Transcript Medium Access Control (MAC) Sublayer

More on the link layer
Logical Link Control (LLC)
Medium Access Control (MAC)
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link layer functions
• services
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– un-ACKed connectionless
– ACKed connectionless
– ACKed connection oriented
framing
encoding
error control
flow control (vs. congestion control?)
MAC
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point-to-point vs. broadcast media
• point-to-point
– PPP for dial-up access
– point-to-point link between Ethernet switch and host
• broadcast (shared wire or medium)
– traditional Ethernet
– 802.11 wireless LAN
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data link protocols (logical)
• unrestricted simplex
• simplex stop-and-wait
• simplex for noisy channel
– discussion
• sliding window protocols
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sliding window protocols
• piggybacked ACKs
– less overhead (bandwidth, interrupts, buffering, ...)
– how to deal with timeouts?
• sequence numbers
• sending window
• receiving window
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1-frame sliding window
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1-bit sliding window
(seq#, ACK#, pkt#); ACK#: last OK frame
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pipelining
• idea
– do not block transmitter during the roundtrip time
– increase the window size
• what happens with errors
– go back n
– selective repeat
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go back n
• if error
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receiver discards subsequent frames
no ACKs for the discarded frames
receiver window size = 1
transmitter times-out, resends unACKed frames
inefficient if the error rate is high
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selective repeat
• if error
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receiver still stores the subsequent good frames
transmitter retransmits the bad frame
receiver window size > 1
efficient at higher error rate
trade-off between bandwidth and buffer space
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sliding window schemes
“go back n”
(a) “go back n” (frames received out of sequence discarded)
(b)( “selective repeat” (frames received out of sequence buffered)
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(after error, receiver may NAK frame 2 to short-circuit sender timeout)
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3-bit sequence numbers
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line utilization
• b = channel capacity in bits per second
• f = frame size in bits
• R = round-trip propagation time
• frame transmission time = (f / b) seconds
• line utilization = f / (f + bR)
• if f < bR, then efficiency < 1/2
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window size
• sender needs n buffers for window size n
• go back n
– sender needs enough buffers for
• timeout
• RTT of frame + NACK
• selective repeat
– window size = floor((maxseq+1)/2)
– why?
• optimal window size, sequence#s
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(review this in detail)
protocol specification by FSM
(a) state diagram; key to state ovals: (SRC), S in {0,1}, R in {0,1}, C in {0,1,A,-}
(b) transition chart
(from Tanenbaum text)
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(review this in detail)
Petri net for simplex “wait for ACK”
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[Tanenbaum 4/e, 233]
places, tokens, transitions, input/output arcs
tokens not conserved
no composite states:
sender, receiver, channel separated
• transitions may be viewed as a “grammar”
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multiple access protocols
• single shared broadcast channel
– multiple nodes can speak at once
– collisions lead to garbled data
• multiple access protocol (medium access control)
– distributed algorithm for sharing the channel
– algorithm determines which node can transmit
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types of MAC
• static bandwidth allocation: channel partitioning
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TDM
FDM
CDMA (see handout)
problems?
• deterministic sharing
– token-passing
– polling
– reservations, scheduling
• contention
– random access: allow collisions, and then recover
– ALOHA, CSMA, more
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“taking turns” MAC protocols
polling
token passing
• master node “invites”
slave nodes to
transmit in turn
• control token passed from one
node to next sequentially
• concerns:
– polling overhead
– latency
– single point of failure
(master)
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• token message
• concerns:
– token overhead
– latency
– single point of failure (token)
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token rings
• (a) transmit token after frame is sent
• (b) transmit token while frame is being stripped
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ring topology
• self-healing ring (SHR)
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random access protocols
• when node has packet to send
– transmit at full channel data rate
– no a priori coordination among nodes
• >= 2 nodes transmit concurrently ➜ “collision”
• random access MAC protocol specification
– collision detection
– collision recovery
• examples
– ALOHA, slotted ALOHA
– CSMA, CSMA/CD, CSMA/CA
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key ideas of random access
• station model
– n independent stations (nodes, terminals)
• single channel
– all transmission/reception on shared channel
– nodes have equivalent ability
– node priority may vary dynamically
• time
– continuous
– slotted (discrete, timed intervals, “master clock”)
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key ideas of random access
• carrier sense (or not)
– listen before speaking, and don’t interrupt
– check if someone else is already sending data ...
