Networks Lecture 7

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

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

Network Manager Headaches Ethernet Ethernet Hardware Outline Network Manager Headaches Ethernet Ethernet Hardware

Network Manager Headaches

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.

Ethernet

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

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

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

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

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

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

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

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

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: 10101011 - 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

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

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

Manufacturer Addresses Examples of Manufacturer IDs Cisco : 00-00-0C- : 00-60-09- 3Com : 00-60-8C- : 00-60-08- Xircom : 00-80-C7- IBM : 08-00-5A Sun : 08-00-20- Nokia : 00-40-43-

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

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

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!!!

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?

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

Transmission Flow Chart

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

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?

Ethernet Hardware

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

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

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