Transcript Bluetooth: 1.Applications, Technology And Performance

Bluetooth:
1.Applications, Technology
And Performance
Bluetooth
 A cable replacement technology
 1 Mb/s symbol rate
Why not use Wireless LANs?
- power
 Single chip radio + baseband
- cost
 at low power & low price point ($5)
 Range 10+ meters
802.11
 Replacement for Ethernet
 Supported data rates
 11, 5.5, 2, 1 Mbps; and recently up to 20+Mbps @ 2.4 GHz
 up to 54 Mbps in 5.7 GHz band (802.11 a)
 Range
 Indoor 20 - 25 meters
 Outdoor: 50 – 100 meters
 Transmit power up to 100 mW
 Cost:
 Chipsets $ 35 – 50
 AP $200 - $1000
 PCMCIA cards $100 - $150
Emerging Landscape
Bluetooth
802.11
New developments are
blurring the distinction
Cordless
headset
 802.11b for PDAs
LAN AP
 Bluetooth for LAN
access
 Which option is technically superior ?
 What market forces are at play ?
 What can be said about the future ?
Bluetooth working group history
 February 1998: The Bluetooth SIG is formed
 promoter company group: Ericsson, IBM, Intel, Nokia,
Toshiba
 May 1998: Public announcement of the Bluetooth
SIG
 July 1999: 1.0A spec (>1,500 pages) is published
 December 1999: ver. 1.0B is released
 December 1999: The promoter group increases to 9
 3Com, Lucent, Microsoft, Motorola
 March 2001: ver. 1.1 is released
 Aug 2001: There are 2,491+ adopter companies
Bluetooth: Today and Tomorrow
 Will Bluetooth become a household name?
New Applications
Synchronization
User benefits
 Automatic synchronization of
calendars, address books, business
cards
 Push button synchronization
 Proximity operation
Cordless Headset
Cordless
headset
User benefits
 Multiple device access
 Cordless phone benefits
 Hands free operation
Usage scenarios examples
 Data Access Points
 Synchronization
 Headset
 Conference Table
 Cordless Computer
 Business Card Exchange
 Instant Postcard
 Computer Speakerphone
BluetoothTechnical
overview and
Protocol
Bluetooth Specifications
Applications
IP
SDP
RFCOMM
Data
Audio
L2CAP
Link Manager
Baseband
RF
Single chip with RS-232,
USB, or PC card interface
 A hardware/software/protocol description
 An application framework
Interoperability & Profiles
 Represents default
Applications
Protocols
solution for a usage
model
 Vertical slice through the
protocol stack
 Basis for interoperability
and logo requirements
 Each Bluetooth device
supports one or more
profiles
Profiles
Bluetooth Profiles (in version 1.2 release)
 Generic Access (discovery of Bluetooth devices)
 Service Discovery (establish connection and Discover available services
and connects devices)
 Cordless Telephone
 Intercom
 Serial Port
 Headset
 Dial-up Networking
 Fax
 LAN Access
 Generic Object Exchange
 Object Push
 File Transfer
 Synchronization
Bluetooth Protocol Stack
Composed of protocols to allow
Bluetooth devices to locate each
other and to create, configure
and manage both physical and
logical links that allow higher
layer protocols and applications
to pass data through these
transport protocols
Applications
IP
SDP
Data
Audio
Transport Protocol Group
RFCOMM
L2CAP
Link Manager
Baseband
RF
Bluetooth Radio Specification
Applications
IP
SDP
RFCOMM
Data
Audio
L2CAP
Link Manager
Baseband
RF
Composed of protocols to allow
Bluetooth devices to locate each other
and to create, configure and manage
both physical and logical links that allow
higher layer protocols and applications to
pass data through these transport
protocols
Radio Frequency (RF)
Sending and receiving modulated bit streams
Baseband
Defines the timing, framing
Flow control on the link.
Link Manager
Managing the connection states.
Enforcing Fairness among slaves.
Power Management
Logical Link Control &Adaptation Protocol
Handles multiplexing of higher level protocols
Segmentation & reassembly of large packets
Device discovery & QoS
RF - Unlicensed Radio Spectrum

33cm
26 Mhz
902 Mhz
12cm
83.5 Mhz
2.4 Ghz
928 Mhz
cordless phones
baby monitors
Wireless LANs
2.4835 Ghz
802.11
Bluetooth
Microwave oven
5cm
125 Mhz
5.725 Ghz
5.785 Ghz
802.11a
HyperLan
RF - Bluetooth radio link
1Mhz
. . .
