1. Networks
Lecture 7

2. Review – Physical Layer
Link
Network
Transport
Application
Physical
Bit-signal
Transformation
Bit-rate
control
Circuit
Switching
Bit
synchronization
Multiplexing
Transmission media

3. Review - Basics Transmission media Communication Channel Properties:
To be transmitted the individual bits must be transformed into electromagnetic signals
Transmission media
Communication Channel Properties:
-- Bandwidth
-- Transmission and Propagation Delay
-- Loss/Error rates
-- Buffering

4. Network Manager Headaches Ethernet Ethernet Hardware
Outline
Network Manager Headaches
Ethernet
Ethernet Hardware

5. Network Manager Headaches

6. More users from hell
The nursing director complains to the system adm that she needs a new monitor. "I went to her office, and sure enough, her screen was discolored and unfocused," says the sys adm. "I lifted her new boombox off the top of the monitor, and the screen cleared up instantly. So she got a short lesson on magnets and computer equipment, and she moved her boombox to another part of the room.“
This company offers several ways to contact IT for help, ranging from an intranet ticketing system to emergency pagers. "I got a panicked call directly from one user who had tracked me down via a company receptionist who was aware of the emergency pager schedule," reports a network adm on the scene. "At the same time, I noticed a 911 emergency call had been placed from the same user's office moments before. Upon investigation, he confessed that he had intentionally dialed 911 in an attempt to contact our emergency technical support line.

7. Ethernet

8. Introduction History Bandwidth Problem
Developed by Xerox PARC, mid 70s
Roots in Aloha packet radio network
Standardized by Xerox, DEC and Intel in 1978
Similar to IEEE 802.3
IEEE 802.3u standard defines Fast Ethernet (100 Mbps)
Switched Ethernet now popular
Bandwidth
10 Mbps – 100 Mbps – 1 Gbps
Problem
Distributed algorithm that provides fair efficient access to a share medium

9. Ethernet – CSMA/CD CS – Carrier Sense MA - Multiple Access
Nodes can distinguish between an idle and a busy link
MA - Multiple Access
A set of nodes send and receive frames over a shared link
CD – Collision Detection
Nodes listen during transmission to determine if there has been interference

10. Ethernet Topologies Bus Topology: Shared All nodes connected to a wire
Star Topology:
All nodes connected to a central repeater

11. Ethernet Specifications
Coaxial Cable
upto 500m
Taps
> 2.5m apart
Transceiver
Idle detection
Sends/Receives signal
Repeater
Joins multiple Ethernet segments
< 5 repeaters between any two hosts
< 1024 hosts

12. 10Mb Ethernet Specification
Broadcast
Encoding
Manchester
10 Mbps -> Transmission at 20Mhz
Framing
Sentinel marks end of frame
Bit oriented (similar to HDLC)
Data-dependent length
Error Detection
32-bit CRC

13. Ethernet in Practice Number of hosts Range Round Trip Time Topology
Limited to 200 in practice, standard allows 1024
Range
Typically much shorter than 2.5km limit in standard
Round Trip Time
Typically 5 or 10 s
Topology
Star easier to administer than bus

14. Ethernet Performance
Ethernet is the most popular network model for LAN
Ethernet is like talking at a dinner table!
Anyone may talk at anytime
If two people attempt to talk at the same time, neither is heard
Each waits (a random amount of time) and then tries to talk again

15. Ethernet Algorithm Sender/Transmitter If line is idle (carrier sensed)
Send immediately
Send maximum of 1500B data (1527B total)
Wait 9.6 s before sending again
If line is busy (no carrier sense)
Wait until line becomes idle
Send immediately (1-persistent)
If collision detected
Stop sending and jam signal
Try again later

16. Ethernet Frame
Just like each of the layers, ethernet adds information to the data packet.
Preamble
SFD DA SA
type
data + layer information
Pad
CRC
Data + layer: 0 – 1500 bytes of data
Type: 2 bytes which indicates protocol of encapsulated data (e.g. IP = 0x0800)
SA & DA: each is a 48-bit globally unique address assigned by manufacturer.
SFD: Start of Frame Delimiter
Preamble: 7 bytes of alternating 1’s and 0’s used to sync the clock
Pad: 0 to 46 bytes of 0’s used to ensure minimum frame length
CRC: 4 byte Cyclic Redundancy Check

