Computer Communications Sunggu Lee EE Dept., POSTECH Sep. 7, 2006 Analog and Digital Signals Analog: real-world signal Digital: “digitized” version of original analog signal - represented as sequence of binary bits - e.g., 12, 7, 7, 6, 10, 14, 15, 15, … 11000111011101101010111011111111… Digital Signal Representation Bit Representation (Logic 0 and 1) in Wireless Communication Channel Typically based on electromagnetic (EM) waves Changes in electrical current flow cause EM waves Example methods Sine wave frequencies: high frequency = 1, low frequency = 0 Sine wave phases: 0 degree = 1, 90 degree = 0 Bit Representation (Logic 0 and 1) in Wired Communication Channel Optical: no light = 0, light = 1 Current: no current = 0, positive current = 1 Voltage Positive logic: 0 = low voltage, 1 = high voltage Noise margins, voltage ranges used to permit small variations in input & output voltage values LSTTL: for inputs, „0‟ = 0.0-0.8V and „1‟ = 2.0-5.0V for outputs, „0‟ = 0.0-0.5V and „1‟ = 2.7-5.0V Wireless Communication Communication “medium” (thing through which the data is communicated) is shared Each “communication connection” is referred to as a “channel” Methods for sharing the communication medium Time division multiple access (TDMA) Frequency division multiple access (FDMA) Code division multiple access (CDMA) Many different varieties Example: use a frequency hopping code Change communication frequencies in a predetermined pattern TDMA [Agrawal 2006] FDMA [Agrawal 2006] CDMA [Agrawal 2006] Frequency Hopping [Agrawal 2006] Binary Codes Meaning of a sequence of binary bits is dependent on the interpretation used Example: 01101111 unsigned integer = 104, character = „o‟ part of a binary program part of a video or audio data stream A “service request” command from a client PC A “service response” reply from a server PC Other Example: 01001000 01100101 01101100 01101100 01101111 When interpreted as character string Hello „H‟ = 01001000, „e‟ = 01100101, „l‟ = 01101100, … Packetization of Data For transmission of a stream of data bits (message), the message is typically partitioned into “packets” A packet consist of Packet header (destination, routing info, etc.) Data payload (the bits of the message) Check bits (redundant bits used to check for errors in the received packet) Communication Protocols For successful transmission/receipt of a packet, the transmitter and receiver must agree on a “communication protocol” Set of rules on how the packet is interpreted How to sample the bits of the packet Signaling method Synchronization of the transmitter/receiver How to determine which parts of the packet are the packet header (destination info, etc.), data payload, check bits, etc. How to interpret the bits of the data payload Integer, floating-point, character string, JPEG picture, etc. Computer Communication Models and Communication Protocol Suites Most commonly used reference base communication model is the Open Systems Interconnection (OSI) model Standardized by the International Organization for Standardization (ISO) Most common implementation of the OSI model is a set of protocols referred to as the TCP/IP protocol suite (or stack) TCP = Transmission Control Protocol IP = Internet Protocol Communication Protocols OSI Model TCP/IP Protocol Suite L7 Application Application TEL L6 Presentation Presentation SMTP FTP NET DNS SNMP NFS HTTP RPC L5 Session Session L4 Transport Transport TCP UDP L3 Network Network Internet Internet ICMP IGMP Protocol Protocol IP ARP RARP (IP) (IP) L2 Data Link Data Link Protocol for Protocol for Token Underlying Underlying Ethernet Ring ATM ... L1 Physical Network Network Physical [Forouzan 2003] Computer Communication Example Send picture image and message to friend sender receiver Hello! Microsoft Outlook Netscape Messenger Hello! Hello! system software system software Communication Channel Layer-by-Layer (OSI Model) View sender receiver MS Outlook Netscape L7 through Hello! Hello! L5 Encoded Data Encoded Data L4 H4 Data1 H4 Data2 H4 Data3 H4 Data1 H4 Data2 H4 Data3 L3 H3 Data1 H3 Other H3 Data2 H3 Data1 H3 Other H3 Data2 L2 H2 Data11 H2 Other H2 Data12 H2 Data11 H2 Other H2 Data12 L1 001100110000111100001111000 001100110000111100001111000 Activities Required (Sender Side) Edit message and enter “send” MS Outlook Express Convert into sequence of bits Tags must be inserted so that original message can be reconstructed at destination E.g., “string” 01001000 … “JPEG” 110011101010 … “end” 11001100100010 … 101011111100 … 01111110 Encrypt message if necessary for privacy Compress