1. Local Area Network
Lesson 7
NETS2150/2850

2. Lesson Outline Common LAN topologies Logical Link Control sublayer
Medium Access Control sublayer
ARP protocol for IP  MAC map
LAN interconnection devices

3. Topologies
LAN topology refers to the ways end systems are interconnected
Common topologies:
Tree
Bus
Special case of tree
Ring
Star

4. LAN Topologies

5. Bus and Tree Transmission propagates throughout medium
Heard by all stations
Need to identify target station
Each station has unique address
Full duplex connection between station and tap
Allows for simultaneous transmission and reception
Need to regulate transmission
To avoid collisions
To avoid hogging
Data in small frames (fragmentation!)
Terminator absorbs frames at end of medium
Prevent from being reflected into the channel

6. Frame Transmission on Bus LAN

7. 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 protocol determines when station can insert frame

8. Frame Transmission Ring LAN

9. Star Topology Each station connected directly to central node
Usually via two point to point links
Central node can broadcast
Only one station can transmit at a time
Or central node can act as frame switch
More stations can transmit at a time

10. IEEE 802 v OSI RM

11. 802 Layers - Physical Encoding/decoding Preamble generation/removal
7 bytes with pattern followed by one byte with pattern
used to synchronise receiver, sender clock rates
Bit transmission/reception
Transmission medium and topology

12. 802 Layers - Logical Link Control
Based on HDLC
Provides interface to higher levels
Transmission of LLC PDU between two stations
Flow and error control
Must support multiaccess, shared LAN media
Link access handled by MAC layer

13. LLC Services Unacknowledged connectionless service
No handshake and no ack (unreliable)
Connection mode service
Use handshake and ack
Acknowledged connectionless service
No handshake but uses ack

14. Media Access Control
Assembly of data into frame with address and error detection fields
Disassembly of frame
Address recognition
Error detection
Govern access to transmission medium

15. MAC Frame Format MAC layer receives data from LLC layer and adds:
MAC control
Destination MAC address (6-octet or 48-bit)
Source MAC address
CRC
MAC layer detects errors and discards frames
MAC broadcast address: FF FF FF FF FF FF16
LLC optionally retransmits unsuccessful frames

16. IEEE 802.3 MAC Frame Format Addresses: 6 octets
Length
Addresses: 6 octets
if adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to net-layer protocol
otherwise, adapter discards frame
Length: length of data field in octets, max frame size is 1518 octets (excluding preamble & SFD)
CRC: checked at receiver, if error is detected, the frame is simply dropped (32-bit CRC)

17. MAC protocols Assume single shared broadcast channel
Two or more simultaneous transmissions by nodes will cause interference
only one node can send successfully at a time
MAC protocol:
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit

18. MAC Protocols: A taxonomy
Three broad classes:
Channel Partitioning or Reservation
divide channel into smaller “pieces” (time slots, frequency, code)
allocate a piece to node for exclusive use
Random Access or Contention
channel not divided, thus can’t avoid collisions
Need to “recover” from collisions
“Taking turns” or Round Robin
tightly coordinate shared access to avoid collisions

19. Address Resolution Protocol (ARP)
Even if you have the IP address of your destination, you need its MAC to get your data across a physical network
So, we need a way to do this mapping
ARP performs dynamic mapping between IP and MAC
Any resolved mapping is stored in a host’s ARP cache

20. ARP operation
McGraw-Hill
The McGraw-Hill Companies, Inc., 2004

21. An ARP request is broadcast; an ARP reply is unicast.
Note:
An ARP request is broadcast; an ARP reply is unicast.
An ARP reply is only generated by the destined node.

22. ARP Packet Format
McGraw-Hill
The McGraw-Hill Companies, Inc., 2004

23. Encapsulation of ARP Packet
Length
McGraw-Hill
The McGraw-Hill Companies, Inc., 2004

24. Interconnecting LAN segments
Hubs
Bridges
Switches

25. Hubs Hub acts as a repeater (physical layer device)
When single station transmits, hub repeats signal on outgoing line to each station
Limited to about 100 m
Optical fibre may be used
Max about 500 m
Physically star, logically bus
Transmission from any station received by all other stations
Forms a single collision domain
Two stations transmit at the same time  collision!!

