Medium Access Control

Channel Allocation Static channel allocation in LANs and MANs FDMA, TDMA, CDMA Dynamic channel allocation in LANs and MANs MAC protocols: with collisions, polling, token

Static Channel Allocation Delay for one fast channel: Delay for multiple FDM slower channels:

Poisson Process Probability of k arrivals in time t: Probability that interarrival time exceeds t:

M/M/1 Queue Queue equations Delay for multiple FDM slower channels:

Delay Little’s formula Delay is:

Static Channel Allocation Delay for one fast channel: Delay for multiple FDM slower channels:

Dynamic Channel Allocation 1.(a) Single channel (b) Multiple channel 2.(a) Collision (b) Collision-free 3. (a) Continuous Time. (b) Slotted Time. 4. (a) Carrier Sense. (b) No Carrier Sense.

Multiple Access Protocols ALOHA Carrier Sense Multiple Access (CSMA) protocols CSMA/CD CSMA/CA Collision-Free protocols Reservation based Token based

Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times.

Pure ALOHA Vulnerable period for the shaded frame.

ALOHA Throughput Throughput is S=GP 0, where P 0 is the probability of successful transmission. The k frames per f frame slots is P 0 =e -fG For pure ALOHA f=2, for slotted ALOHA f=1, so:

Pure and Slotted ALOHA Throughput versus offered traffic for ALOHA systems.

Carrier Sense Multiple Access 1-Persistant CSMA Nonpersistant CSMA P-Persistant CSMA

Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

Wireless LAN: CSMA with Collision Avoidance A wireless LAN. (a) A transmitting. (b) B transmitting.

Wireless LAN: CSMA-CA The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.

DOCSIS ( Data Over Cable Service Interface Specification

Collision-Free Protocols:Reservations The basic bit-map protocol.

Collision-Free Protocols: Bidding The binary countdown protocol. A dash indicates silence.

Collision Free Protocols: Fiber Distributed Data Interface (FDDI) Station transmits only when it has a token Timers count the time while the token is away Two timers determine how much data a station may transmit, so that the token delay is limited

Limited-Contention Protocols Acquisition probability for a symmetric contention channel.

Adaptive Tree Walk Protocol The tree for eight stations.

WDMA Networks Stations may be connected to the optical star coupler or onto the optical ring and utilize multiple wavelengths. Stations are equipped with the tunable/fixed transmitters and tunable/fixed receivers. Stations are tuned to the common control channel where they compete for the medium.

Ethernet Ethernet, IEEE Base (10Mbps) Fast Ethernet (100Mbps) Gigabit Ethernet

Ethernet MAC Sublayer Protocol Frame formats. (a) DIX Ethernet, (b) IEEE

CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

Performance of Non-Persistant CSMA/CD Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990

Performance of 1-Persistant CSMA/CD Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990

Comparison of CSMA and CSMA/CD Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990

Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

Back-Off Mechanism After a collision, user accesses medium with probability 1/W where W is the window size. With each collision W doubles.

10Mbps Ethernet Cabling The most common kinds of Ethernet cabling.

10Mbps Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

Ethernet Cabling Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

10Mb Ethernet Coding (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

10 Mb Ethernet Collision Detection 10Base5 cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

10 Mb Ethernet Collision Detection 10Base2 and 10BaseT cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

Fast Ethernet The original fast Ethernet cabling.

Fast Ethernet Auto negotiation enables communication with 10Mb Ethernet Manchester code → 4B/5B code Full duplex mode is optional with using PAUSE command

Switched Ethernet A simple example of switched Ethernet.

Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

Gigabit Ethernet Gigabit Ethernet cabling.

Gigabit Ethernet Prioritization of fiber over copper 4B/5B coding → 8B/10B coding Full duplex mode is preferred with PAUSE message Carrier extension, and frame bursting introduced in half-duplex mode

IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats.

IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats.

