Chapter 4. The Medium Access Control Sublayer. Points and Questions to Consider. Multiple Access Protocols. The Channel Allocation Problem.

Transcription

1 Dynamic Channel Allocation in LANs and MANs Chapter 4 The Medium Access Control Sublayer 1. Station Model. 2. Single Channel Assumption. 3. Collision Assumption. 4. (a) Continuous Time. (b) Slotted Time. 5. (a) Carrier Sense. (b) No Carrier Sense. 1 4 Points and Questions to Consider a) Broadcast networks Key issue: medium access control (MAC) b) What is the purpose of the MAC protocol? c) What are the assumptions and design issues? d) Solutions Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Limited-Contention Protocols Wireless LAN Protocols e) Important notes MAC protocols typically used in LANs and ad hoc networks WANs (except satellite) used point-to-point links 2 5 The Channel Allocation Problem Pure ALOHA Static Channel Allocation (e.g., FDM) in LANs and MANs Advantages: simple and efficient Disadvantages: large number of users may result in denial of service for lack of bandwidth bursty traffic results in wasted bandwidth Dynamic Channel Allocation in LANs and MANs In pure ALOHA, frames are transmitted at completely arbitrary times. 3 Question: What fraction of all transmitted frames escape collisions under these chaotic circumstances? 6 1

2 Pure ALOHA (2) Carrier Sense Multiple Access Protocols a) 1-persistant CSMA Sense the carrier: listen to the channel to see if anyone is transmitting. If the channel is idle, begin transmission. If channel is busy, wait until channel becomes idle. When station becomes idle, transmit frame with probability 1. b) Non-persistant CSMA Sense the carrier. If the channel is idle, begin transmission. If the channel is busy, wait a random period of time and then repeat algorithm. Vulnerable period for the shaded frame. 7 c) P-persistant CSMA (for slotted channels) Sense the channel If the channel is idle, transmit with probability p. Defer until next time slot with probability q=(1-p). If channel is busy, wait until next time slot and apply algorithm again. 10 Slotted ALOHA Persistent and Nonpersistent CSMA a) A station cannot transmit whenever it wants b) It must wait until the beginning of the next slot. c) The vulnerable period is now half that of pure aloha. 8 Comparison of the channel utilization versus load for various random access protocols. 11 Performance of ALOHA CSMA with Collision Detection CSMA/CD CSMA/CD can be in one of three states: contention, transmission, or idle. Throughput versus offered traffic for ALOHA systems. 9 Vital question to consider: Suppose that two stations both begin transmitting at exactly time t 0. How long will it take them to realize that there has been a collision? The answer to this question is vital to determining the length of the contention period and hence what the delay and throughput will be. Hint: The minimum time to detect the collision is then just the time it takes the signal to propagate from one station to the other. 12 2

3 Collision-Free Protocols Limited-Contention Protocols The basic bit-map protocol. 13 Acquisition probability for a symmetric contention channel. 16 Collision-Free Protocols (2) Adaptive Tree Walk Protocol The binary countdown protocol. A dash indicates silence. 14 The tree for eight stations. 17 Contention vs. Collision-free Approach a) Performance measures Delay at low loads Channel efficiency at high loads b) Which approach is preferable at low loads? Why? Wireless LAN Protocols Hidden Station Problem Exposed Station Problem c) Which approach is preferable at high loads? Why? d) Can we combine the best properties of the contention and collision free protocols? That is, the protocol should: use a contention-based approach at low loads to provide low delay and, a collision free technique at high loads to improve channel efficiency at high loads. A wireless LAN. (a) A transmitting. (b) B transmitting

4 Wireless LAN Protocols (2) Ethernet Cabling (2) The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A. 19 Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T. 22 Ethernet Ethernet Cabling (4) Ethernet Cabling Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm Ethernet Performance Switched Ethernet Fast Ethernet Gigabit Ethernet IEEE 802.2: Logical Link Control Retrospective on Ethernet 20 Transition here indicates 0 Lack of a transition here indicates 1 (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. 23 Ethernet Cabling Ethernet MAC Sublayer Protocol The most common kinds of Ethernet cabling. Frame formats. (a) DIX Ethernet, (b) IEEE

5 Ethernet MAC Sublayer Protocol (2) Ethernet Performance Collision detection can take as long as 2 τ. Efficiency of Ethernet at 10 Mbps with 512-bit slot times Minimum Length of an Ethernet Frame Switched Ethernet a) Consider a Ethernet LAN with the following characteristics: 10-Mbps transmission speed Round trip propagation time for a 2500 meter cable = 50µsec Question: What is the minimum frame size? Solution: Minimum frame size = How fast can the station transmit * How long must it transmit Minimum frame size = transmission speed * round trip propagation delay b) What if the transmission speed increases? Decreases? c) What if the cable length increases? Decreases? A simple example of switched Ethernet The Binary Exponential Backoff Algorithm Fast Ethernet a) After a collision, time is divided into discrete slots Slot length = the worst-case round-trip propagation time (2τ). For Ethernet, slot time = 512 bit times or 51.2µsec b) After the first collision, each station waits 0 or 1 slot times before trying again. c) If two stations select the same random number, a collision occurs. d) After the second collision, each one picks either 0, 1, 2, or 3. e) If a third collision occurs, then the number of slots to wait is chosen at random from the interval 0 to f) In general after i collisions, a random number between 0 and 2 i 1. g) The interval is frozen at h) After 16 collisions, stations gives up 27 The original fast Ethernet cabling. 30 5

