Lectures The Medium Access Control Sub-Layer

Transcription

1 Computer Communications Lectures The Medium Access Control Sub-Layer The Channel Allocation Problem Consider a broadcast channel (sometimes also called multi-access or random access channel) How to allocate the channel among multiple users? The channel allocation protocol reside in medium access control (MAC) sub-layer within the data link layer MAC Sub-layer 2 1

2 Static Channel Allocation Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM) Both are inefficient when: Number of users large and time-varying Traffic bursty Compare with Statistical Multiplexing and Packet-Switching 3 Dynamic Channel Allocation Model Station Model Single Channel assumption Collision assumption Continuous vs. Slotted Time Carrier Sense vs. No Carrier Sense 4 2

3 Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Ethernet Wireless LAN Protocols IEEE based Wireless LANs Collision-Free Protocols Limited-Contention Protocols Wavelength Division Multiple Access Protocols 5 Pure ALOHA A user transmits whenever there is data to send In pure ALOHA, frames are transmitted at completely arbitrary times. 6 3

4 Pure ALOHA (2) Vulnerable period for the shaded frame. 7 Pure ALOHA vs. Slotted Aloha S = G * P 0 Throughput versus offered traffic for ALOHA systems. 8 4

5 Carrier Sense Multiple Access (CSMA) Sense (listen to) the carrier (channel) and wait if busy 1-persistent CSMA Wait until channel is idle and transmit (i.e., with probability 1) Upon collision, retry after a random wait period Performance worsens with increasing propagation delay p-persistent CSMA Assumes slotted time Wait until channel is idle and transmit with probability p If collision or someone else grabs the channel, retry after a random wait period Non-persistent CSMA If channel busy, wait a random period before retrying 9 Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols. 10 5

6 CSMA with Collision Detection (CSMA/CD) Abort transmissions as soon as collision is detected Basis of Ethernet LAN Inherently half-duplex Inefficient when large propagation delays and short frames CSMA/CD can be in one of three states: contention, transmission, or idle. 11 Ethernet Required Reading: Tanenbaum (section and chapter 4) 6

7 Original Ethernet and IEEE Standard Architecture of the original Ethernet (1976). Thick coaxial multidrop cable up to 2.5km long (with repeaters every 500 meters) Connect up to 256 computers 2.94Mbps line speed CSMA/CD Abort transmissions as soon as collision is detected and jam the cable to alert others DIX Standard for 10Mbps Ethernet (1978) IEEE Standard (1983): DIX Standard with two minor modifications 13 Ethernet Cabling The most common kinds of Ethernet cabling. Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T. 14 7

8 Ethernet Cabling (2) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. 15 Manchester Encoding Differentiate idle and busy periods Sender-receiver synchronization (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. 16 8

9 Ethernet Frame Structure Frame formats. (a) DIX Ethernet, (b) IEEE Addressing mechanism supports group (multicast/broadcast) addresses and distinguishing local from global addresses Minimum frame length requirement (64 bytes) To allow proper detection of collisions Distinguish valid frames from noise 17 Need for Minimum Frame Length in Ethernet Collision detection can take as long as 2 τ. Minimum frame length = 500 bits rounded to 512 bits (64 bytes) Worst case round trip propagation delay on 2.5km cable with 4 repeaters = 50 micro seconds Network speed = 10Mbps each bit takes 100 nano seconds Higher network speed greater minimum frame length or shorter maximum cable length 18 9

10 Binary Exponential Backoff (BEB) Algorithm Mechanism to dynamically adapt number of contending nodes After i collisions, choose a random number of slots between 0 and 2 i -1 before retrying After 10 collisions, randomization interval is frozen at 1023 slots Give up retrying after 16 collisions 19 Ethernet Performance Channel efficiency = P / (P + 2 τ /A) longer the cable, longer the contention interval and lower the channel efficiency P: mean frame transmission time 2τ : slot duration (worst case round-trip propagation delay) A: probability that some station acquires the channel in a given slot Channel efficiency = 1 / (1+2BLe/cF) for optimal case of e contention slots per frame F: frame length B: network bandwidth L: cable length c: signal propagation speed Efficiency of Ethernet at 10 Mbps with 512-bit slot times

11 IEEE 802.2: Logical Link Control Hide differences between various 802 networks by providing a single format and interface to network layer Can support error control and flow control over 802 networks Three service options: unreliable datagram, acknowledged datagram and reliable connection-oriented Based on HDLC (a) Position of LLC. (b) Protocol formats. 21 Switched Ethernet Switch: high-speed backplane plug-in line cards each with 1-8 connectors/ports A simple example of switched Ethernet. Handling simultaneous transmissions from hosts connected to same plug-in card Ports connected together to form an on-card LAN each card a separate collision domain» Handle collisions within a card and retransmissions using CSMA/CD with BEB Buffer at each input port each port a separate collision domain» No collisions 22 11

