Mạng máy tính 1 - Lecture 3: Networking technologies

Computer Networks 1 (Mạng Máy Tính 1) Lectured by: Dr. Phạm Trần Vũ Lecture 3: Networking Technologies Reference: Chapter 4 - “Computer Networks”, Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003. Faculty of Computer Science and Engineering - HCMUT Content Channel Allocation Problem Multiple Access Protocols Faculty of Computer Science and Engineering - HCMUT Two types of transmission technology Point-to-Point Link Broadcast Link Faculty of Computer Science and Engineering - HCMUT Channel Allocation Problem Who's first? Faculty of Computer Science and Engineering - HCMUT Channel Allocation Problem Static Method Dynamic Method Faculty of Computer Science and Engineering - HCMUT Static Channel Allocation in LANs and MANs KEY: Each user is assigned with a equal-portion of the bandwidth Plus: 1. Simple 2. No interference Minus: 1. Ineffective bandwidth utilization 2. Some users will be denied in high traffic Faculty of Computer Science and Engineering - HCMUT Dynamic Channel Allocation in LANs and MANs 1. Station Model 2. Single Channel Assumptions 3. Collision Assumption 4. Continuous Time vs Slotted Time 5. Carrier Sense vs No Carrier Sense Key assumptions Faculty of Computer Science and Engineering - HCMUT Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Faculty of Computer Science and Engineering - HCMUT ALOHA Idea: Users can transmit whenever they have data to send Two types of ALOHA: Pure ALOHA Slotted ALOHA The main difference between them is time and time and time Aloha!! Faculty of Computer Science and Engineering - HCMUT Pure ALOHA continuous time no global time synchronization frame Send & then wait for collision collision? end n o wait a random time yes Faculty of Computer Science and Engineering - HCMUT Pure ALOHA – vulnerable period t – frame time, to – starting time Faculty of Computer Science and Engineering - HCMUT Slotted ALOHA 1. Time is divided to equal intervals (slots) 2. Need time synchronization 3. Frames can only be transmitted at starts of time slots Faculty of Computer Science and Engineering - HCMUT Slotted ALOHA – who's next? Faculty of Computer Science and Engineering - HCMUT Channel utilization Faculty of Computer Science and Engineering - HCMUT CSMA with Collision Detection (CSMA/CD) Idea: A station stop transmitting its frame immediately after a collision is detected to save time and bandwidth This is the basis of Ethernet LAN How? But: half-duplex Faculty of Computer Science and Engineering - HCMUT Propagation Delay & CD Faculty of Computer Science and Engineering - HCMUT Collision-free Protocols The basic bit-map control Idea: At the contention period, all station announce their needs to transmit. And then at the transmission period all registered stations take turn to send their frames. Drawback: suffer n bits delay in contention period Faculty of Computer Science and Engineering - HCMUT Collision-Free Protocols The binary countdown protocol Unique binary station addresses with the same length Rules: •Compete by “OR” with other bits in the same order •If there is a “0 to 1 transformation” in the result → give up Faculty of Computer Science and Engineering - HCMUT The Adaptive Tree Walk Protocol Distributed Depth-first tree walk Faculty of Computer Science and Engineering - HCMUT The Adaptive Tree Walk Protocol - improvement At heavy traffic, skip nodes father nodes 1,2,3,... Number of nodes at level i: 2^i Assume that, p is number of ready stations. Then k = p/2^i is number of ready stations per node at level i Optimal: k = 1 or p/2^i= 1 from this i = log2(p) Faculty of Computer Science and Engineering - HCMUT Wavelength Division MA Protocols (WDMA) To allow multiple transmissions the spectrum is divided into channels (wavelength bands) Total internal reflection: n1>n2 i1>ilimit Faculty of Computer Science and Engineering - HCMUT WDMA Each station is assigned two channels: control & data and two transmitters and two receivers 1. A fixed-wavelength receiver for listening to its own control channel. 