26. Interconnecting with hubs
Backbone hub interconnects LAN segments
Extends max distance between stations
But individual segments’ collision domain become one large collision domain
when a node in CS and a node in EE transmit at same time  collision!!
Can’t interconnect 10BaseT & 100BaseT

27. Bridges Link layer device (layer-2 device)
stores and forwards Ethernet frames
examines frame header and selectively forwards frame based on MAC dest address
transparent
stations are unaware of presence of bridges
plug-and-play, self-learning
bridges do not need to be configured

28. Bridges: traffic isolation
Bridge installation breaks LAN into LAN segments
bridges filter packets:
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
bridge
collision
domain
= hub
= station
LAN segment
LAN segment
LAN

29. Forwarding How to determine to which LAN segment to forward frame?
Looks like a routing problem...

30. Self learning A bridge has a bridge table entry in bridge table:
(Station MAC Address, Bridge Interface, Timestamp)
stale entries in table dropped (TTL can be ~ 60 min)
bridges learn which hosts can be reached through which interfaces
when frame received, bridge “learns” location of sender: incoming LAN segment
records sender/location pair in bridge table

31. Bridge example
Suppose C sends frame to D and D replies back with frame to C.
Bridge receives frame from from C
updates bridge table, C is on interface/port 1
because D is not in table, bridge sends frame into interfaces 2 and 3
frame received by D

32. Bridge Learning: example
C 1
D generates frame for C, and sends it
bridge receives frame
notes in bridge table that D is on interface 2
bridge knows C is on interface 1, so selectively forwards frame to interface 1

33. Interconnection without backbone
Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over CS segment

34. Backbone configuration
Recommended !
Note: A bridge does not change the physical (MAC) addresses in a frame.

35. Loop of Bridges

36. Spanning Tree Algorithm
Address learning works for tree layout
i.e. no closed loops (or cycles)
But not for cyclic connected graph!
Spanning Tree Algo. builds a network including all the nodes with selected links (i.e. edges) without closed loops
Known as a spanning tree!

37. Spanning Tree
for increased reliability, desirable to have redundant, alternative paths from source to dest but need to avoid cycles
solution: organize bridges in a spanning tree by disabling subset of interfaces
Disabled

38. Some bridge features
Isolates collision domains resulting in higher total max throughput (i.e. amount of data transmitted within an interval)
Transparent (“plug-and-play”): no configuration necessary

39. Routers vs. Bridges (1) both store-and-forward devices
routers: network layer devices (examine network layer headers)
bridges are link layer devices
routers maintain routing tables, implement routing algorithms
bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

40. Routers vs. Bridges (2) Bridges pros (+) and cons (-)
+ Bridge operation is simpler requiring less data unit processing
+ Bridge tables are self learning
- All traffic confined to spanning tree, even when alternative bandwidth is available
- Bridges do not offer protection from broadcast storms (i.e. forwarding of broadcast traffic)

41. Routers vs. Bridges (3) Routers + and -
+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)
+ provide protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher packet processing
bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

42. Ethernet Switches Essentially a multi-interface bridge
layer 2 (frame) forwarding, filtering using LAN addresses
Incoming frame from particular station switched to appropriate output line
Unused lines can switch other traffic
More than one station can transmit at a time
Multiplying capacity of LAN

43. Shared Hub and Switch 

44. Types of Ethernet Switches
Store-and-forward switch
Accepts frame on input line
Buffers it briefly, then forwards it to appropriate output line
Error checking, boosts integrity of network
Cut-through switch
Takes advantage of dest address appearing at beginning of frame
Switch begins repeating frame onto output line as soon as it recognizes dest address
Highest possible throughput
Risk of propagating bad frames
Switch unable to check CRC prior to retransmission
Netgear GS108UK GB Switch
Latency ~ 10 µs for 64-byte frames
Throughput 32 Mfps
MAC database (8000 entries)

45. Ethernet Switch Benefits
No change to attached stations to convert bus LAN or hub LAN to switched LAN
For Ethernet LAN, each station uses Ethernet MAC protocol
Each station has dedicated capacity equal to original LAN
Assuming switch has sufficient capacity to keep up with all devices
Switch scales easily
Con: still has broadcast storm problem!

46. Subnetwork with layer-3 device!
Solution: break up network into subnetworks connected by routers or layer-3 switch (faster!)
Packet forwarding done in the hardware
MAC broadcast frame limited to stations and switches contained within a single subnetwork

47. Typical Large LAN Organization
Thousands to tens of thousands of stations
Desktop systems links 10 Mbps to 100 Mbps
Into layer 2 switch
Wireless LAN connectivity available for mobile users
Layer 3 switches at local network's core
Form local backbone
Interconnected at 1 Gbps
Connect to layer 2 switches at 100 Mbps to 1 Gbps
Servers connect directly to layer 2 or layer 3 switches at 1 Gbps

48. Typical Large LAN Organization Diagram

49. Summary comparison

50. Summary LAN topologies IEEE 802 reference model Types of MAC protocols
Interconnection Devices
Hubs, bridges, switches, routers
Read Stallings chapter 15
Next: Specific MAC protocols