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Shared Access Networks
Outline
Ethernet
Wireless
1
Ethernet Overview
• History
–
–
–
–
developed by Xerox PARC in mid-1970s
roots in Aloha packet-radio network
standardized by Xerox, DEC, and Intel in 1978
similar to IEEE 802.3 standard
• CSMA/CD
– carrier sense
– multiple access
– collision detection
• Frame Format
64
48
48
16
Preamble
Dest
addr
Src
addr
Type
32
Body
CRC
2
3
Ethernet (cont)
• Addresses
–
–
–
–
unique, 48-bit unicast address assigned to each adapter
example: 8:0:e4:b1:2
broadcast: all 1s
multicast: first bit is 1
• Bandwidth: 10Mbps, 100Mbps, 1Gbps, 10Gbps
• Length: 2500m (500m segments with 4 repeaters)
• Problem: Distributed algorithm that provides fair access
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Transmit Algorithm
• If line is idle…
– send immediately
– upper bound message size of 1500 bytes
– must wait 9.6us between back-to-back frames
• If line is busy…
– wait until idle and transmit immediately
– called 1-persistent (special case of p-persistent)
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Algorithm (cont)
• If collision…
– jam for 32 bits, then stop transmitting frame
– minimum frame is 64 bytes (header + 46 bytes of data)
• RTT required for detecting collision
• RTT for 2.5Km max length Ethernet is 51.2 us, equivalent to 512
b (64B) @10 Mbps (must accommodate slowest version)
– delay and try again
• 1st time: 0 or 51.2us
• 2nd time: 0, 51.2, or 102.4us
• 3rd time51.2, 102.4, or 153.6us
• nth time: k x 51.2us, for randomly selected k=0..2n - 1
• give up after several tries (usually 16)
• exponential backoff
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Collisions
A
B
A
B
A
B
A
B
(a)
(b)
(c)
(d)
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Wireless Networks
Bluetooth
802.15.1
WiFi
802.11
WiMax
802.16
3G Cellular
Typical link
length
10m
100m
10km
Tens of km
Typical
bandwidth
2.1Mbps
(shared)
54Mbps
(shared)
70Mbps
(shared)
384+Kbps
(per
connection)
Typical use
Connect
peripheral to
computer
Provide
network access
to notebook
Provide
network access
to building
Provide
network access
to cell phone
Wired
technology
analogy
USB
Ethernet
Coaxial cable
DSL
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IEEE 802.11 (WiFi) Protocol Stack
9
The 802.11 Physical Layer
(Earlier Versions)
• Infrared:
– 1 or 2 Mbps, good indoor isolation (do not penetrate walls), poor
outdoor performance (sunlight interferes), rarely used
• Frequency Hopping Spread Spectrum (FHSS):
– 79 1MHz channels, at 2.4 GHz ISM band, both sender and receiver
frequency jumps according to a pseudorandom number generator
with the same seed.
– Advantage: secure, resistant to fading and interference.
– Disadvantage: low bandwidth
• Direct Sequence Spread Spectrum (DSSS)/CDMA
– uses 11chip Baker sequence, modulation is PSK at 1 MHz, each
baud could encode 1 or 2 bits, giving data rates at 1 or 2 Mbps.
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The 802.11 Physical Layer
(Later Versions)
• High Rate DSSS (HR-DSSS, 802.11b)
– 11 Mbps at 2.4 GHz ISM band, can adapt to lower rates to cope
with high load/noise but rarely needed, uses Walsh/Hadamard
sequence.
• Orthogonal Frequency Division Multiplexing (OFDM, 802.11a):
– 54 Mpbs at 5 GHz ISM band, signal is split into 52 frequency
bands (48 data plus 4 synchronization), can use noncontiguous
bands. Modulation: PSK for below 18 Mbps and QAM above. At
54 Mbps, 288 bits QAM symbol (216 bits data).
– Better resistance to fading and interference, better spectral
efficiency, but the range is 7 times shorter than 802.11b.
• OFDM (802.11g):
– a compromise between 802.11a and b, 54 Mbps at 2.4 GHz.
• 802.11n: Uses MIMO, ~200 Mbps.
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MAC Collision: Hidden and Exposed
Station Problem
(a) The hidden station problem, (b) The exposed station problem.