2. A fixed-wavelength transmitter for outputting data frames. 3. A tunable transmitter for sending on other stations' control channels. 4. A tunable receiver for selecting a data transmitter to listen to. Faculty of Computer Science and Engineering - HCMUT WDMA Require global time synchronization for both channels Faculty of Computer Science and Engineering - HCMUT Wireless LAN Protocols CSMA is not appropriate Interference at the receiver, not at the sender Hidden station problem Exposed station problem Faculty of Computer Science and Engineering - HCMUT Wireless LAN Protocols - MACA A sends RTS to B and begins transmission only after receiving CTS reply from B Faculty of Computer Science and Engineering - HCMUT Wireless LAN Protocols - MACA 1. C doesn't see CTS and thus is free to send its RTS 2. C and B may want to send RTS at the same time so cause collision. Faculty of Computer Science and Engineering - HCMUT Wireless LAN Protocols - MACAW Improvements: 1. ACK frame after each successful data frame 2. Use CSMA for RTS frame 29 Ethernet  Ethernet Cabling  Manchester Encoding  The Ethernet MAC Sublayer Protocol  Switched Ethernet  Fast Ethernet  Gigabit Ethernet 30 Ethernet Cabling The most common kinds of Ethernet cabling. 31 Ethernet Cabling (2) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T. 32 Ethernet Cabling (3) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. 33 Using voltage to encode binary code:  0 volts ~ 0 bit and 5 volts ~ 1 bit 0001000 = 10000000 or 01000000, because the difference between an idle sender (0 volts) and a 0 bit (0 volts) is ambiguous  -1 and +1? out of synchronization, long sequence of 0 or 1 Manchester Encoding 34 Using two voltages to encode one bit:  1 ~ high – low; 0 ~ low - high 010 = (-1,1)(1,-1)(-1,1) Plus: every bit has a transition in the middle → determine bit boundary & sync Minus: double bandwidth, 10Mbs ~ 20 mil changes per second Manchester Encoding 35 Differential Manchester encoding 1 bit ~ absence;0 bit ~ presence of a transition at the start of the interval. In both cases, there is a transition in the middle. Plus: complex equipment Minus: better noise immunity. (the 802.5 token ring) 36 Manchester Encoding (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. 37 Ethernet MAC Sublayer Protocol s 38 Ethernet MAC Sublayer Protocol Unicast: 1 receiver Multicast: group of receivers Broadcast: all 281,474,976,710,656 possible MAC addresses 39 802.3 Ethernet frame structure 40 802.3 Ethernet frame structure Header: source and destination MAC address; Ethertype protocol identifier field; optional IEEE 802.1Q VLAN tag Frame check sequence: 32-bit CRC (cyclic redundancy check) 41 In addition to there being a maximum frame length, there is also a minimum frame length. While a data field of 0 bytes is sometimes useful, it causes a problem. When a transceiver detects a collision, it truncates the current frame, which means that stray bits and pieces of frames appear on the cable all the time. To make it easier to distinguish valid frames from garbage, Ethernet requires that valid frames must be 42 Switched Ethernet A simple example of switched Ethernet. 43 Fast Ethernet 802.3u vs 802.12 802.3u:  backward compatible  unforeseen problems  technology change period Ideas:  keep all the old stuffs  reduce the bit time from 100 nsec to 10 nsec 44 Fast Ethernet The original fast Ethernet cabling. 45 Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet. 46 Gigabit Ethernet (2) Gigabit Ethernet cabling. 47 Wireless LAN Protocols  Hidden station problem  Exposed station problem A wireless LAN. (a) A transmitting B. (b) B transmitting A. 48 The 802.11 MAC Sublayer Protocol (1) The use of virtual channel sensing using CSMA/CA 49 The 802.11 MAC Sublayer Protocol (2) A fragment burst. 50 The 802.11 Frame Structure The 802.11 data frame. 51 802.11 Services • Association • Disassociation • Reassociation • Distribution • Integration Distribution Services 52 802.11 Services • Authentication • Deauthentication • Privacy • Data Delivery Intracell Services