Rechnernetze und -Organisation Michael Hutter Karl C. Posch.

Transkript

1 und -Organisation 2010 Michael Hutter Karl C. Posch 1 Classification - Local Area Networks (LAN) - Wide Area Networks (WAN) - Personal Area Networks (PAN) 2 1

2 Overview - Motivation - The Basics of Computer Networks - Network Topologies - Cables and Connectors - Factors that Impact the Network Performance - Components of a Typical Network Interface Card - How is Data Transmitted over the Network? - How to avoid Collisions? - The Ethernet Frame - Thinking in Layers The OSI Model - Network Devices in LANs - The Hub and the Switch 3 Motivation Computer networks are sets of computers that are linked together Communication Resource sharing Achieving connectivity Links and nodes Unidirectional vs. bidirectional connections Point-to-point vs. multipoint (multiple access) connection Node addressing: unicast, broadcast, and multicast 4 2

3 Switching Modes Circuit-Switched Networks Fixed connection is established before communication Resources are reserved during communication Delay during set-up, no delay and overhead during transfer Constant bandwidth Continuous data stream E.g. Telephone lines, mobile-telephone networks Packet-Switched Networks Data is divided into packets Packets can take different paths (no sequential arrival order) No connection set-up, no resource reservation Delay: packet-header overhead, packet processing delay, router store-and-forward delay Better bandwidth utilization E.g. LAN, Internet Network Topologies Star Nodes are connected to a central device Easy structure, good performance (with switches) Network gets broken if central device fails Bus Nodes are connected to a single cable Ethernet: 50Ω terminator necessary Each node listens to every packet transmitted Collisions can occur due to multiple access Ring Each node connects to two other nodes Data travels from node to node (one pathway) Large networks possible (nodes act as repeaters) Network can break if one single node fails Others: Line, Tree, Mashed, 6 3

4 Cables and Connectors CAT-5 Unshielded twisted-pair (UTP) cable 4 pairs of wires (send...green, receive orange, ) Full-duplex transmission possible RJ-45 connector Patch vs. crossover (EIA/TIA-568A/B) Coaxial Shielded cable with only 1 copper wire Half-duplex transmission (collisions possible) Bayonet Neill Concelman (BNC) connector, T-connector, terminator Fibre optics Transmission of light through a center glass core Monomode vs. multimode cable Larger distances possible, low signal loss, high bandwidths 7 Network Performance Bandwidth (throughput) Number of bits that can be transmitted over the network in a certain period of time Measured in bits per second (bps or bit/s) Link bandwidth (constant bandwidth) Physical links have a deterministic value, e.g. 10 Gbps End-to-end bandwidth (variable bandwidth) Bandwidth depends on network delay, traffic, congestion, protocol overhead, processing speed of network devices, Latency (delay) How long does it take to transmit a message from one node to another Measured in time (e.g. milliseconds) Latency = propagation delay + transmit delay + queuing delay Round Trip Time (RTT) is the time that takes to transmit a message from sender to receiver and back to the sender. 8 4

5 Network Interface Cards (NIC) Physical Layer (PHY) Coding of signals Media Access Controller (MAC) Address handling (MAC address) Multiple-access control (CDMA/CD) Controller Microcontroller, FPGA, or ASIC Send and Receive Buffer (TX/RX) First-In-First-Out (FIFO) approach I/O-Bus Interface (e.g. PCI) Read/Write to host memory (DMA, PIO) Interrupt handling or polling µc/ FPGA Networks need both hardware and software support RAM RX FIFO PHY Ethernet PCI MAC TX FIFO 9 How is Data Transmitted over the Network? Signal Coding - Data is encoded before transmission over the network - Ethernet uses Manchester encoding Manchester Encoding Each bit has at least one signal transition Clock signal can be recovered Simple XOR gate in hardware Data Clock XOR Manchester Encoding How to synchronize sender and receiver? Send clock signal in a separated line (too expensive?) Bitstuffing: embed clock-signal information in transmitted data (e.g. USB) Use of a preamble, e.g. Ethernet Data Clock NRZ NRZI Manchester

6 Ethernet - Collision Handling Collisions occur when multiple nodes try to send data over one common network connection Carrier Sense Multiple Access / Collision Detection - CSMA/CD is a mechanism to handle collisions in networks - Used in Ethernet (standardized in IEEE 802.3)? Bus 11 Carrier Sense Multiple Access / Collision Detection 1. Listen ( Carrier Sense ) - The node tests if it can send data over the cable - If no other node is sending, goto step 2 else goto step Send data and sense cable for collisions - Succeeded: deliver data to the higher-layer application, goto step 5. - Collision occurred: abort transmission, send a jam signal, and goto step 3. Case 1: Case 2: Succeeded Collision occurred 12 6