A Sample HFC System Secondary Hub o HOME o 5-42 MHz 550 MHz 750 MHz RF Spectrum on coax: return 80 broadcast channels30 QAM channels (~150 video channels) Downstream: 500 MHz shared by ~50,000 (broadcast) 200 MHz by 1200 (narrowcast) Upstream: ~37 MHz shared by 300 broadcast narrowcast Fiber Node up b n (4 n /fiber) Sheryl Woodward, AT&T Labs-Research

Justification for Using Shared Medium Equivalent circuit rate (ECR) on a cable with many users is the rate of a dedicated link that would provide the same e.g. average delay (similar results is obtained for 90 th percentile page delay). By Shankar, Jiang and Mishra: where t ON is the transmission tim, and t OFF is the think time, r is the channel rate, t ON /(t ON +t OFF )<<1, on periods have an exponential distribution.

Justification for Using Shared Medium Let’s calculate how many users can be allocated one DOCSIS channel of 32Mbps to get the same experience as DSL user with dedicated rate of 2Mbps. According to traffic statistics page size is 68KB on average, and t OFF is 14.5s on average, which is much more than 32/2=16 users. Price: high user speed.

DOCSIS MAC Protocol Traffic that is transmitted downstream to the users is controlled by CMTS (cable modem termination system) in headend. It polices and shapes the traffic, and perform algorithms such are WFQ and RED. Users requests are resolved at headend, and they are informed about the resolution through the downstream channel. If there is a collision of requests, users repeat their requests according to exponential back-off mechanism, otherwise they send data in specified time slot(s).

QoS in DOCSIS ServiceQoS parametersAccess ModeApplications UGSUnsolicited grant size, interval, jitter IsonchronousVideoconferencing, VoD, VoIP UGS-ADUnsolicited grant size, interval, jitter;polling interval, jitter Isonchronous, periodic request polling VoIP with silence supression rtPSPolling interval, jitterPeriodic request polling, piggybacking reservation VoIP

QoS in DOCSIS ServiceQoS parametersAccess ModeApplications nrtPSPolling interval, min reserved rate, max sustained rate, priority Periodic request polling, piggybacking reservation, immediate access Demanding FTP BEmin reserved rate, max sustained rate, priority Normal, piggybacking reservation, immediate access Telnet, FTP, WWW CIRUnspecified

Performance for BE service in DOCSIS Assume that requests form a Poisson process with rate g, T is time slot duration, and T p is a packet duration. The throughput equals S=T p /(T p +I), where I is the average time between packet transmissions. The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is gTe -gT. The average time between transmissions is

Performance for BE service in DOCSIS The throughput is It tends to 0 when g increases. Exercise: Find the mapximum value of S in terms of T and T p, and plot graphs of S versus g, and different T/T p. Protocol is unstable like ALOHA. g S 1 2

Wireless LANs Distributed coordination function (DCF) Point coordination function (PCF)

The Protocol Stack Part of the protocol stack.

The MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem.

The MAC Sublayer Protocol The use of virtual channel sensing using CSMA/CA.

The MAC Sublayer Protocol A fragment burst.

The MAC Sublayer Protocol Interframe spacing in

The Frame Structure The data frame.

Services Association Disassociation Reassociation Distribution Integration Privacy Data delivery

Broadband Wireless Fixed wireless Larger bandwidth

The Protocol Stack The Protocol Stack.

The Physical Layer The transmission environment.

The Physical Layer Frames and time slots for time division duplexing.

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 Frame Structure (a) A generic frame. (b) A bandwidth request frame.

Bluetooth Used for communication of the equipment in the house, office Interferes with IEEE

Bluetooth Architecture Two piconets can be connected to form a scatternet.

Bluetooth Applications The Bluetooth profiles.

The Bluetooth Protocol Stack The version of the Bluetooth protocol architecture.

The Bluetooth Frame Structure A typical Bluetooth data frame.

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

Data Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

Bridges from 802.x to 802.y Operation of a LAN bridge from to

Bridges from 802.x to 802.y (2) The IEEE 802 frame formats. The drawing is not to scale.

Local Internetworking A configuration with four LANs and two bridges.

Spanning Tree Bridges Two parallel transparent bridges.

Spanning Tree Bridges (2) (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.

Remote Bridges Remote bridges can be used to interconnect distant LANs.

Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers.

Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) (a) A hub. (b) A bridge. (c) a switch.

Virtual LANs A building with centralized wiring using hubs and a switch.

Virtual LANs (2) (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.

The IEEE 802.1Q Standard Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.

The IEEE 802.1Q Standard (2) The (legacy) and 802.1Q Ethernet frame formats.

Summary Channel allocation methods and systems for a common channel.