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 packet duration exceeds t: Note that λ is the average packet arrival rate, and 1/μ is the average packet duration.

M/M/1 Queue Queue equations, pj(t) is the probability that the number of packets at time t in a queue is j The solution of the previous recursion for stationary probabilities to which pj(t) converge

Delay Little’s formula for average delay E[D] E[Q] is the average number of packets in a queue Delay is:

Static Channel Allocation Delay for one fast channel: where C is the channel bit-rate and E[L] is the average packet length. Delay for multiple FDM N times slower channels:

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

Vulnerable period for the shaded frame. Pure ALOHA Vulnerable period for the shaded frame.

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

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

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

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

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

Ethernet Ethernet, IEEE 802.3 10Base (10Mbps) Fast Ethernet (100Mbps) Gigabit Ethernet

Ethernet MAC Sublayer Protocol Frame formats. (a) DIX Ethernet, (b) IEEE 802.3. Preamble-synchronization, Type-upper layer protocol, Pad-to make the minimum packet size 64B

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

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

Ethernet Performance

Throughput of CSMA/CD (Ethernet) Assume that requests form a Poisson process with rate g, T is time slot duration, and Tp is a packet duration. The throughput equals S=Tp/(Tp+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 Ps=gTe-gT. The average time between transmissions is

Throughput of CSMA/CD The throughput is It tends to 0 when g increases . Protocol is unstable like ALOHA. g S l 1 2

Throughput of CSMA/CD The throughput is If p is the packet generation probability and k is the number of active users Ps=kp(1-p)k-1 The maximum throughput is achieved for p=1/k and it is tends to e when k tends to infinity

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

The most common kinds of Ethernet cabling. 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.

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

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

A simple example of switched Ethernet.

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

A Sample HFC System HOME Secondary Hub RF Spectrum on coax: Downstream: 500 MHz shared by ~50,000 (broadcast) 200 MHz by 1200 (narrowcast) Upstream: ~37 MHz shared by 300 HOME o o o o o o o o lup lb ln (4ln/fiber) Secondary Hub Fiber Node o o o o o o o o o o RF Spectrum on coax: return Capacity of the coax is “wasted.” No architecture really looks exactly like this, but it is a well-defined starting point. I could have used Lightwire as my starting point (in fact I originally did use Lightwire as my starting point) but systems like this are currently more common. ********OLD NOTES FROM HFC BASICS SLIDE*************** 1. Drop is a coax bus, therefore all downstream traffic is broadcast, and terminal equipment in home selects the desired content. 2. Because it is coax, RF transmission is used. 3. Coax has a limited bandwidth, but is capable of maintaining a high SNR. 4. Fiber has virtually unlimited bandwidth, but it is difficult to maintain a high SNR (high SNR requires high power, which runs into non-linearities) This point is best illustrated by the fact that over longer distances the fiber carrying the analog AM-VSB signals doesn’t carry any other wavelengths. 5. The RF demarcation between analog broadcast signals and narrowcast signals is flexible. The traditional method to increase capacity is to reduce the node size (analogous to cell size). Upstream frequently split into 4 parts. RF band is 5-42 MHz, this band can carry multiple RF channels. Channel width downstream 6MHz, channel spacing upstream is 2MHz. 5-15 MHz is plagued with ingress noise. All frequencies suffer from the funnel effect. Bandwidth is shared. Coax drop downstream has 5Gbps capacity. Services can be segregated by RF frequency. For data the standard is DOCSIS (Data Over Cable Service Interface Specification). Up to 10 Mbps transmission per RF channel is provided in the standard, but a peak rate of ~3 Mbps is more realistic. A cable modem or set top box resides in the home, a CMTS, which coordinates traffic, resides in the headend. 80 broadcast channels 30 QAM channels (~150 video channels) narrowcast broadcast 5-42 MHz 550 MHz 750 MHz 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 90th percentile page delay). By Shankar, Jiang and Mishra: where tON is the transmission tim, and tOFF is the think time, r is the channel rate, tON/(tON+tOFF)<<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 tOFF 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 Service QoS parameters Access Mode Applications UGS Unsolicited grant size, interval, jitter Isonchronous Videoconferencing, VoD, VoIP UGS-AD Unsolicited grant size, interval, jitter;polling interval, jitter Isonchronous, periodic request polling VoIP with silence supression rtPS Polling interval, jitter Periodic request polling, piggybacking reservation VoIP

QoS in DOCSIS Service QoS parameters Access Mode Applications nrtPS Polling interval, min reserved rate, max sustained rate, priority Periodic request polling, piggybacking reservation, immediate access Demanding FTP BE min reserved rate, max sustained rate, priority Normal, piggybacking reservation, immediate access Telnet, FTP, WWW CIR Unspecified

Performance for BE service in DOCSIS Assume that requests form a Poisson process with rate g, T is time slot duration, and Tp is a packet duration. The throughput equals S=Tp/(Tp+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 . Protocol is unstable like ALOHA. g S l 1 2

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

Part of the 802.11 protocol stack.

The 802.11 MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem.

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

The 802.11 MAC Sublayer Protocol The use of virtual channel sensing using CSMA/CA.

The 802.11 MAC Sublayer Protocol A fragment burst.

The 802.11 MAC Sublayer Protocol Interframe spacing in 802.11.

The 802.11 Frame Structure The 802.11 data frame. Address 3 and 4-for source and dest base stations, Seq-fragment sequence number, Type-data or control, Subtype-RTS or CTS, MF-more fragments, More-more frames, W-WEP, O-frame sequence maintained

802.11 Services Association Disassociation Reassociation Distribution Authentication Integration Privacy