1. LAN topologies and access techniques (with a focus on Ethernet)
BSAD 141
Dave Novak
Chap 5
Network+ Guide to Networks, Dean

2. Topics Covered LAN Topologies Bus Star Ring Hierarchical Star Mesh
Wireless

3. Topics Covered Media access techniques (4) Network backbone
Ethernet – CSMA/CD
Priority access
Token passing
Wireless/Localtalk – CSMA/CA
Network backbone

4. Network Topologies
Topology – physical / spatial arrangement or design used to connect devices to network medium
How is topology different from architecture?

5. Network Topologies
Topology is typically related to the networking technology and the specific standards used
Cannot adopt every standard and technology to any topology
Can create separate LANs using different standards and topologies and connect them using bridges, switches, or routers

6. Network Topologies Three basic network topologies 1) Bus 2) Star
3) Ring

7. Bus “Legacy” Ethernet standards support this topology
Each device is cabled directly to the device next to it
Early Ethernet used bus topology with coaxial cable: Thicknet and Thinnet

8. Bus
Signal is sent along connecting media and all attached devices receive the signal
Signal travels in both directions along the bus
Ends of bus medium must be terminated
Devices must coordinate to ensure that only one device transmits a signal at a time

9. Star All devices attached to central point (hub or switch)
Separate cable from each device to hub

10. Star
Most Ethernet LANs (as well as LANs using other access technologies and layer 2 protocols) employ some variation of star topology
Can use different types of cable including twisted pair and fiber
Functionally, star-wired Ethernet LAN with hub uses shared medium
Basic hub propagates signals it receives out all other ports

11. Ring
In terms of signal transmission, ring is like bus – each device logically connected to the next
Two ends connected forming an endless loop

12. Ring
The ring is a LOGICAL loop, not necessarily a physical ring layout
Ring topology LAN physically look like a star topology LAN
The “ring” is a logical function of the MAU to which all PCs are connected
Each device acts as a repeater
Signal regenerated and forwarded
Packets passed to devices one at a time (sequentially)
Packets flow in one direction

13. Hierarchical Star / Star Bus
Variation of the base star
Once all ports on hub are used, new hub is added to the network
Hubs connected using standard cable - one end of cable plugged into the uplink port of one of the hubs

14. Mesh
In terms of many LAN / WAN connections, more theoretical than actual solution
Not used in practice, but is a valid topology
Each device has dedicated connection to all other devices on network

15. Mesh Not practical for most wired LANs Number of connections required:
Number of NICs required

16. Mesh May be used as a model for WAN wiring (internetworking)
Mesh internetwork has multiple paths between two destinations using redundant routers

17. Wireless
Term “topology” also used to describe wireless communication patterns
1) Ad hoc topology
2) Infrastructure network

18. Ad Hoc Topology
Group of wireless devices communicate directly with one another
Inside the communication range of the wireless technology
Free to communicate
Free to roam
Works for small number of devices in a small geographical setting

19. Infrastructure Topology
Wireless devices communicate with wired network using wireless transceivers
Wireless devices communicate directly with transceivers (physical access points to the wired network)
Allows communication between both wired and wireless devices through transceivers
Typical in larger network with mix of wireless and wired devices

20. Compare / Contrast Base Topologies

21. Sharing medium on a LAN LANs typically rely on shared media
Devices attached to network must coordinate use of the network
Coordination requires communication
Communication requires time
Time increases as distance between computers grows
Shared networks with long delay are inefficient
Spend more time coordinating actions than sending data

22. Shared versus Dedicated?
Shared – media / channel capacity is used jointly by multiple users or applications as needed
Envision a toll road
Dedicated – media / channel capacity is used exclusively by a single user or application
Envision a toll road where users pay to reserve their own lane and no one else can use that lane while occupied

23. Public versus Private? Public – users pay fees to use a shared network
Often a “pay-as-you-go” approach
Parts of the physical infrastructure network are available to the general public for sharing
Private – users pay fees to obtain a dedicated portion of the network
Often a “flat fee” approach
Parts of the physical infrastructure network are dedicated just for the private user

24. Point-to-Point and PSTN
In many cases we are talking about “classification” or a stratification of the same physical infrastructure network…
For example, AT&T infrastructure networks can provide users with both:
Dedicated / private service
Shared / public service

25. Locality of Reference
LAN technology is inexpensive and widely available
Locality of reference principle:
1) Communicate most frequently with computers nearby
2) Communicate repeatedly and most frequently with the same subset of computers

26. Access control techniques
Describe how devices share media
The process or method devices follow for sharing media
Data Link (2) Layer has 2 sublayers
1) LLC
2) MAC
Access control defined by MAC mechanism used

27. Access control techniques
Access control defined by Data Link Layer protocols (Layer 2)
1) CSMA/CD
2) CSMA/CA
3) Token passing
4) Demand priority

28. Ethernet (IEEE 802.3)
Most commonly used data link standard / protocols
Does NOT have a central control structure controlling when devices can transmit
Provides many Physical Layer (1) Specifications including (see Table 5.1 on page 121):
100BaseTX – Cat 5 UTP, Star, 100 Mbps, 100 meters
1000BaseSX (160 MHz) – 62.5/125 multimode fiber, Star, Mbps, 220 meters
10Base2 – RG-58 coaxial, Bus, 10 Mbps, 185 meters