17. Ethernet Addresses
Unique, 6 bytes or 48-bit address assigned to each adapter by manufacturer.
It is read in : notation, for example: 8:0:e4:b1:2
An address with all 1s is a broadcast address.
multicast: first bit is 1
In order to make the address unique, first 24 bits are assigned to manufacturers and the last 24 bits are assigned locally.
Each adaptor accept the packet if the destination address is its own address, broadcast address or multicast to which this adaptor belongs

18. Keeping Addresses Straight
32-bit IP address:
network-layer address
used to get datagram to destination IP network
LAN (or MAC or physical or Ethernet) address:
used to get datagram from one interface to another physically-connected interface (same network)
48 bit MAC address (for most LANs)
burned in the adapter ROM

19. Manufacturer Addresses
Examples of Manufacturer IDs
Cisco : C :
3Com : C :
Xircom : C7-
IBM : A
Sun :
Nokia :

20. Discover the MAC Address
How do you find the MAC address on a windows machine?

21. Transmission Algorithm
Sender/Transmitter
If line is idle (carrier sensed)
Send immediately
Send maximum of 1500B data (1527B total)
Wait 9.6 s before sending again
If line is busy (no carrier sense)
Wait until line becomes idle
Send immediately (1-persistent)
If collision detected
Stop sending and jam signal
Try again later

22. Receiver Algorithm Security Problem!!!
Sender handles all access control
Receiver simply pulls the frame from the network
Ethernet controller/card
Sees all frames
Selectively passes frames to host processor
Acceptable frames
Addressed to host
Addressed to broadcast
Addressed to multicast address to which host belongs
Anything (if in promiscuous mode)
Need this for packet sniffers/TCPDump
Security Problem!!!

23. Collision Behavior I
While the transmitting station is sending the frame, it monitors the medium for a collision
If a collision is detected, the transmitting station stops sending the frame data and sends a 32-bit "jam sequence
The sequence jam is transmitted to ensure that the length of the collision is sufficient to be noticed by the other transmitting stations
After sending the jam sequence the transmitting station waits a random period of time
This process is called "backoff"
How?

24. Collision Behavior II
If repeated collisions occur, then transmission is repeated
But the random delay is increased with each attempt
This process repeats until a station transmits a frame without collision
Once a station successfully transmits a frame, it clears the collision counter it uses to increase the backoff time after each repeated collision

25. Transmission Flow Chart

26. Exponential Backoff I
If a deterministic delay, that is a fixed delay for each machine, is used after a collision then collisions will occur again in lockstep
If a random delay (with fixed mean) is applied by each machine then
Few senders  needless waiting
Too many senders  too many collisions
Goal: adapt retransmission attempts to estimated current load
heavy load: random wait will be longer

27. Exponential Backoff II
Exponentially increasing random delay
Infer senders from # of collisions
More senders  increase wait time
First collision: choose K from {0,1}; delay is K x 512 bit transmission times
After second collision: choose K from {0,1,2,3}…
After ten or more collisions, choose K from {0,1,2,3,4,…,1023}
What is the
formula for
the range of K?

28. Ethernet Hardware

29. Repeaters
A repeater is an amplifier used to expand the maximal distance of an Ethernet-LAN segment
Regenerate signals on the receiving port, amplify them, and sent these signals to all connected net segments
No buffering, just a short delay
Local repeaters directly connect two coaxial segments
Repeater

30. Network Devices I
Ethernet devices are connected to one another through intermediary devices in order to form networks
There are several classes of these network devices, but they primarily fall into one of four categories:
Hubs – the most basic device, it is a repeater, simply copying data coming in on one of its ports as the data outgoing on all of its other ports
Switches – A hub structure becomes ineffective for large networks simply because it copies redundant data.
A switch is essentially a smart hub, only providing relevant data at a destination port

31. Network Devices II
Routers – At a higher level of complexity, the routers act as switches between networks
Routers are different from switches in that they connect networks together, whereas switches are used to connect devices together on a local LAN
Gateways – At even a higher level of complexity are the gateways
They are used to interconnect networks at a higher level by mapping addresses from one network to another
They are also used to perform the required protocol conversions from one network to another