Chapter 4 media access layer1 Medium Access Sublayer Wireless and other broadcast media Who goes...

chapter 4 media access layer 1

Medium Access Sublayer

• Wireless and other broadcast media• Who goes next?• Centralized control

– Ex: polling• Disadvantages of overhead, bottleneck, and central pt of failure• 802.12 (100 Base) VG-AnyLAN

• Distributed control• Static assignment

– TDM, FDM, WDM» Waste of bandwidth when station has nothing to send

– FHMA, CDMA

• Bandwidth on demand


Static assignment delay characteristics

• Queueing theory– (Note assumptions)– Service rate denoted by – Arrival rate denoted by (or G)– T (mean delay time) = 1/(-)– Text uses C for service rate, where

• C is capacity of channel in bps• is frame/bit ratio

– Then mean delay time for FDMA with N channels is N times delay with multiple servers and a single queue


From Abhilash Dhongdisubstitute servers for counters

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From Abhilash Dhongdi

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Aloha (1970s)

• Pure and slotted Aloha are contention systems– Aloha has a radio packet network on the islands of

Hawaii

– In pure Aloha, stations can send at any time

– What happens if collision occurs?• As offered traffic increases past 50% of available time,

throughput starts decreasing (rapidly) for pure Aloha

• Maximum of 18.4% of channel capacity achievable


Slotted Aloha

• Stations synchronize into the beginning of the frame (central station sends timing info)– As offered traffic increases past 100% (i.e., on average

one frame per slot), throughput starts decreasing rapidly for slotted Aloha

– Maximum of about 36% of channel capacity is achievable

– This degradation is called instability– Slotted Aloha is commonly used for access to a control

channel in cellular phones


Carrier Sense Multiple Access (CSMA)

LANS – stations can sense media and, if channel is busy, do not transmit– 1-persistent

• When channel becomes idle, station sends immediately

– p-persistent• When channel becomes idle, station waits some random

interval of time before transmitting

• Less collisions occur when p decreases, but delays are longer


CSMA/CD (collision detection)

• Stations can sense collisions– 2 slot time (2Tp)– For resolving collision, slotted time is used– Longer channels can take longer for collisions to be

detected– Binary exponential backoff to resolve order of

transmission

• CSMA/CA (collision avoidance)– See 802.11, Apple’s LocalTalk network– Avoid collision, successive collision


Collision-Free Protocols

• TDMA narrow band used for reservations (out-of-band)– Must be synchronized– Binary countdown for bit map

– Address bits determine priority; may share bits– Limit on number of stations by number of bits and voltage allowed

– Reservation schemes

• Token passing• Tree walk• Issue of recovering from synchronization or hardware

errors (resiliency)


802.11 (CSMA/CA)

• Carrier sense is imperfect in wireless LANs– For example: Two stations out of range of each other can be

sending to same access point (hidden station problem)– Two stations not competing with each other may be within range

of each other’s radio (exposed station problem)

• Collision detection is not possible– 802.11b Radios cannot listen and receive at same time

• RTS and CTS (short frames include information on length of broadcast, sender)– These given higher priority than regular traffic


IEEE 802.3 (ISO 8802.3)

• Physical layer (media dependent sublayer)– 10 base-T; UTP with hubs

• 100 to 150 meter max segment• 1024 max nodes/segment

– 10 base5 and 10 base2• coax bus• 500, 180 meters max (without repeaters)• 100, 30 stations max• 2500 m max length (3000 with drop cables)

– 10 base-F, fiber with hub• 2000 m. max; 1024 stations max

– 10 Broad36


IEEE 802.3 physical layer

• Media independent sublayer

• Manchester encoding– 20 Megabaud; 10 Mbps

• In text,– 1 is HL, 0 is LH– H is +.85 volts, L is -.85 volts


802.3 Media Access sublayer

• Frame consists of– 7 + 1 bytes for preamble + frame start

• 10101010 … 10101011• Frame synchronization

– Address (hardwired MAC address if bit 46 is on)• 6 bytes typically used• Destination broadcast (all 1s), multicast addressing (bit 47 is 1)• Source address

– Frame length (2 bytes)– Data (0-1500 bytes) – LLC data unit– Pad (0-46 bytes)– CRC (FCS) – 4 bytes


http://www.firewall.cx/ethernet-frames-802.3.php

( discuss preamble, pad, datalink header)



http://www.geekonwheels.com/SiteGraphics/Image/ethernetformat.gif

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802.3 Media Access sublayer

• 1-persistent CSMA/CD• Binary exponential backoff (to 210)

• Up to 16 successive collisions• Formula p. 286 for channel efficiency (i.e.,

utilization)– Efficiency = P (i.e., Tx)

» P + 2 /A

» where A is probability that a single station gets the channel

(kp (1-p) k-1 )