79
12 3
83.5 Mhz
 frequency hopping spread spectrum
 2.402 GHz + k MHz, k=0, …, 78
 1,600 hops per second
 GFSK modulation
 1 Mb/s symbol rate
 transmit power
 0 dbm (up to 20dbm with power control)
GFSK Differences & Advantages over FSK
Modulation
 Q: What are the physical Differences between an FSK & GFSK
Modulator, and how do their results vary?
 A1: An FSK Modulator is much the same as a GFSK Modulator, but
GFSK uses a Gaussian filter as well. In a GFSK modulator
everything is the same as a FSK modulator except that before the
baseband pulses (-1, 1) go into the FSK modulator, it is passed
through a gaussian filter to make the pulse smoother so to limit its
spectral width
 A2: Gaussian filtering is one of the very standard ways for
reducing the spectral width, it is called Pulse Shaping. If we use -1
for fc-fd and 1 for fc+fd, once when we jump from -1 to 1 or 1 to -1,
the modulated waveform changes rapidly, which introduces large
out-of-band spectrum. If we change the pulse going from -1 to 1 as
-1, -.98, -.93 ..... .96, .99, 1, and we use this smoother pulse to
modulate the carrier, the out-of-band spectrum will be reduced.
Middleware Protocol Group
Additional transport protocols to allow
existing and new applications to operate
over Bluetooth. Packet based telephony
control signaling protocol also present.
Also includes Service Discovery
Protocol.
Applications
IP
SDP
RFCOMM
Data
Middleware
MiddlewareProtocol
ProtocolGroup
Group
Audio
L2CAP
Link Manager
Baseband
RF
Middleware Protocol Group (contd.)
Service Discovery Protocol (SDP)
Means for applications to discover device info, services and its
characteristics.
TCP/IP
Network Protocols for packet data communication, routing
RFCOMM(Radio Frequency Communications)
Cable replacement protocol, emulation of serial ports over
wireless network
Application Group
Applications
Application Group
IP
SDP
Consists of Bluetooth aware as well as
un-aware applications.
RFCOMM
Data
Audio
L2CAP
Link Manager
Baseband
RF
Review Format
And Power Mode
Master - Slave
Master
Device in Piconet whose clock and hopping sequence are used
to synchronize all other devices (slaves) in the Piconet.
It also carries out Paging procedure and also Connection
Establishment.
Slaves
Units within the piconet that are syncronized to the master via
its clock and hopping sequence.
After connetion establishment, Slaves are assigned a temporary
3 bit member address to reduce the no. of addresing bits
required
Piconets
Point to Point Link
Master - slave relationship
Bluetooth devices can function as masters or
slaves
m
s
Piconet
It is the network formed by a Master and one or
more slaves (max 7).
Each piconet is defined by a different hopping
channel to which users synchronize to.
Each piconet has max capacity (1 Mbps).
Hopping pattern is determined by the master.
m
s
s
s
Piconet Structure
Master
Active Slave
Parked Slave
Standby
Physical Link Types
Synchronous Connection Oriented (SCO)
Point to Point Full Duplex between Master & Slave
Established once by master & kept alive till released by Master
Typically used for Voice connection ( to guarantee continuity )
Master reserves slots used for SCO link on the channel to
preserve time sensitive information
Asynchronous Connection Link (ACL)
It is a momentary link between master and slave.
No slots are reserved.
It is a Point to Multipoint connection.
Symmetric & Asymmetric links possible
Packet Types
Data/voice
packets
Control
packets
ID*
Null
Poll
FHS
DM1
Voice
HV1
HV2
HV3
DV
Access Code
Header
Payload
data
DM1
DM3
DM5
DH1
DH3
DH5
Packet Structure
54 bits
72 bits
Access
Code
Header
0 - 2744 bits
Payload
Voice
Data
No CRC
No retries
FEC (optional)
ARQ
CRC
FEC (optional)
Access Code
Purpose
Synchronization
DC offset compensation
Identification
Signaling
Types
Channel Access Code (CAC)
•
Identifies a piconet.
Device Access Code (DAC)
•
Used for signalling procedures like paging and response paging.
Inquiry Access Code (IAC)
•
General IAC is common to all devices, Dedicated IAC is for a
dedicated group of Bluetooth devices that share a common
characteristic.