– … wait till the other node is done
• collision detection (or not)
– if someone else starts talking at the same time, stop
– if the data on the wire is garbled ...
– ... another node is transmitting, too, so stop
• randomness
– don’t start talking again right away
– wait for a random time before trying again
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pure ALOHA
in pure ALOHA, frames are transmitted at completely
arbitrary times
temporal collisions destroy colliding frames
send when data arrives; if collision, random delay, resend
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ALOHA
Vulnerable period for the shaded frame.
shaded frame vulnerability period modeled as two frame times
intuition: collisions of partially-overlapping frames
slotted ALOHA:
during contention, frames collide in some single slot/frame time
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slotted ALOHA
assumptions
operation
• all frames same size
• when node obtains fresh
frame, transmits in next slot
• time divided into equal slots
(time to transmit a frame)
• nodes start to transmit
frames only at start of slots
• nodes are synchronized
• if two or more nodes
transmit, all nodes detect
collision
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• no collision: node can send
new frame in next slot
• collision: node retransmits
frame in each subsequent
slot with probability p until
success
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pure vs. slotted ALOHA
Throughput versus offered traffic for ALOHA
systems.
max throughput: ALOHA 1/(2e) ~ 18%; slotted ALOHA 1/e ~ 37%
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CSMA: carrier sensing multiple access
• 1-persistent CSMA:
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if idle, send; ... ; if collision, random delay, sense ...
propagation delay → collision
“two nodes waiting for idle” → collision
idea behind Ethernet LAN protocol
• non-persistent CSMA:
– if idle, send; else, random delay
• p-persistent CSMA (slotted time):
– if idle, send with probability p; if collision, random
delay
– slotted transmission discipline
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persistent and non-persistent CSMA
non-persistent CSMA: very good throughput under high load
0.01-persistent CSMA: best throughput
tolerance for delay can enhance throughput in chaotic environments
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collision-free protocols
basic bit-map protocol
- contention slots are constant overhead
- overhead means less as frames get large
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collision-free protocols
addresses:
(AND of addresses)
binary countdown protocol (log2n bits); dash indicates silence.
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limited-contention protocols
throughput of
contention protocols
under high load
Acquisition
probability
for a symmetric
contention channel.
~ 1/e
- motivation for hybrid deterministic/probabilistic protocols
- idea: allow contention at low load, use taking-turns at high load
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adaptive tree walk protocol
tree for eight stations: depth first search, LR
nodes
- at idle, each station ready to send data signals 1 or not,
according to a clever plan
- example: only station H ready to send:
slot1 = 1 (candidate sender is under 1),
s2 = 0 (not under 2), s3 = 0 (not under 6),
s4 = 0 (not G), s5 = 1 (H).
- for binary tree, sender is chosen in O(log2(n)) time
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WDM access protocol (one example out of 100s)
Wavelength division multiple access.
