1. EEL 5718 Computer Communications
Local Area Networks (LAN)
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2. Local Area Network (LAN)
Cover short distance
Provide very high speed
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3. (b). Network Interface Card (NIC)
(a). LAN
(b). Network Interface Card (NIC)
RAM
Ethernet Processor
RAM
ROM
Figure 6.10

4. LAN Applications Personal computer LANs
Back end networks and storage area networks
Interconnecting large systems (mainframes and large storage devices)
High data rate
High speed interface
Distributed access
Limited distance
Limited number of devices
High speed office networks
Backbone LANs
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5. LAN Topologies Tree Bus Ring Star Special case of tree
One trunk, no branches
Ring
Star
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6. LAN Topologies
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7. Bus and Tree Multipoint medium
Transmission propagates throughout medium
Heard by all stations
Need to identify target station (with unique address)
Full duplex connection between station and tap
Allows for transmission and reception
Need to regulate transmission
To avoid collisions
Terminator absorbs frames at end of medium
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8. Frame Transmission - Bus LAN
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9. 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 in frames
Circulate past all stations
Destination recognizes address and copies frame
Frame circulates back to source where it is removed
MAC determines when station can insert frame
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10. Frame Transmission Ring LAN
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11. Star Topology Each station connected directly to central node
Usually via two point to point links
Central node can broadcast
Physical star, logical bus
Only one station can transmit at a time
Central node can act as frame switch
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12. Protocol Architecture
Lower layers of OSI model
IEEE 802 reference model
Physical
Logical link control (LLC)
Media access control (MAC)
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13. IEEE 802 vs. OSI
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14. IEEE 802 OSI Network Layer Network Layer LLC
Logical Link Control
Data Link
Layer
802.11
Wireless
LAN
802.3
CSMA-CD
802.5
Token Ring
Other
LANs
MAC
Physical
Layer
Physical
Layer
Various Physical Layers
IEEE 802
OSI

15. IEEE 802 Layers - Physical Encoding/decoding
Preamble generation/removal
Bit transmission/reception
Transmission medium and topology
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16. 802 Layers - Media Access Control (MAC)
Multiple Access Control or Multiaccess Control
Provides connectionless transfer of datagrams
Govern access to transmission medium
For the same LLC, several MAC options may be available
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17. MAC MAC: Coordinating the transmission on shared channel/medium
Round robin: Token bus, token ring, polling
Reservation: DQDB (IEEE 802.6)
Good for stream traffic
Contention- CSMA/CD(802.3),CSMA/CA (802.11)
Good for bursty traffic
All stations contend for time
Simple to implement
Efficient under moderate load
Tend to collapse under heavy load
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18. IEEE 802 Layers - Logical Link Control (LLC)
Interface to higher levels
Provides a standard set of services
Hides the details of the underlying MAC protocols
Interface to MAC layer
Enhances the datagram service offered by MAC
Three types of services based on MAC datagram services
Type1: unacknowledged connectionless service
Type 2: reliable connection-oriented service (e.g., ARQ)
Type 3: acknowledged connectionless service
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19. LLC (cont.) Flow and error control Segmentation and assembly
How large packets are handled in a LAN with smaller maximum PDU size
Segmentation of a packet
IP address and offset number
Form a frame with address, CRC, and offset number
Assembly of packet segments into the packet
Address recognition
Error detection
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20. LLC (cont.) Addressing NIC can be set to run in “promiscuous” mode
Network Interface Card (NIC) or LAN adapter card
Physical address: unique and burned into the ROM
MAC address: 48 bits
First three bytes: vendor
The rest bytes: a unique number to that vendor
Can set to recognize physical address, broadcast address, and multicast address
NIC can be set to run in “promiscuous” mode
Listen to all transmissions over the LAN
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21. LLC (cont.) Additional addressing
One NIC card serves multiple applications (data exchanges) simultaneously
SAP: Service Access Point
LSAP: LLC Service Access Point
Hexadecimal identification
06: frames containing IP packets
E0: frames containing Novell IPX
FE: frames containing OSI packets
AA: frames containing SNAP PDUs
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22. LLC (cont.)