Switched (802.3) LANs

• Fast Ethernet (4B/5B)– 100Base-T4, 100Base-TX, 100Base-FX– Limit collision domain or supply buffer for

overlapping frames– Can be combined with hubs, intelligent hubs

into tree structure– Must use switches for fiber (2000m)

• Gigabit Ethernet (8B/10B)


Retrospective on Ethernet

• Ethernet dominates the wired LAN market– Compatible with 802.11 for wireless

• Advantages– Maturity – trained personnel– In Place– Cost– Simplicity, resiliency, availability– Speed has been updated – Connectionless (is this a plus?)– Switches for deterministic access, fiber


Retrospective on Ethernet

• Disadvantages– Nondeterministic (access is based on assumed

probability)– No priorities– Degrades in heavy traffic (if bus, passive bus is

used)


802.5 token ring

• Physically a star– (hub or wire center, STP)

• Differential Manchester Encoding• J (same,same),K (opposite,same)

– Starting & ending delimiters are– SD(JK0JK000), ED(JK1JK1IE)

• I for final frame, E for any station finding error

• 4, 16Mbps, 100 Mbps; +/- 3-4.5 volts• Logically a ring- each frame is “broadcast”• Repeater at each station- 1 bit delay


802.2 LLC• Adapts HDLC but typically uses

unnumbered frames for the control field– Sequence #s, ack #s are an option– DSAP (8 bits), SSAP (8 bits), control (8-16

bits)

• Can contain additional fields– Ex: IEEE 802.5 inserts bridge addresses– Ex: SNAP (Sub Network access protocol) – for

larger port # (when used with IP)


802.11 physical layer

• Which band is allocated – Size of band and sharing issues– Speed depends on bandwidth!!

• Type of modulation scheme– Note that analog signals are used

• Signaling over multiple frequencies • Physical layer attaches fields to frame that

assist bit synchronization; modulation


Wireless LANs – early 802.11

• Physical layer – 1-2 Mbps – Infrared – FHSS (short-range radio frequency hopping spread

spectrum)• Relatively immune to multi-path fading• Sender and receiver have same seed to pseudo-random number

generator for frequency hopping sequence – must stay synchronized

– DSSS (short-range radio direct sequence spread spectrum)

• 11 channels


802.11a

• Physical layer – 54 Mbps; 5 GHz ISM band

• OFDM (Orthogonal Frequency Division Multiplexing)– 48 frequencies for data; 4 for synchronization

• QAM– combinations of phase and amplitude shift

keying


802.11b

• Physical layer

• Up to 11 Mbps; actually preceded 802.11a in obtaining standardization

• 2.4GHz band

• HR-DSSS (High Rate Direct Sequence Spread Spectrum)


802.11g

• Up to 54Mbps

• Uses OFDM (802.11a modulation scheme)– But in 802.11b band (2.4GHz)

• 2.4Ghz and 5GHz bands are ISM– Industrial, Scientific, Medical bands (although

not considered ISM devices)• Unlicensed in most countries

• Some limitations (such as power emission)


802.11n• Standardized 2009

– Same 2.4 GHz and 5GHz bands

• 600Mbps datarate; 100 Mbps throughput– Multimedia applications

• Stress on power savings

• Designed for multiple radios – input and output (or multiple input/output) can

occur simultaneously on 40 MHZ combined band


802.11 MAC sublayer

• Carrier Sense is imperfect; collision detection is not possible– Hidden station problem– Exposed station problem– Early radios couldn’t send and receive at the same time

• DCF (Distributed Coordination Function) –CSMA/CA– stations sense (and defer if channel is busy)- transmit

entire frame; collision causes binary exponential backoff

– short RTS (contains length field); CTS


802.11 MAC sublayer

• PCF (point coordination function)– Optional

– Allows for polling by base station

– Supports DCF with lower priority• DCF frames must wait for 3rd time slot after transmission

completes

• 1st slot time for CTS, acks, fragment bursts

• 2nd slot time; poll or beacon frames from base station

• 4th slot time for error reporting


Wireless LAN issues

• noisy; interference with other applications in same band

• If ber is .0001, probability of obtaining a 12144 bit frame correctly is (1-.0001) 12144 – Less than 30%

• Frames may be fragmented; each with its own checksum– Stop and wait protocol with seq#, ack#

• Fragment bursts can be sent (similar to sliding window)


http://www.webcastmy.com.my/unimasresearchgateway/thesis/thesis_0061/chap3.htm#figure3.4


802.11 MAC frame (p.300)

• Frame control (2 bytes)– Protocol version– Frame type (data, control, mgt)– Subtype (RTS, CLS, etc.)– To DS; from DS for intercell traffic– MF (more fragments)– R- Retry (retransmission to signal possible duplicates)– P- Power (base station/access point) can put receiver into sleep

mode (conserve battery)– M- More (more frames follow)– W – signals use of WEP (wired equivalent privacy )– O – sequence counts