- fixed data output & control input
- tunable data input & control output
- on control channel: control slots; on data channel, status slot
- classes of traffic: CBR, VBR, datagram
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IEEE 802.3: Ethernet
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[Metcalfe, Boggs, 1976]: first LAN, “the Ethernet”
TCP/IP/Ethernet: a connectionless stack
simple to use, reliable, cheap, scalable
LAN collision domains (broadcast) (bus, hub)
store-and-forward switches, point-to-point links
10 Mbps, 100 Mbps, gigabit, 10 gigabit
becoming very rare for network paths not to
traverse any Ethernet links
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“original flavor” 10 Mbps Ethernet
common kinds of Ethernet cabling
only twisted pair and fiber are still being deployed
(except in specialized environments)
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10 Mbps (“plain”) Ethernet PHY
(a) and (b) (10base5, 10base2) seldom deployed
(c) 10baseT hub is a collision domain
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10 Mbps Ethernet PHY topologies
- broadcast medium
- same logical topologies
- no loops (rings) allowed
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-no path has > 4 repeaters
- network diameter <= 2500 m
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intermission
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Ethernet MAC sublayer protocol
- no two nodes are farther apart than A and B
- τ is the diameter of the network,
the one-way propagation time between the farthest nodes
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Ethernet: CSMA/CD (Collision Detection)
contention slot time = 2τ = max(signal round-trip time)
(10 Mbps Ethernet slot = 51.2 microsec = 512 100-nanosec-wide bits)
contention period: series of slot-length collisions/jamming frames
half-duplex: cannot receive while listening for own transmission collision
 sense; if idle, send; if collision, abort, random delay
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performance of Ethernet
efficiency of 10 Mbps Ethernet, 512-bit slot times
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switched Ethernet vs. hub
half-duplex
“collision domain”
full-duplex point-to-point links
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100 Mbps (“fast”) Ethernet cabling
T4: 4 each, “cat 3” unshielded twisted pairs,
3 pairs simplex forward, 1 pair simplex reverse (dynamic)
ternary encoding (trits, not bits)
TX: 2 each cat 5 unshielded twisted pairs (opposing simplex)
FX: 2 multimode fiber (opposing simplex), point-to-point links only
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gigabit Ethernet cabling
100 m and 25 m copper segments used,
e.g., in data center or POP of ISP
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Ethernet MAC sublayer framing
(a) DIX (Digital, Intel, Xerox)
(b) IEEE 802.3
preamble: for receiver clock sync
dest addr: 0* unicast, 1* multicast, all 1s broadcast
type: network protocol to call at dest,
OR
length: # data bytes (“type” embedded in data)
pad: frame length (without preamble) >= 64 bytes = 512 bits
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Why MAC is a sublayer
• Ethernet (one of the MAC protocols) interfaces
directly to network layer (IP)
• the Ethernet MAC offers best-effort, no-guarantee,
datagram service
• this is great for TCP/IP, nothing else is needed
• but, other network layer protocols expect link layer
error control and flow control services
• IEEE 802.2 (LLC) supports these services, built on
various MAC sublayers (e.g., Ethernet)
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IEEE 802.2: logical link control
- Ethernet MAC sends best effort datagrams
- LLC supports flow-control & error-control
PHY
PHY
(a) network layer sees the same LLC, regardless of type of MAC
(b) LLC encapsulates network layer packet,
MAC encapsulates LLC frame before passing to PHY
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wireless LANs
• IEEE 802.11 (“WiFi”)
• Distributed Coordination Function (DCF )
– CSMA-CA
• Point Control Function (PCF)
– centrally controlled by basestation (access point)
• short-range RF (rooms, battlefields)
• ad hoc and basestation flavors
• many PHY layer options
– 802.11, 802.11a, 802.11b, 802.11g, 802.11n (pre-std)
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The 802.11 Protocol Stack
1-2 Mbps
54 Mbps 11 Mbps 54 Mbps
1997
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2001
CSMA fails off the wire
why CSMA falls short in packet radio networks
and mobile ad hoc networks
C wants to send to B
C wants to send to D
channel sounds clear to C
channel sounds busy to C
C cannot hear A
D cannot hear B
the “hidden station problem”
the “exposed station problem”
(not “unhidden station problem”)
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wireless LAN protocols (CSMA-CA)
MACA(W), multiple access collision avoidance:
A sends RTS to B, B sends CTS to A
all potential “interrupters” hear B's CTS, wait for frame
(“W”ireless: sense first, ACK every frame, sophisticated backoff)
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802.11 MAC sublayer protocol
The use of virtual channel sensing using
CSMA/CA.
- C hears A’s RTS … D hears B’s CTS
(NAV: transmitter quiet time)
- notice how politely C and D each set aside time for A and B
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802.11 MAC sublayer protocol
A fragment burst.
- A sends a burst to B … C and D wait for it
- after each data or control frame, a system of delay intervals …
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802.11 delay timing
• how can PCF and DCF protocols coexist?
