CS2302- COMPUTER NETWORKS RAJALAKSHMI ENGINEERING COLLEGE DEPARTMENT OF INFORMATION TECHNOLOGY
UNIT I INTRODUCTION: • A computer network is a group of interconnected computers • A collection of computers and devices connected to each other. • Allows computers to communicate with each other and share resources and information.
Building a Network • To build a network Identify the set of constraints and requirements based on Application programmer Network designer Network provider
Requirements: • Connectivity • point to point or multiple access • Links - physical medium • Nodes,clouds - computer • Switched Network • Circuit Switched • Packet Switched • Uses store and forward • Establishes dedicated circuit • More efficient in working
Routing • Provides Systematic procedure for forwarding messages • Unicasting • Multicasting • Cost effective Resources sharing How system resource is shared effectively by multiple users multiplexing
Multiplexing methods • STDM - Synchronous time division multiplexing • FDM - Frequency division multiplexing
Network Architecture • Provides a general, effective, fair, and robust connectivity of computers • Provides a blueprint • Types • OSI Architecture • Internet Architecture
OSI ARCHITECTURE • Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984 OSI model defines the communications process into Layers Provides a standards for communication in the network Primary architectural model for inter-computing and Inter networking communications. network communication protocols have a structure based on OSI Model
Internet Architecture • TCP/IP Architecture • Four Layer model • TCP,UDP,FTP,HTTP,SMTP Protocols used • Internet Protocol Graph
Direct Links: Outline • Physical Layer • Link technologies • Encoding • Link Layer • Framing • Error Detection • Reliable Transmission (ARQ protocols) • Medium Access Control: • Existing protocols: Ethernet, Token Rings, Wireless
Link Technologies • Cables: • Cat 5 twisted pair, 10-100Mbps, 100m • Thin-net coax, 10-100Mbps, 200m • Thick-net coax, 10-100Mbps, 500m • Fiber, 100Mbps-2.4Gbps, 2-40km • Leased Lines: • Copper based: T1 (1.544Mbps), T3 (44.736Mbps) • Optical fiber: STS-1 (51.84Mbps), STS-N (N*51.84Mbps)
Link Technologies • Last-Mile Links: • POTS (56Kbps), ISDN (2*64Kbps) • xDSL: ADSL (16-640Kbps, 1.554-8.448Mbps), VDSL (12.96Mbps-55.2Mbps) • CATV: 40Mbps downstream, 20Mbps upstream • Wireless Links: Cellular, Satellite, Wireless Local Loop
FRAMING • An efficient data transmission technique • It is a message forwarding system in which data packets, called frames, are passed from one or many start-points to one
Approaches • Byte oriented Protocol(PPP) BISYNC Binary Synchronous Communication DDCMP Digital Data Communication Message Protocol • Bit oriented Protocol(HDLC) • Clock based Framing(SONET)
Byte oriented Protocol(PPP) BISYNC FRAME FORMAT PPP Frame Format
Bit Oriented Protocol(HDLC) • Collection of Bits 1.HDLC High-Level Data Link Control 2.Closed Based Framing(SONET) Synchronous Optical Network
HDLC Frame Format Bit Stufffing After 5 consecutive 1s insert 0 Next bit is 0 – stuffed removed Next bit is 1 –end of frame or erorr
Closed Based Framing(SONET) • STS-1 Frame 9 rows of 90 byte each First 3 byte for overhead rest contains data Payload bytes scrambled- exclusive OR Supports Multiplexing Payloads 9 rows 90 columuns
ERROR DETECTION • Detecting Errors In Transmission Electrical Interference, thermal noise Approaches Two Dimensional Parity Internet Checksum Algorithm Cyclic Redundancy Check
Transmission sent using even parity: • A wants to transmit: 1001 • A computes parity bit value: 1^0^0^1 = 0 • A adds parity bit and sends: 10010 • B receives: 10010 B computes parity: 1^0^0^1^0 = 0 • B reports correct transmission after observing expected even result.
Transmission sent using odd parity: • A wants to transmit: 1001 • A computes parity bit value: ~(1^0^0^1) = 1 • A adds parity bit and sends: 10011 • B receives: 10011 • B computes overall parity: 1^0^0^1^1 = 1 • B reports correct transmission after observing expected odd result.
