
local routing tables can be derived from the central table

41. Example Fixed Routing LAN E to LAN F LAN E Bridge 107 LAN A Bridge 102
LAN C
Bridge 105
LAN F

42. Spanning Tree Routing Bridge automatically develops routing tables
Automatically updates in response to changes
Station locations are learned from packets
forwarding databases are constructed for each port of bridges
If the location of a station is known, packet is forwarded accordingly
Otherwise packet is forwarded to all ports
to guarantee delivery
to allow learning

43. Address Learning
When frame arrives at port X, it has come from the LAN attached to port X
Use the source address to update forwarding database for port X to include that address
Example: when Bridge 102 receives a packet from LAN A port with source address 2, it learns that station 2 is on LAN A port
Examples of port databases
Bridge 102 A port
Stations 1, 2, 3, 4 and 5
Bridge 102 C port
Stations 6 and 7

44. Other Considerations Address learning works for tree layout
i.e. no closed loops
a spanning tree must be found in case of a loop
Response to topology changes is required
database entries are not permanent
i.e. learning algorithm is repeated periodically

45. Loop of Bridges

46. William Stallings Data and Computer Communications
Chapter 14
LAN Systems (CSMA/CD - Ethernet, Ring Systems)

47. Ethernet (CSMA/CD)
Carriers Sense Multiple Access with Collision Detection
Xerox - Ethernet
IEEE 802.3
Random Access (ALOHA)
Stations access medium randomly
Contention
Stations contend for time on medium

48. ALOHA Packet Radio (applicable to any shared medium)
When station has frame, it sends
collisions may occur
Station listens for max round trip time
If no collision, fine. If collision, retransmit after a random waiting time
Max utilization 18% - very bad

49. Slotted ALOHA Divide the time into discrete intervals (slots)
equal to frame transmission time
need central clock (or other sync mechanism)
transmission begins at slot boundary
Collided frames will do so totally or will not collide
Max utilization 37%

50. CSMA First listen for clear medium (carrier sense)
If medium idle, transmit
If busy, continuously check the channel until it is idle and then transmit
If collision occurs
Wait random time and retransmit
Collision probability depend on the propagation time
Longer propagation delay, worse the utilization
Collision occurs even if the propagation time is zero. WHY?
1-persistent CSMA
Better utilization than ALOHA

51. Nonpersistent CSMA Patient CSMA If channel idle, send
If not, do not continuously seize the channel
instead wait a random period of time
Better utilization, longer delay

52. CSMA/CD
With CSMA, collision occupies medium for duration of transmission
Stations listen while transmitting
If medium idle, transmit
If busy, listen for idle, then transmit
If collision detected, cease transmission and wait random time then start again
Binary exponential back off
random waiting period but consecutive collusions increase the mean waiting time
low delay with small amount of waiting stations
large delay with large amount of waiting stations

53. CSMA/CD Operation

54. Collision Detection
On baseband bus, collision produces much higher signal voltage than signal
Collision detected if cable signal greater than single station signal
Signal attenuated over distance
Limit distance to 500m (10Base5) or 200m (10Base2)

55. FCS excludes Preamble and SFD
IEEE Frame Format
>=
>=
SFD is
FCS excludes Preamble and SFD

56. 10Mbps Specification (Ethernet)
10Base5 and 10Base2
Coax (thick/thin)
Bus topology
500/200 meters max segment length
100/30 max stations
10BaseT
Twisted pair (regular telephone wiring)
Star topology with central hub or switch
Point to point with cross cables
max 100 meters segment length
10BaseF
Optical fiber
star topology or point to point
on/off manchester encoding
too expensive for 10 Mbps

57. 100Mbps (Fast Ethernet) 100BaseT4 100Base-TX 100Base-FX
to use voice grade cat 3 cables
3 pairs in each direction (with 33.3 Mbps on each)
total 4 pairs (2 of them bidirectional)
cat 3 cables carry 25MHz signal
Manchester encoding does not work with 3 pairs
How many pairs?
A different coding is used (8B6T = 8 bits map to 6 trits)
Can be used with cat 5 cables (but waste of resources)
100Base-TX
STP or cat 5 UTP only (one pair in each direction)
at 125 Mhz with special encoding that has 20% overhead
4 bits are encoded using 5-bit time
100Base-FX
Optical fiber (one at each direction)
Similar encoding

58. Gigabit Ethernet Configuration

59. Gigabit Ethernet Backward compatible Carrier extension Frame bursting
CSMA/CD protocol
same frame format
Carrier extension
a mechanism to make shortest frame 512 bytes
Frame bursting
ability to send multiple short frames consecutively

60. Gigabit Ethernet - Physical
1000Base-SX, LX
Optical fiber options
1000Base-CX
A special STP (<25m)
one for each direction
1000Base-T
4 pairs, cat 5 UTP (bidirectional)

61. Token Ring (802.5) MAC protocol
Small frame (token) circulates when idle
Station waits for token
Changes one bit in token to make it start-of-frame sequence for data frame
Append rest of data frame
Other stations must wait since there is no token circulating

62. Token Ring (802.5) MAC protocol (cont’d)
Frame makes round trip and is absorbed by transmitting station
Station then inserts new token when
transmission has finished and
leading edge of returning frame arrives
Under light loads, some inefficiency
Under heavy loads
round robin
fair

63. Token Ring Operation

64. Token Ring Characteristics Variations 4/16/100/1000 Mbps
maintenance needed
loss tokens
multiple tokens
priorities and reservations possible
Variations
Early token release
sender does not wait the leading edge of the frame to release the token
Dedicated Token Ring (DTR)
central hub acts as switch (concentrator)
dedicated connection between stations and concentrator
4/16/100/1000 Mbps

65. FDDI 100Mbps LAN and MAN applications Token Ring
Optical fiber or cat 5 UTP or STP
LAN and MAN applications
Token Ring

66. FDDI MAC Protocol As for 802.5 except:
Station seizes token by aborting token transmission
Once token captured, one or more data frames transmitted
New token released as soon as transmission finished (early token release in 802.5)

67. FDDI Operation

68. Token Bus IEEE 802.4 Physical bus, logical ring old standard (1990)
inactive working group
Physical bus, logical ring
preferred by manufacturers with linear production lines
Complex MAC protocol