7 Carrier Sense Multiple Access / Collision Detection 3. Wait a certain amount of time - Choose a random delay (exponential backoff time) and restart (goto step 1) - Goto step 4 if number of tries exceeds a maximum. 4. Abort transmission - Inform higher-layer application that a transmission error occurred. 5. End. Z Z Z 13 Error Detection 1. Send two complete copies of the data - Very inefficient (n-bit redundancy for a n-bit message) - Faulty bits in both copies are undetectable (rather poor error-detection method) 2. Parity Bits - One extra bit is added to the data block - Bit corresponds to the sum of all ones ( 1 ) in the block 3. Cyclic Redundancy Check CRC - Used in nearly all link-layer protocols (Ethernet, IP, TCP, ) - CRC calculation over binary field GF(2) using a polynomial long division - Can be easily implemented in hardware (LFSR with XOR gates, ) 4. Error Correcting Codes (ECC) - Detection and also correction of faulty bits - Forward Error Correction (FEC) 14 7

8 The Ethernet Frame Encapsulates an IP Packet, Ethernet Frame IP Packet TCP Segment Data (UDP Datagram) 15 The Ethernet Frame SFD Präambel Ziel-MAC Quell-MAC Typ Daten CRC IP-Header Ethernet Block >= Ziel-IP Quell-IP Ziel- Quell- TCP TCP IP Paket TCP-Header+ Daten TCP Paket TCP- CRC IP- CRC 16 8

9 Thinking in Layers - The OSI Model 7 6 Layer Application Presentation Units Data Protocols HTTP, FTP, SMTP, Telnet, Session Transport Network Data Link Physical Segment/ Datagram Packet Frame Bit TCP, UDP IP, ICMP Ethernet, Token Ring, FDDI, 17 The Hub Connects nodes in a network in a physical star topology All nodes are connected to the central hub It acts like a simple multiport repeater (signal amplification, filtering,...) Can only handle electronic signals and makes no decoding/demodulation of the signals (thus it does not understand frames, packets, segments, ) Connects nodes in a network in a logical bus topology Network cable as a common resource (bandwidth is shared between the nodes) Communication is listen by every node (low performance in large networks) Point-to-multipoint connection Only half-duplex communication where collisions can occur Hubs work at the physical layer (layer 1) of the OSI model Hub 18 9

10 The Switch Nodes are connected in a physical and logical star topology Switches read out the MAC address of Ethernet frames to switch packets to the appropriate destination Point-to-point connection between nodes (dedicated bandwidth for each device) Allows full-duplex communication with no collisions Switches maintain a source-address table (SAT) Stores the MAC address of the connected device for each port MAC addresses of new devices are learned through traffic Broadcasting of frames if MAC is unknown Switches work at the data-link layer (layer 2) of the OSI model Switch 19 Destination Source = 00:1F:F3:45:3D:73 00:1F:F3:87:E2:4F 00:1F:F3:87:E2:4F 00:1F:F3:34:5A:27 Network Example Hub 00:1F:F3:9A:12:7C Switch 00:1F:F3:45:3D:73 00:1F:F3:19:BA:

11 00:1F:F3:87:E2:4F Network Example Hub 00:1F:F3:9A:12:7C 00:1F:F3:34:5A:27 Switch 00:1F:F3:45:3D:73 00:1F:F3:19:BA: :1F:F3:87:E2:4F Network Example Hub 00:1F:F3:9A:12:7C 00:1F:F3:34:5A:27 Destination Port 00:1F:F3:19:BA: :1F:F3:45:3D:73 Port 2 source-address table Switch 00:1F:F3:45:3D:73 00:1F:F3:19:BA:

12 Network Packet Analyzer - Wireshark 23 Summary Network data is encapsulated in (Ethernet) frames There exist different network layers Hubs operate with electrical signals in the physical layer Switches read out the MAC address of frames to switch received data Hubs and Switches usually don t have an own MAC address 24 12

13 What should you know by now? Understand the terms; understand the connections between them; be able to operate with them: Computer Networks Circuit switching Packet switching Network topologies Network Cables Bandwidth Latency Round Trip Time NIC Components Manchester Encoding Sender/Receiver Synchronization Bitstuffing Collision Handling CSMA/CD Exponential Backoff Error Detection Methods Ethernet OSI model Hub Switch Hub vs. Switch Source Address Table 25 13