Packet Header
Addressing ( 3 bits )
Packet type (4 bits )
Flow Control ( 1 bit )
1-bit ARQ
Sequencing ( 1 bit )
HEC ( 8 bit )
For filtering retransmitted packets
Verify header integrity
Connection State Machine
Inquiry
Page
Standby
Connected
Transmit data
Park
Hold
Sniff
Connection State Machine (contd.)
Inquiry Scan
A device that wants to be discovered will periodically enter
this mode and listen for inquiry packets.
Inquiry
Device sends an Inquiry packet addressed to GIAC or DIAC
Transmission is repeated on the inquiry hop sequence of
frequencies.
Inquiry Response
When an inquiry message is received in the inquiry scan state,
a response packet (FHS) containing the responding device
address must be sent after a random number of slots.
Connection State Machine (contd.)
Inquiry Response
Connection State Machine (contd.)
Page
The master uses the clock information, about the slave to be paged,
to determine where in the hop sequence, the slave might be listening
in the page scan mode.
The master sends a page message
Page Scan
The page scan substate can be entered by the slave from the standby
state or the connection state. It listens to packets addressed to its
DAC.
Page Response
On receiving the page message, the slave enters the slave page
response substate. It sends back a page response consisting of its ID
packet which contains its DAC, at the frequency for the next slot
from the one in which page message was received.
Security
Security Measures
Limited/Restricted Access to authorized users.
Both Link Level Encryption & Authentication.
Personal Identification Numbers (PIN) for device access.
Long encryption keys are used (128 bit keys).
These keys are not transmitted over wireless. Other parameters
are transmitted over wireless which in combination with certain
information known to the device, can generate the keys.
Further encryption can be done at the application layer.
Security values
Device Address-Public
Authentication Key(128 bits)-Private
Encryption Key(8-128 bits)-Private
Random Number
Frequency Hop Spread-Spectrum
 Bluetooth channel is represented
by a pseudo random hopping
sequence through the entire 79
RF frequencies
 Nominal hop rate of 1600 hops
per second
 Channel Spacing is 1 MHz
Time-Division Duplex Scheme
 Bluetooth devices use a Time-Division Duplex (TDD) scheme
 Channel is divided into consecutive slots (each 625 s)
 One packet can be transmitted per slot
 Subsequent slots are alternatively used for transmitting and
receiving
 Strict alternation of slots b/t the master and the slaves
 Master can send packets to a slave only in EVEN slots
 Slave can send packets to the master only in the ODD slots
Review of basic
concepts
Baseband
Applications
SDP
IP
RFCOMM
Data
Audio
L2CAP
Link Manager
Baseband
RF
Bluetooth Physical link
 Point to point link
 master - slave relationship
 radios can function as masters or slaves
m
m
 Piconet
 Master can connect to 7 slaves
 Each piconet has max capacity =1 Mbps
 hopping pattern is determined by the master
s
s
s
s
Connection Setup
 Inquiry - scan protocol
 to learn about the clock offset
and device address of other
nodes in proximity
Inquiry on time axis
Slave1
f1
f2
Inquiry hopping
sequence
Master
Slave2
Piconet formation
 Page - scan protocol
 to establish links with
nodes in proximity
Master
Active Slave
Parked Slave
Standby
Addressing
 Bluetooth device address (BD_ADDR)
 48 bit IEEE MAC address
 Active Member address (AM_ADDR) (see power mode)
 3 bits active slave address
 all zero broadcast address
 Parked Member address (PM_ADDR) (see power mode)
 8 bit parked slave address
Piconet channel
FH/TDD
f1
f2
m
s1
s2
625 sec
1600 hops/sec
f3
f4
f5
f6
Multi slot packets
FH/TDD
f1
f4
f5
m
s1
s2
625 µsec
Data rate depends on type of packet
f6
Physical Link Types
 Synchronous Connection Oriented (SCO) Link
 slot reservation at fixed intervals
 Asynchronous Connection-less (ACL) Link
 Polling access method
m
s1
s2
SCO
ACL
ACL
SCO
ACL
ACL
SCO
ACL
ACL
Packet Types
Data/voice
packets
Control
packets
ID*
Null
Poll
FHS
DM1
Voice
HV1
HV2
HV3
DV
data
DM1
DM3
DM5
DH1
DH3
DH5
Packet Format
72 bits 54 bits
Access
code
slave
Payload