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MACAW
• Sender transmits RequestToSend (RTS) frame
• Receiver replies with ClearToSend (CTS) frame
• Neighbors…
– see CTS: keep quiet
– see RTS but not CTS: ok to transmit
• Receive sends ACK when has frame
– neighbors silent until see ACK
• Collisions
– no collisions detection
– known when don’t receive CTS
– exponential backoff
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The 802.11 MAC Sublayer
• 802.11 supports two modes:
– DCF: distributed coordination function, ad hoc networking
– PCF: point coordination function, base station based, optional
• DCF: uses CSMA/CA (collision avoidance). Two methods
of operation:
– Physical channel sensing: sender senses the channel, transmit if
idle but stop sensing during the transmission, not transmit if busy,
exponential backoff if collision.
– Virtual channel sensing: MACAW like RTS/CTS scheme
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Supporting Mobility
• Case 1: ad hoc networking
• Case 2: access points (AP)
– tethered
– each mobile node associates with an AP
Distribution system
AP-1
AP-3
F
AP-2
A
B
G
H
C
E
D
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Mobility (cont)
• Scanning (selecting an AP)
–
–
–
–
–
node sends Probe frame
all AP’s w/in reach reply with ProbeResponse frame
node selects one AP; sends it AssociateRequest frame
AP replies with AssociationResponse frame
new AP informs old AP via tethered network
• When
– active: when join or move
– passive: AP periodically sends Beacon frame
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The 802.16 (WiMax) Protocol Stack
•
•
QPSK and QAM are used for the physical layer, new modulation schemes are also
proposed: 802.16a uses OFDM at 2-11 GHz, 802.16b at 5 GHz ISM band.
MAC sublayer common part is where the main protocol resides, handles channel
allocation, controlled by the base station, connection oriented. The convergence sublayer
handles interoperation with IP or ATM network.
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The 802.16 Physical Layer
The 802.16 transmission environment.
•
•
•
•
Unlike wireless LAN, directional antenna is used, users are divided into sections that do
not interfere with each other.
Since signal falls off sharply in the millimeter bands, different modulation schemes are
used at different distances. At typical 25 MHz spectrum, closer users get 150 Mbps, distant
ones only get 50 Mbps.
FDD (frequency division duplexing ) and TDD (time division duplexing) to provide 2-way
communication. Channel allocation for upstream and down streams can be dynamically
changed according to traffic demands.
Hamming codes are used for error correction, nearly all other networks only use checksum
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to detect error.
The 802.16 MAC Sublayer Protocol:
Class-Based
• Constant bit rate service: for uncompressed voice, channels are statically
allocated.
• Real-time variable bit rate service: for compressed multimedia, base station
polls the uses at fixed intervals asking how much bandwidth is needed.
• Non-real-time variable bit rate service: for large file transfers, the base
station polling is not at fixed rapid interval, and the users can ask for
channels. If the user does not respond to the polling k times in a row, it is
put into a multicast polling group, where anyone can respond to a poll,
contention for the service is possible but bandwidth waste is reduced.
• Best efforts service: for everything else, no polling, users contend for
service by making the channels in the upstream map, if successful, will be
noted in the next downstream map, if unsuccessful, exponential backoff.
• Bandwidth can be allocated per station or per connection.
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802.15.1(Bluetooth)
•
•
•
•
The basic unit of Bluetooth is a piconet, which consists of a master node and up to
seven active slave nodes within a distance of 10 meters. Multiple piconets are
connected through a bridge node forming a scatternet.
In addition to the slave nodes, each piconet can have up to 255 parked nodes, which
the master node puts to sleep to save power. In the parked state, a node can only
respond to an activation or beacon signal from the master node.
The master assigns time slot in a centralized TDM scheme, all communication is
between master and slave, no slave-slave communication.
The master-slave design makes possible for dumb and inexpensive slaves. The target
price for Bluetooth cards is under $5.
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Bluetooth Architecture
Two piconets can be connected to form a scatternet.
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Bluetooth Physical Layer
• Low power radio (2.5 mW) operates at 2.4 GHz ISM band, 10 m
range, interferes with 802.11.
– In comparison, WiFi requries 100s mW, 3G cellphone a little less.
• The band is divided into 79 1MHz channels, FSK is used, 1 bit per
symbol, so the raw data rate per channel is 1 Mbps, but much of that is
used for overhead.
• FHSS is used, with hopping frequency of 1600 hops/sec, and a dwell
time of 625 us, the master node dictates the hoping sequence.
• Time slots are 625 us long. The master’s transmission starts in even
slots, the slaves’ in odd ones, so they get half bandwidth each.
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4G Cellphone
• Speed: 100 Mbps for high mobility
communications (cars) 1 Gbps for low mobility
communications.
• All-IP protocol, support IPv6
• Physical layer: OFDM and MIMO
• Earlier version rolled out in S. Korea,
Scandinavia, Sprint Nextel in US with lower
speed.
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