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23. LLC PDU Structure 1 byte 1 1 or 2 Destination SAP Address Source
Control
Information
Destination SAP Address
Source SAP Address
I/G
C/R 1
7 bits 1
7 bits
I/G = Individual or group address
C/R = Command or response frame
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24. LLC PDU and MAC Frame IP Packet LLC PDU IP LLC Header Data MAC Header
FCS
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25. Typical Frame Format
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26. Bus: Transmission Media
Twisted pair
Not practical in shared bus at higher data rates
Baseband coaxial cable
Used by Ethernet
Broadband coaxial cable
Included in specification but no longer made
Optical fiber
Expensive
Difficulty with availability
Few new installations
Replaced by star based twisted pair and optical fiber
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27. Bus: Baseband Coaxial Cable
Uses digital signaling
Manchester or Differential Manchester encoding
Entire frequency spectrum of cable used
Single channel on cable
Bi-directional
Few kilometer range
Ethernet (basis for 802.3) at 10Mbps
50 ohm cable
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28. 10Base5
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
10Base5
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29. Ring LANs
Each repeater connects to two others via unidirectional transmission links
Single closed path
Data transferred bit by bit from one repeater to the next
Repeater regenerates and retransmits each bit
Repeater performs data insertion, data reception, data removal
Repeater acts as attachment point
Packet removed by transmitter after one trip round ring
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30. Ring Repeater States
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31. Star Ring LANs Feed all inter-repeater links to single site
Concentrator
Provides central access to signal on every link
Easier to find faults
Can launch message into ring and see how far it gets
Faulty segment can be disconnected and repaired later
New repeater can be added easily
Bypass relay can be moved to concentrator
Can lead to long cable runs
Can connect multiple rings using bridges
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32. Star Ring LANs (cont) Use unshielded twisted pair wire (telephone)
Minimal installation cost
May already be an installed base
All locations in building covered by existing installation
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
Fiber optic - up to 500m
Logical bus - with collisions
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33. Legacy LANs Ethernet Token Ring and FDDI
ATM LANs (lost the battle in LAN to Ethernet)
Wireless LANs (will be discussed in EEL 6935: Wireless Networks)
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34. LAN Generations First (10 Mbps) Second (100 Mbps) Third (Gbps)
CSMA/CD and token ring
Terminal to host and client server
Moderate data rates
Second (100 Mbps)
FDDI
Backbone
High performance workstations (HIPPI)
Third (Gbps)
ATM
Aggregate throughput and real time support for multimedia applications
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35. LAN Generations(cont)
Third
ATM
Aggregate throughput and real time support for multimedia applications
Support for multiple guaranteed classes of service
Live video may need 2Mbps
File transfer can use background class
Scalable throughput
Both aggregate and per host
Facilitate LAN/WAN internetworking
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36. Ethernet
Carriers Sense Multiple Access with Collision Detection (CSMA/CD)
Xerox - Ethernet (Metcalfe’s Thesis)
IEEE standard
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37. 10Mbps Specification (Ethernet)
<data rate><Signaling method><Max segment length>
10Base5 10Base2 10Base-T 10Base-FP
Medium Coaxial Coaxial UTP 850nm fiber
Signaling Baseband Baseband Baseband Manchester
Manchester Manchester Manchester On/Off
Topology Bus Bus Star Star
Nodes
Wire length (m)
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38. 100Mbps (Fast Ethernet) 100Base-TX 100Base-FX 100Base-T4
2 pair, STP 2 pair, Cat 5UTP 2 optical fiber 4 pair, cat 3,4,5
MLT-3 MLT-3 4B5B,NRZI 8B6T,NRZ
100Mbps Mbps Mbps Mbps
100m m m m
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39. Gigabit Ethernet Configuration
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40. Gigabit Ethernet - Differences
Carrier extension
At least 4096 bit-times long (512 for 10/100)
Frame bursting
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41. Gigabit Ethernet - Physical
1000Base-SX
Short wavelength, multimode fiber
1000Base-LX
Long wavelength, Multi or single mode fiber
1000Base-CX
Copper jumpers <25m, shielded twisted pair
1000Base-T
4 pairs, cat 5 UTP
Signaling - 8B/10B
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42. Token Ring (802.5) MAC protocol
Small frame (token) circulates when idle
Station waits for token
Changes one bit in token to make it SOF for data frame, append rest of data frame
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
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43. 802.5 Physical Layer Data Rate 4 16 100 Medium UTP,STP,Fiber
Signaling Differential Manchester
Max Frame
MAC TP or DTR TP or DTR DTR
TP: token passing protocol
DTR: Dedicated Token Ring (a mode of operation of token ring)
Note: 1Gbit in development
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44. Fiber Distributed Data Interface
FDDI
Token Ring
100Mbps
LAN and MAN applications
Backbone for LANs
Fiber or twisted pair
4B/5B/NRZI or MLT-3
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45. Internetworking Necessity of globalization
LAN: a collection of nodes in short range
LAN-to-LAN connections lead to larger network
Simple connection: Repeaters, bridges (layer 2)
Complex connection: routers (layer 3)
Connectors: amplifier, switch, repeater, bridge, router, ...
