Wireless network architecture

1
Wireless network architecture

• Base station approach
• Ad-hoc network approach
• Bluetooth

2
Bluetooth basics

• Early development effort by Ericsson, 1994.
  Bluetooth SIG started in 1998
• Specifications of bluetooth
• Idea
• Universal radio interface for ad-hoc wireless
  connectivity
• Interconnecting computer and peripherals,
  handheld devices, PDAs, cell phones
• Embedded in other devices
• Short range (10 m), low power consumption,
  license-free 2.45 GHz
• Voice and data transmission, approx. 1 Mbit/s
  gross data rate

3
Bluetooth

• Several Bluetooth devices can form an ad hoc
  network called a Piconet
• In a Piconet, one device acts as a master (sets
  frequency hopping behavior) and the others as
  slaves
• Example A conference room with many laptops
  wishing to communicate with each other

• Interference from wireless LANs, digital cordless
  phones, microwave ovens
• frequency hopping to minimize interference
• MAC protocol supports
• Forward Error Correction (FEC)
• Automatic Repeat reQuest (ARQ)

4
Bluetooth vs 802.11

• Bluetooth
• Designed for quick, seamless short range networks
• High mobility, and ah hoc connectivity
• Features low power consumption, small protocol
  stack, robust data voice transfer
• Cheap price
• WPAN (Wireless Personal Area Networks)
• 802.11
• Designed for infrequent mobility, IP-based data
  transmission
• Medium range and high data rate
• More money
• WLAN (Wireless Local Area Networks)

5
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• Wireless links and LANs
• 5.7 PPP
• 5.8 ATM

6
Point to Point Data Link Control

• one sender, one receiver, one link easier than
  broadcast link
• no Media Access Control
• no need for explicit MAC addressing
• no multiple access protocols
• e.g., dialup link, ISDN line
• popular point-to-point DLC protocols
• PPP (point-to-point protocol)
• HDLC High level data link control (Data link
  used to be considered high layer in protocol
  stack!

7
Basics

• Link layer protocol, supports multiple protocols,
  not just the IP network protocol.
• IP addresses agreed at connection time.
• How PPP works
• The PC first dials up the ISP, when a router
  answers the call, a physical layer connection is
  established.
• The Data Link Layer on the PC and on the router
  exchange control packets to select the best PPP
  parameters to use.
• When this is done, the network layer on both
  sides exchange packets to configure the network
  layer (i.e. agree on the network layer protocol
  to use, typically IP)

8
PPP Design Requirements RFC 1557

• packet framing encapsulation of network-layer
  datagram in data link frame
• carry network layer data of any network layer
  protocol (not just IP) at same time
• ability to demultiplex upwards
• bit transparency must carry any bit pattern in
  the data field, no constraints
• error detection (CRC)
• connection liveness detect, signal link failure
  to network layer
• network layer address negotiation endpoint can
  learn/configure each others network address
• simplicity

9
PPP non-requirements

• no error correction/recovery
• no flow control
• out of order delivery OK
• no need to support multipoint links (e.g.,
  polling)

Error recovery, flow control, data re-ordering
all relegated to higher layers!
10
PPP Data Frame

• Flag delimiter (framing)
• Address does nothing (only one option)
• Control does nothing in the future possible
  multiple control fields
• Protocol upper layer protocol to which frame
  delivered (eg, PPP-LCP, IP, IPCP, etc)

11
PPP Data Frame

• info upper layer data being carried
• check cyclic redundancy check for error
  detection

12
Byte Stuffing

• data transparency requirement data field must
  be allowed to include flag pattern lt01111110gt
• Q is received lt01111110gt data or flag?
• Sender adds (stuffs) extra lt 01111101gt byte
  before each lt 01111110gt data byte.
  lt01111101gtcontrol escape
• Receiver
• bytes 01111101, 01111110 bytes in a row discard
  first byte, continue data reception
• single 01111110 flag byte
• Two 01111101 bytes?

13
Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
Unpredictable frame size due to byte
stuffing. Malicious users may send 0x7D 0x7E to
inflate bandwidth!
14
Actual coding

• Any occurrence of flag or control escape inside
  of frame is replaced with 0x7D followed by
  original octet XORed with 0x20 (00100000)

Data to be sent
32
31
7D
7E
33
34
35
5D
32
7D
5E
33
34
35
7E
31
7D
7E
After stuffing and framing
15
PPP Link Control Protocol (LCP)

• Before exchanging network-layer data, data link
  peers must
• configure PPP link (max. frame length,
  authentication)
• learn/configure network
• layer information
• for IP carry IP Control Protocol (IPCP) messages
  (protocol field 8021) to configure/learn IP
  address