Reliable Transmission Deliver Frames Reliably Accomplished by Acknowledgements and Timeouts ARQ-Automatic Repeat Request Mechanism: Stop and Wait Sliding Window Concurrent Logical Channels
Stop And Wait ARQ • The source station transmits a single frame and then waits for an acknowledgement (ACK). • Data frames cannot be sent until the destination station’s reply arrives at the source station. • It discards the frame and sends a negative acknowledgement (NAK) back to the sender • causes the source to retransmit the damaged frame in case of error
Stop & wait sequence numbers Sender Receiver Sender Receiver Sender Receiver Frame 0 Frame 0 Frame 0 imeout imeout ACK 0 ACK 0 ACK 0 T T Frame 0 Frame 1 Frame 0 imeout ACK 0 imeout ACK 1 T ACK 0 T Frame 0 (c) (d) ACK 0 (e) • Simple sequence numbers enable the client to discard duplicate copies of the same frame • Stop & wait allows one outstanding frame, requires two distinct sequence numbers
Sliding Window • bi-directional data transmission protocol used in the data link layer (OSI model) as well as in TCP • It is used to keep a record of the frame sequences sent • respective acknowledgements received by both the users.
£ SWS … … LAR LFS Sliding Window: Sender • Assign sequence number to each frame (SeqNum) • Maintain three state variables: • send window size (SWS) • last acknowledgment received (LAR) • last frame sent (LFS) • Maintain invariant: LFS - LAR <= SWS • Advance LAR when ACK arrives • Buffer up to SWS frames
Sequence Number Space • SeqNum field is finite; sequence numbers wrap around • Sequence number space must be larger then number of outstanding frames • SWS <= MaxSeqNum-1 is not sufficient • suppose 3-bit SeqNum field (0..7) • SWS=RWS=7 • sender transmit frames 0..6 • arrive successfully, but ACKs lost • sender retransmits 0..6 • receiver expecting 7, 0..5, but receives the original incarnation of 0..5 • SWS < (MaxSeqNum+1)/2 is correct rule • Intuitively, SeqNum “slides” between two halves of sequence number space
£ RWS … … LFR LFA Sliding Window: Receiver • Maintain three state variables • receive window size (RWS) • largest frame acceptable (LFA) • last frame received (LFR) • Maintain invariant: LFA - LFR <= RWS • Frame SeqNum arrives: • if LFR < SeqNum < = LFA accept • if SeqNum < = LFR or SeqNum > LFA discarded • Send cumulative ACKs – send ACK for largest frame such that all frames less than this have been received
UNIT II LAN Technology • LAN (Local Area Network) refers to a group of computers interconnected into a network • Objective: • they are able to communicate, exchange information and share resources (e.g. printers, application programs, database etc). • the same computer resources can be used by multiple users in the network, regardless of the physical location of the resources.
LAN Architecture Describes the way in which the components in a Local Area Network are connected LAN Topologies: Star Ring Bus Tree
Star • All stations are connected by cable (or wireless) to a central point, such as hub or a switch. • central node is operating in a broadcast fashion such as a Hub • transmission of a frame from one station to the node is retransmitted on all of the outgoing links.
Ring All nodes on the LAN are connected in a loop and their Network Interface Cards (NIC) are working as repeaters. No starting or ending point. Each node will repeat any signal that is on the network regardless its destination. The destination station recognizes its address and copies the frame into a local buffer. The frame continues to circulate until it returns to the source station, where it is removed. Example:Token Ring (IEEE 802.5) FDDI (IEEE 802.6) another protocol used in the
Bus • All nodes on the LAN are connected by one linear cable, which is called the shared medium. • Every node on this cable segment sees transmissions from every other station on the same segment. • At each end of the bus is a terminator, which absorbs any signal, removing it from the bus. • This medium cable apparently is the single point of failure. • Example:Ethernet (IEEE 802.3)
Tree • Is a logical extension of the bus topology. • The transmission medium is a branching cable • no closed loops. • The tree layout begins at a point called the head-end • one or more cables start, and each of these may have branches. • The branches in turn may have additional branches to allow quite complex layouts.
Token Ring • All stations are connected in a ring and each station can directly hear transmissions only from its immediate neighbor. • Permission to transmit is granted by a message (token) that circulates around the ring. • Token Ring as defined in IEEE 802.5 is originated from the IBM Token Ring LAN technologies. • Token-passing networks move a small frame, called a token • Possession of the token grants the right to transmit. • The information frame circulates the ring until it reaches the intended destination station, which copies the information for further processing. • The information frame continues to circle the ring and is finally removed when it reaches the sending station. • The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination.
Ehernet • local-area network (LAN) covered by the IEEE 802.3. • two modes of operation: • half-duplex • full-duplex modes. .
Three basic elements : 1. the physical medium used to carry Ethernet signals between computers, 2. a set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel, 3. an Ethernet frame that consists of a standardized set of bits used to carry data over the system
Wireless • The process by which the radio waves are propagated through air and transmits data • Wireless technologies are differentiated by : • Protocol • Connection type—Point-to-Point (P2P) • Spectrum—Licensed or unlicensed
Types • Infrared Wireless Transmission • Tranmission of data signals using infrared-light waves • Microwave Radio • sends data over long distances (regions, states, countries) at up to 2 megabits per second (AM/FM Radio) • Communications Satellites • microwave relay stations in orbit around the earth.