1

Shared Access Networks

OutlineEthernet

Wireless

2

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

Destaddr

64 48 32

CRCPreamble Srcaddr Type Body

1648

3

4

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

5

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)

6

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

7

Collisions

(a)

(b)

(c)

A B

A B

A B

A B

(d)

8

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

9

IEEE 802.11 (WiFi) Protocol Stack

10

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.

11

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.

12

MAC Collision: Hidden and Exposed Station Problem

(a) The hidden station problem, (b) The exposed station problem.

13

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

14

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

15

Supporting Mobility• Case 1: ad hoc networking• Case 2: access points (AP)

– tethered– each mobile node associates with an AP

BH

A

F

G

D

AP-2

AP-3AP-1

C E

Distribution system

16

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

17

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.

18

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 to detect error.

19

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.

20

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.

21

Bluetooth Architecture

Two piconets can be connected to form a scatternet.

22

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.

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.

23