6 Gigabit Ethernet IEEE 802.2: Logical Link Control (a) A two-station Ethernet. (b) A multistation Ethernet. 31 (a) Position of LLC. (b) Protocol formats. 34 Gigabit Ethernet (2) Wireless LANs The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Services Gigabit Ethernet cabling Quiz #3 10/24/03 a) (5pts) When designing a medium access control protocol for a wireless network in which all communication takes place over a single channel, the hidden terminal problem must be addressed. Explain how this problem is addressed in the IEEE MAC-layer protocol. The Protocol Stack b) (6pts) A minimum frame size is required on networks using CSMA/CD such as Ethernet. Why? c) (8 pts) A particular LAN uses the CSMA/CD medium access control protocol. The network operates at 1Gbps over a 1km cable and has a minimum frame size of 10,000 bits. What is the one-way signal propagation time (in seconds) in the cable? d) (6pts) What is the purpose of the Network Allocation Vector used in the IEEE medium access control protocol? e) (5pts) Medium access control protocols can be generally classified as contentionbased or collision-free. Which approach is preferable at low loads? Why? Part of the protocol stack

7 The MAC Sublayer Protocol The MAC Sublayer Protocol (4) (a) The hidden station problem. (b) The exposed station problem. 37 Interframe spacing in The MAC Sublayer Protocol (2) The Frame Structure The use of virtual channel sensing using CSMA/CA. The data frame The MAC Sublayer Protocol (3) Services 1. Distribution Services: provided by base station and deal with station mobility as they enter and leave cells. A fragment burst. Association Disassociation Reassociation Distribution Integration

8 Handoff Practical Implementation of an AP Services Control Frames in MAC (1) Intracell Services: use after association has taken place Authentication Deauthentication Privacy Data Delivery System Architecture (1) Control Frames in MAC (2) a) Duration Field RTS Frame time in microseconds, required to transmit the next data frame, plus one CTS frame, plus one ACK frame, plus three SIFS (Short InterFrame Space) intervals. CTS Frame time in microseconds, the time obtained from RTS minus the time transmit CTS and one SIFS. ACK Frame If more fragment flag is 0: duration=0 Otherwise: the time obtained from previous frame minus the time to transmit ACK frame and one SIFS

9 Broadband Wireless The Physical Layer (2) Comparison of and The Protocol Stack The Physical Layer The MAC Sublayer Protocol The Frame Structure Frames and time slots for time division duplexing The Protocol Stack The MAC Sublayer Protocol Service Classes Constant bit rate service Real-time variable bit rate service Non-real-time variable bit rate service Best efforts service The Protocol Stack The Physical Layer The Frame Structure The transmission environment. (a) A generic frame. (b) A bandwidth request frame

10 Bluetooth The Bluetooth Protocol Stack Bluetooth Architecture Bluetooth Applications The Bluetooth Protocol Stack The Bluetooth Radio Layer The Bluetooth Baseband Layer The Bluetooth L2CAP Layer The Bluetooth Frame Structure The version of the Bluetooth protocol architecture Bluetooth Architecture The Bluetooth Frame Structure Two piconets can be connected to form a scatternet. A typical Bluetooth data frame Bluetooth Applications Data Link Layer Switching Bridges from 802.x to 802.y Local Internetworking Spanning Tree Bridges Remote Bridges Repeaters, Hubs, Bridges, Switches, Routers, Gateways Virtual LANs The Bluetooth profiles

11 Data Link Layer Switching Local Internetworking Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN. 61 A configuration with four LANs and two bridges. 64 Bridges from 802.x to 802.y Spanning Tree Bridges Operation of a LAN bridge from to Two parallel transparent bridges. 65 Bridges from 802.x to 802.y (2) Spanning Tree Bridges (2) The IEEE 802 frame formats. The drawing is not to scale. 63 (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree

12 Remote Bridges Virtual LANs Remote bridges can be used to interconnect distant LANs. 67 A building with centralized wiring using hubs and a switch. 70 Repeaters, Hubs, Bridges, Switches, Routers and Gateways Virtual LANs (2) (a) Which device is in which layer. (b) Frames, packets, and headers. 68 (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches. 71 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) The IEEE 802.1Q Standard (a) A hub. (b) A bridge. (c) a switch. 69 Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not

13 The IEEE 802.1Q Standard (2) The (legacy) and 802.1Q Ethernet frame formats. 73 Summary 74 Channel allocation methods and systems for a common channel. Quiz #3 10/28/13 a) (3 pts) When designing a medium access control protocol for a wireless network in which all communication takes place over a single channel, the hidden terminal problem must be addressed. Explain how this problem is addressed in the IEEE MAC-layer protocol. b) (3pts) A particular LAN uses the CSMA/CD medium access control protocol. The network operates at 1Gbps over a 1km cable and has a minimum frame size of 10,000 bits. What is the one-way signal propagation time (in seconds) in the cable? c) (3pts) Medium access control protocols can be generally classified as contention-based or collision-free. Which approach is preferable at low loads? Why? d) (3pts) Assume that the probability that k frames are generated during a given frame time (time slot), in which G frames are expected, is given by the Poisson distribution: P(k) = G k e -G /k! Write an equation defining the throughput when using ALOHA vs. Slotted ALOHA