802.11 MAC frame

• Duration (2 bytes) – length of transmission• Source and Destination Addresses (6/6 bytes)• Source and destination base station addresses for

intercell traffic (6/6 bytes)

• Sequence# 2 bytes (12bits for frame; 4 for fragment)

• Data (up to 2312 bytes)• Checksum (4 bytes)


Some 802.11 services

• Association (connect to base station)– Accept; authenticate

• Disassociation– Either base station or client can disconnect

• Reassociation– Handoff while roaming

• Distribution– Routing, specifically when connecting to wired network

• Integration– Translation to another protocol’s format

• Privacy – encryption• Data delivery (not guaranteed; does not imply that the above are

guaranteed)


802.16- WLL or WiMAX

• Broadband for the local loop by wireless

• Services within buildings (fixed wireless)

• Uses full duplex radios (more expensive equipment than most wireless LANs)

• Needs large spectrum (consider an entire building’s needs)– 10-66GHz band

• Quality of service (QoS) requirements– To support telephony and other soft real-time applications


802.16 layers

• Physical layer (new standards closer to 11b)– modem signaling method (QAM-16, QPSK, etc.)– transmission convergence sublayer

• Translates signal to encoding method

• Data Link Layer– Security sublayer

• Encryption, decryption, authorization

– MAC sublayer• Connection oriented• Base stations send downstream traffic• Supports Polling• Each frame has beginning bytes for “free” map of upstream traffic slots

– Convergence sublayer• Merging datagrams to connections


802.16 classes of service (similar to ATM)

• Constant bit rate (highest priority)– Uncompressed voice, video

• Real-time variable bit rate service– Compressed multimedia

• Discuss sports video vs soap operas

• Non-real time variable bit rate service– Large file transfers

– Bit set to request a poll• (poll taken away if it does not respond in k times)

• Best-efforts service• Contend with other best-effort users for slots marked available by “free” map


Bluetooth

• Standardized in 1999 for wireless transmission– IEEE 802.15 adoption of physical and data link layer

• Architecture

– Scatternet – interconnection of piconets

• Piconet – one master node and up to seven active slave nodes within 10 meters (others can be “parked” at low power)

• Piconets can be linked into a scatternet

– All communication is through master (different for each piconet)

• Primary goals– Low cost, small size, low power for connecting devices and

computers

– These factors limit the diameter of a piconet


Applications (profiles) Bluetooth must/may handle

• Must provide– Generic access (establish the network links)– Service discovery

• May provide– Cordless telephony– Intercom– Headset– FAX– LAN access– Dial-up networking– Replace serial port for devices– Link management functions– File transfer– Data (object) transfer


Some issues with Bluetooth

• 13 protocol stacks for different profiles– Complexity

• Uses same 2.4 GHz ISM band as 802.11b, g, n – FHSS at 1600 hops/sec

• Probably it will interfere with 802.11• There have been security problems as Bluetooth

was incorporated into smartphones, etc.


Relays

• At physical layer we use repeaters, amplifiers, passive hubs

• At data link layer we use bridges; second layer switches

• At network layer we use routers; 3rd layer switches• At transport and application layer we use

gateways• (the above is a simplification)


Bridges• Operate in promiscuous mode• Extend geographic limitations• Connect separate LANs• Filter traffic

• May increase throughput• Increase reliability

– rest of LAN can continue if node malfunctions

• Increase security• Address isolation

• Each bridge contains line cards (perhaps for different LANs with own collision domain)


Transparent Bridges

• Defined for Ethernet LANs• Complexity is in the bridges – transparent to the

hosts• Bridges cooperate to form a spanning tree

– (that means that you can add a bridge and possibly nothing will be routed through it)

• Backward learning + flooding• Tables maintained at bridges; periodically purged• Think of appropriateness for mobile hosts


Connecting 802.3; 802.11; 802.16• Each bridge strips off data link header and adds its

own– Different frame formats– Checksum must be recomputed

• Different speeds– Buffers may overflow

• Different frame sizes– Must discard too long frames

• Security – what to do with encryption that is passed to 802.3?

• No QoS in 802.3


Second layer switches

• Each switch port (usually) goes to a single computer– No collisions– (Limited) buffer space for frames

• Cut-through switches– Can begin forwarding frame after destination

field has been received


Routers

• Can fragment packets

• Route packets; sometimes individually

• Resolve MAC addresses with IP addresses

• Translate between LAN and WAN protocols, different network protocols (Gateways)


Virtual LANs (VLAN)

• LAN composition can be decided logically instead of geographically– Perhaps by organizational hierarchy

• Provide better security; more thruput for “important users”– Switches can create logical LANs – forward ONLY to

users on logical LAN• Table for identifying MAC address with VLAN (similar to

bridge routing tables)• 802.1Q for VLAN tag

– Already present in bridges and switches– On newer 802.3 cards

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Chapter 4 media access layer1 Medium Access Sublayer Wireless and other broadcast media Who goes...

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