12 Fast Ethernet (IEEE 802.3u, 1995) 100Mbps 10 times faster yet backward compatible with earlier Ethernet Supports flow control Encoding schemes different from Manchester encoding used Use hubs or switches The original fast Ethernet cabling. 23 Gigabit Ethernet (IEEE 802.3z, 1998) 1Gbps 10 times faster than fast Ethernet yet backward compatible with all existing Ethernet standards Supports flow control Uses a different set of encoding schemes 10-gigabit Ethernet also standardized in 2002 (IEEE 802.3ae) (a) A two-station Ethernet. (b) A multistation Ethernet. Gigabit Ethernet cabling

13 Retrospective on Ethernet Simple reliable, cheap and easy to maintain Flexible Interworks easily with TCP/IP (both IP and Ethernet connectionless) Evolved without need for software changes 25 Wireless LANs and IEEE Standard Required reading: Tanenbaum (section 1.5.4, section and section 4.4) 13

14 Wireless LAN Protocols CSMA does not work when all nodes are not within range of each other Need to know interference at receiver, but know only about interference at sender with CSMA Hidden node problem: two non-neighboring nodes simultaneously transmit to a common neighbor, causing a collision Exposed node problem: a node unnecessarily getting blocked from transmitting due to an on-going transmission from a neighbor A wireless LAN. (a) A transmitting. (b) B transmitting. 27 Wireless LAN Protocols (2) MACA (Multiple Access with Collision Avoidance) protocol Alleviates hidden station problem, but does not completely eliminate it Collisions still possible (e.g., RTS collisions)» Retry after random wait period based on BEB algorithm MACAW (MACA for Wireless) Add ACKs and carrier sense To improve fairness, apply backoff algorithm for each source-destination pair To improve performance:» congestion info exchange between nodes» decrease sensitivity of backoff algorithm to temporary problems The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A

15 IEEE Wireless LAN Standard Challenges Identifying a suitable, worldwide frequency band Sufficient bandwidth and economic viability Protect users privacy Impact of host mobility Coping with limited battery life Dealing with human safety issues Several inherent differences with Ethernet CSMA is not sufficient in wireless LANs Presence of multipath fading Need for host mobility support Dealing with mobility-unaware software standard (1997) 2.4GHz ISM band 1Mbps or 2Mbps (DSSS/FHSS/Infrared) b standard (1999) 2.4GHz ISM band 11Mbps (HR-DSSS) a standard (1999) 5GHz ISM band 54Mbps (OFDM) g (2001) 2.4GHz ISM band 54Mbps (OFDM) 29 IEEE Wireless LAN Standard (2) (a) Infrastructure mode. (b) Ad hoc mode. A multicell network

16 The Protocol Stack Part of the protocol stack. 31 The MAC Sublayer Protocol Distributed Coordination Function (DCF) Distributed Compulsory CSMA with Collision Avoidance (CSMA/CA)» When channel busy, set a random backoff counter but keep it frozen while channel busy» No carrier sensing while transmitting» Upon frame loss due to collisions or channel errors, retry after random wait period based on Ethernet BEB algorithm Virtual carrier sensing» RTS-CTS mechanism» Network Allocation Vectors (NAVs) Point Coordination Function (PCF) Centralized: base station or Access Point (AP) controls intra-cell channel allocation Optional AP uses a polling mechanism» Periodic beacon frames (10-100/second) contain system parameters (e.g., clock synchronization) and invitation to join polling service» Poll frames» Polling frequency, polling order and service priorities not specified in standard Also useful for power management Can co-exist with DCF via inter-frame spacing mechanism in

17 The MAC Sublayer Protocol (2) Virtual channel sensing mechanism. 33 The MAC Sublayer Protocol (3) A fragment burst. Stop-and-wait protocol applied to each fragment Fragment size not set by the standard 34 17

18 The MAC Sublayer Protocol (4) Interframe spacing in The Frame Structure Data, management and control frames Management frames similar to data frames, but with one less AP address Control frames even shorter with only one or two addresses, no Data or Sequence fields The data frame

19 Services Distribution (Inter-Cell) Services for managing cell membership and interacting with nodes outside a cell Association Disassociation Reassociation Distribution Integration Station (Intra-Cell) Services for activity within a single cell Authentication Deauthentication Privacy Data Delivery 37 Collision-Free Protocols Assume: Exactly N stations, each with a unique address from 0 to N-1 Propagation delay negligible The basic bit-map protocol (comes under the category of reservation protocols)

20 Collision-Free Protocols (2) The binary countdown protocol. A dash indicates silence. 39 Limited-Contention Protocols Adapt assignment of stations to slots according to load Acquisition probability for a symmetric contention channel

21 Adaptive Tree Walk Protocol The tree for eight stations. 41 Wavelength Division Multiple Access Protocols Dynamic variants of FDM/TDM methods Assume global synchronization Wavelength division multiple access