29. Ethernet (IEEE 802.3) Provides Data Link Layer (2) Specifications:
Encapsulates data received from Network Layer into Ethernet frame which specifies:
MAC address
Size and format of frame

30. Ethernet (IEEE 802.3) Carrier Sense Multiple Access (CSMA)
Uses activity on cable to determine status
Cable is free, no activity – used, activity
Device checks for activity before transmitting
If activity, the device waits
Checking for activity is called Carrier Sensing

31. CSMA/CD
Sharing on legacy Ethernet (think about a bus topology for conceptual reasons)
One device has exclusive use of cable during transmission
After one is done, another can transmit

32. CSMA/CD
Collision detection (CD) – most important phase of transmission
Electrical and fiber
Collisions do not harm hardware
Result in data being destroyed or corrupted
Devices detect collisions using collision detection
Once a collision is detected, any device currently transmitting stops and sends jamming signal
After collision occurs, devices wanting to transmit must wait for the wire to become idle again

33. Binary exponential backoff
In the event of a collision how do devices using CSMA/CD know when to attempt to transmit again?

34. CSMA/CD Collisions are normal part of Ethernet operation
More devices and more activity result in more collisions
Collisions result in retransmission and delay
Ethernet does not perform well when heavily utilized
Install switch, bridge, router

35. Wireless (IEEE )
Generally considered to be slower and less reliable than wired technologies
Provides Physical Layer (1) specifications:
supports different modulation techniques at Physical Layer
1) Direct Sequence Spread Spectrum (DSSS)
2) Frequency Hopping Spread Spectrum (FHSS)

36. Wireless (IEEE )
1) DSSS
2) FHSS

37. Wireless (IEEE 802.11) Provides Data Link (2) Layer specifications
Frame - use standard frame
CSMA/CA (collision avoidance) as opposed to CD (collision detection)
CD requires full duplex – not available in wireless
Performs layer 2 error detection on incoming packets – if no errors, sends ACK indicating no collisions
If sender does not receive ACK, assumes a collision and retransmits

38. Token Passing (IEEE 802.5) Far less common than Ethernet
Hardware is generally more expensive
Single shared media
Logical ring, physical star
Passive MAU – packets are forwarded to single device at a time in order

39. Token Passing (IEEE 802.5)
Provides NO Physical Layer (1) specifications
Original IBM Token Passing employed a variety of cable specifications
Modern generally use Cat 5/5e/6 and RJ45 connectors

40. Token Passing (IEEE 802.5) Provides Data Link Layer (2) specifications
Token Ring Frames
4 different frame formats used in communication (Ethernet employs only 1): 1) Data Frame, 2) Token Frame, 3) Command Frame, and 4) Abort Delimiter Frame

41. Token Passing (IEEE 802.5)
Devices must wait for the token before transmitting
Device can only transmit if it has token
When sending device is finished, the token is passed to the next device on the ring

42. Token Passing (IEEE 802.5)
Logical operation of MAU

43. Token Passing (IEEE 802.5)
Considered more efficient than CSMA/CD because access technique works well even under heavy load
Provides each device with equal opportunity to transmit
Collision free environment

44. FDDI / CDDI
Prior to Fast Ethernet, FDDI was only data link protocol to offer 100 Mbps transmission
Provides redundancy to avoid failure
Two rings
Only one used when network is functioning properly
Counter rotating
Data flow in opposite directions on the two rings
Self healing
Hardware can detect failure and recover automatically
Failure is bypassed

45. FDDI
Provides unique Layer 1 and 2 specifications - frame format different from token ring, although access method (token passing) the same

46. Demand priority Access method REQUIRES use of star topology
Each device sends request to active hub
Hub processes requests and assigns a communication order – one at a time (FCFS)
Fair access
Central hub controls traffic
Devices do not compete for use of media

47. Comparison of Access Methods
CSMA/CD
CSMA/CA
Token passing
Demand priority

48. Topology and Access methods
Three basic topologies
Bus
Star
Ring
Four basic access methods
CSMA/CD
CSMA/CA
Token passing
Demand priority

49. Mapping Do topologies map to the access methods on a one-to-one basis?
CSMA/CD
Demand
Priority
Token
Passing
CSMA/CA
Topology
Star, bus
Ad hoc
infrastructure
Any
Star
Architecture
LAN
technology
802.3 Ethernet
Wireless,
Localtalk
802.5 HSTR
802.4 Token bus
Demand
priority

50. Backbone Networks
Serial
Distributed
Collapsed
Parallel

51. Serial Backbone
Simplest backbone consisting of two or more devices connected directly to each other via a single medium – daisy chain
Dean, Network+ Guide to Networks (2013)

52. Distributed Backbone
Hierarchy consisting of multiple intermediate devices connected to one or more central devices
Dean, Network+ Guide to Networks (2013)

53. Distributed Backbone
Provides the ability to segregate specific workgroups or functional areas
Dean, Network+ Guide to Networks (2013)

54. Collapsed Backbone
Single router or switch is central connection point for multiple LANs or subnetworks
Dean, Network+ Guide to Networks (2013)

55. Parallel Backbone
Most robust backbone consisting of redundant connections from all central routers to all switches and network segments
Dean, Network+ Guide to Networks (2013)

56. Lecture Summary LAN topologies Media access techniques
Network backbone