• end of frame or ACK starts series of timers
• {Short, PCF, DCF, Expanded} InterFrame Spacing
• Short for burst fragment, receiver ACK, or CTS
• PCF for central control (beacons, polling)
• DCF for contention (RTS)
• Extended for bad frame reporting (NAK)
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802.11 MAC sublayer protocol
PCF and DCF coexist in a single collision domain:
Interframe spacing in 802.11.
fragment
CTS
FRAME
dead
ACK
central ctl
RTS
NAK
ACK
the starting guns go off at different times for different frame types
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RF signaling
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RF signal strength at node's own antenna
2-antenna implementation?
shared channel
RF signal strength from distant antennae
how to detect?
– interference
– fading
– multipath
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IEEE 802.11
• access point infrastructure largely 802.11
– 100 Million 802.11 chipsets per annum (out of date statistic)
– strong application development efforts
• IEEE 802.11 spec: CSMA-CA
– RTS/CTS channel reservation, ACK
• explicit ACK
– CSMA sender will not hear interference, fading, multipath
• contention: short RTS frames, collisions waste less
• if sender’s CTS times out, it knows the RTS failed
– random backoff (countdown proceeds while channel is idle)
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802.11 distribution services
• association (connect to access point cell)
– beacons in the “jungle”; “there can only be one”
– next, DHCP discovery
• disassociation
• reassociation (cell-to-cell handover)
• distribution (how to route frames)
• integration (802.11  external network format)
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802.11 intracell services
• authentication (challenge frame, key, encryption)
– if invoked, a pre-condition for association
• deauthentication
• privacy (data encryption)
• data delivery (best effort)
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802.11 data frame
- type: data, ctl, mgt; subtype: RTS, CTS, probe (scanning for new AP)
- to DS/from DS: activation of address 3, 4 for “distribution system” APs
- MF: “more frags”; retry; more (frames)
- duration, sequence
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broadband wireless comparison ...
• 802.11 (WiFi)
– mobile Ethernet LAN
– centralized infrastructure (APs and cell architecture)
– or, distributed architecture
• ad hoc
• mobile ad hoc (MANET)
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best effort delivery
short range, half-duplex
power concerns
limited budget (commodotized)
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... broadband wireless comparison
• 802.16 (WiMax)
– wireless “local loop” for buildings
– metro area coverage, full duplex
– many users aggregated per endpoint
– connection oriented
– FEC (Hamming codes), security
– base station control (centralized control)
– fixed, directional antennas
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802.16 Protocol Stack
modulation schemes vary with range to end-points
(what is wrong with this picture?)
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The 802.16 Physical Layer
The 802.16 transmission environment.
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The 802.16 Physical Layer
- shown: TDD (time division duplexing) PHY frames
- not shown: FDD (frequency division duplexing)
- millimeter RF waves are not omnidirectional
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The 802.16 Frame Structure
(a) A generic frame.
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(b) A bandwidth request
frame.
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802.16 MAC sublayer protocol
service classes
• constant bit rate service (regular slots)
• real-time variable bit rate service (regular polling)
• non-real-time variable bit rate service (frequent polling)
• best effort service (contend for request slots)
• aggregator switch at subscriber building may negotiate
with base station for all users, and arbitrate received
bandwidth between users
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personal area networks (PANs)
• Bluetooth (Ericsson, IBM, Intel, Nokia, Toshiba)
• “cable replacement”
• specifies complete networking stack
• TDM; 10 m; 2.4 Ghz FHSS; 79 1-MHz channels
• IEEE 802.15
– only PHY & LL
• interferes with 802.11 (2.4 GHz)
• master/slave “piconets”
• slaves can bridge piconets to form “scatternets”
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remarks
• networks run on various link layer technologies
– point-to-point links vs. shared media
– wide varieties within each class
• link layer performs key services
– encoding, framing, and error detection
– optionally error correction and flow control
• shared media introduce interesting challenges
– decentralized control over resource sharing
– partitioned channel, taking turns, and random access
– Ethernet as a wildly popular example
• next: switches and bridges
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summary/glossary
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