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46. Internetworking Bridge Router
Connects two LANs using similar LAN protocols
Address filter passing on packets to the required network only
OSI layer 2 (Data Link)
Router
Connects two (possibly dissimilar) networks
Uses internet protocol present in each router and end system
OSI Layer 3 (Network)
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47. Bridges Ability to expand beyond single LAN
Provide 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
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48. Why Bridge? Reliability: One fault may disable all
Performance: too many nodes one one wire may produce too much traffic (too many collisions)
Security: one break-in affects too many (firewall)
Geography: one enterprise may have many locations
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49. 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
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50. Bridge Operation
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51. Connection of Two LANs
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52. Two Types of Bridges Two popular bridges Transparent Bridge
Source routing bridge
Defined by IEEE 802.1d committee
Stations are completely unaware of the presence of the bridge
Bridge operations
Forwards frames from one LAN to another
Learns where stations are attached to the LAN
Prevents loops in the topology
The bridge is configured in the promiscuous mode
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53. Transparent Bridge Bridge learning Spanning tree algorithm
Construct forwarding table: MAC address and port number
Spanning tree: overcome the loop
Spanning tree algorithm
Spanning tree: a graph containing all nodes without loops
Treat all bridges as a node and disable some bridges to remove loops
Each bridge has a unique bridge ID, each port within a bridge has a unique port ID, all bridges on a LAN recognize a unique MAC group address
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54. Transparent Bridge (cont)
Spanning tree algorithm
Selected a root bridge among all bridges in the bridged LAN. Minimum number ID is for root
Determine the root port for each bridge except the root: the root port is the port with the least-cost path to the root. In case there is a tie, the the one with minimum ID is chosen. The link cost can be
Hop count
Other assigned cost (such as a function of LAN speed)
Select a designated bridge for each LAN: the one with the least cost to the root
This procedure can be implemented via a distributed algorithm
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55. Transparent Bridge (cont)
Example
LAN1
LAN2
LAN3 B1 B2 B3 B4 B5
LAN4
(1)
(2)
(1)
(2)
(3)
(2)
(1)
(2)
(1)
(2)
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56. Transparent Bridge (cont)
Spanning topology (minimum hop-count)
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57. Source Routing Bridges
Developed by IEEE 802.5
Primarily used in interconnected token-ring networks
Main idea
Each station determines the route to destination when it wants to send a frame and therefore include the route information in the header of the frame
The first frame sent to the destination will be gathered and stored at the source for future use
Route discovery messages/frames may be issued to find the routes via broadcast!
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58. Source Routing Bridges
Frame format
Routing
Route-1
Route-2
Route-m
Control
Designator
Designator
Designator
2 bytes
2 bytes
2 bytes
2 bytes
Destination
Source
Routing
Data
FCS
Address
Address
Information
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59. Reading Textbook Chapter 6 (6.1, 6.2, 6.6, 6.7) Stallings Chapter 13
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