if necessary IP address Partition into packets of fixed maximum size Attach header information (Packet ID, destination, checksum, …) Intersperse with packets from messages created by other applications On first link of path, Partition each packet into fixed-size frames (with headers) Send each frame out onto the network Activities Required on Network Route each packet to its destination During each “hop” of the path Send signals back and forth to coordinate the sending and receiving of the stream of bits corresponding to a frame Handshaking Check each frame for errors Request retransmission in the case of errors Arrange received frames into the proper order Wait for all frames of the packet to be received Once each packet reaches its destination node,IP address Store packet in a memory buffer at destination Send signal to destination CPU to inform it of the arrival of the new packet Port Number Activities at Destination Node Receive packets Check each packet for errors and request retransmission in the case of errors Arrange received packets into the proper order Once all packets have been received, form a complete message Decompress if necessary Decrypt if necessary Check for errors Use tags in the bit stream to reconstruct the message Show message to user using email tool (e.g., MS Outlook Express) Network Addresses IP (Internet Protocol) address Address used to identify a computing node on the internet Network layer (L3) address E.g., 126.96.36.199 (Look up “properties” on “TCP/IP” on “Network”) MAC (Medium Access Control) address Address used to identify a LAN card – cannot be changed Data link layer (L2) address E.g., abcd1234 (Enter “ipconfig /all” from MS Windows “cmd” window) Port address Address used to identify a network interface point for an application prog. Corresponds to a memory buffer Send a message - write to a memory buffer on a remote computer Receive a message – read from a memory buffer on the local computer Example: 39 (for FTP), 3000 (for a user-defined port) Connection-Oriented and Connectionless Networking Connection-oriented networking Uses a specific network path that is established for the duration of a connection Three phases: connection establishment, data transfer, connection termination Main advantage: reliable communication Main implementation method: TCP (transfer control protocol) Connectionless networking Finds a new path for each packet sent Main advantage: fast communication for short messages Main implementation method: UDP (user datagram protocol) Communication Performance Parameters (1) Throughput (데이터 처리량) Actual number of bits transmitted per second Note 1: different from latency (지연시간) Note 2: different from bandwidth (대역폭) Most important communication performance parameter Typical measurement method Send a data file from a source node to a destination node Record the time t1 when the first byte of the data is received Record the time t2 when the last byte of the data is received Divide amount of data received by (t2 – t1) Note: Mbps = mega-bits-per-second (not bytes) Communication Performance Parameters (2) Bandwidth Maximum number of bits that can be transmitted per second Note 1: different from latency (지연 시간) Note 2: different from throughput (데이터 처리량) Measures performance of network only (not the computer hardware or software) Typical measurement method Difficult to measure since effects of small data amounts, software and hardware at source and destination nodes must be removed The “rated” figure stated in the specifications for the relevant communication protocol is most commonly used E.g., 11 Mbps for IEEE 802.11b Communication Performance Parameters (3) Latency Time required for the first byte of a message to be transferred from the source to the destination node Should include software processing time Typical measurement method At time t1, source node sends a very small message to destination node Destination node receives message and sends it back to the source node Source node receives message and records the time t2 One-way communication latency is (t2 – t1) / 2 Why can‟t we measure latency directly (record time t3 at destination and measure latency as t3 – t1)? Communication Performance Parameters (4) Other parameters also sometimes measured Example: Packet loss rate Number of packets dropped by the network Most relevant to wireless networks References Behrouz A. Forouzan, TCP/IP Protocol Suite, 2nd Ed., McGraw-Hill, Boston, 2003. D. P. Agrawal and Q.-A. Zeng, Introduction to Wireless and Mobile Systems, 2nd Ed., Nelson, Toronto, 2006.