Voice
Data
No CRC
No retries
FEC (optional)
ARQ
625 µs
master
Header
0 - 2744 bits
CRC
FEC (optional)
Access Code
72 bits
Access
code
Header
Purpose
Payload
Types
 Synchronization
 Channel Access Code (CAC)
 DC offset compensation
 Device Access Code (DAC)
 Identification
 Inquiry Access Code (IAC)
 Signaling
X
Packet Header
54 bits
Access
code
Header
m
Payload
s
s
s
Purpose
 Addressing
Max 7 active slaves

16 packet types (some unused)




(3)
Packet type (4)
Flow control (1)
1-bit ARQ
(1)
Sequencing (1)
HEC
(8)
total
Broadcast packets are not ACKed
For filtering retransmitted packets
Verify header integrity
18 bits
Encode with 1/3 FEC to get 54 bits
Voice Packets (HV1, HV2, HV3)
Data Packet Types
Symmetric
2/3 FEC
DM1
108.8 108.8 108.8
DM3
258.1 387.2
54.4
DM5
286.7 477.8
36.3
Symmetric
No FEC
Asymmetric
DH1
DH3
DH5
Asymmetric
172.8 172.8 172.8
390.4 585.6 86.4
433.9 723.2 57.6
Inter piconet communication
Cordless
headset
mouse
Cordless
headset
Cell phone
Cell phone
Cell phone
Cordless
headset
Scatternet
Scatternet, scenario 2
How to schedule presence in
two piconets?
Forwarding delay ?
Missed traffic?
Baseband: Summary
Device 1
Device 2
L2CAP
L2CAP
LMP
Data link
LMP
Baseband
Baseband
Physical
 TDD, frequency hopping physical layer
 Device inquiry and paging
 Two types of links: SCO and ACL links
 Multiple packet types (multiple data rates with
and without FEC)
Link Manager Protocol
Applications
IP
SDP
RFCOMM
Data
Audio
Setup and management
of Baseband connections
L2CAP
Link Manager
Baseband
RF
LMP
• Piconet Management
• Link Configuration
• Security
Piconet Management
 Attach and detach slaves
 Master-slave switch
 Establishing SCO links
 Handling of low power modes ( Sniff, Hold, Park)
Paging
s
s
req
response
Slave
s
Master
m
Low power mode (hold)
Hold offset
Slave
Hold duration
Master
Hold Mode
By this capacity can be made free to do other things like scanning,
Slave temporarily (for Thold sec) does not support ACL packets on
the channel (possible SCO links will still be supported).
paging, inquiring, or attending another piconet.
The slave unit keeps its active member address (AM_ADDR)
Low power mode (Sniff)
Sniff offset
Sniff duration
Slave
Sniff period
Master
Sniff Mode (Traffic reduced to periodic sniff slots)
This is a low power mode in which the listening activity of the slave is
reduced.
In the sniff mode, the slave listens for transmissions only at fixed intervals
Tsniff, at the offset slot Dsniff for Nsniff times. These parameters are given by
the LMP in the master when it issues the SNIFF command to the slave.
Low power mode (Park)
Slave
Beacon instant
Master
Beacon interval
Park Mode (Power saving + keep more than 7 slaves in a piconet)
 This is a very low power mode with very little activity.
 The slave however, stays synchronized to the channel.
 The parked slaves regularly listen for beacon signals at intervals decided by the
beacon structure communicated to the slave during the start of parking.
 The parked slave has to be informed about a transmission in a beacon channel
which is supported by the master to keep parked slaves in synchronization and
send them any other information.
 Any message to be sent to a parked member are sent over the broadcast
channel. Communication via broadcast LMP messages.
 It also helps the master to have more than seven slaves
Connection establishment & Security
 Goals
 Authenticated access
 Only accept connections from trusted
devices
 Privacy of communication
Paging
 prevent eavesdropping
 Cannot rely on PKI
 Simple user experience
LMP Accepted
Security procedure
LMP_setup_complete
LMP_setup_complete
Slave
 $10 headsets, joysticks
LMP_host_conn_req
Master
 Constraints
 Processing and memory
limitations
Authentication
 Authentication is based on link key (128 bit shared
secret between two devices)
 How can link keys be distributed securely ?
Link key
response
accepted
Claimant
Verifier
challenge
Link key
Pairing (key distribution)
 Pairing is a process of establishing a trusted secret
channel between two devices (construction of
initialization key Kinit)
 Kinit is then used to distribute unit keys or combination
keys
PIN
+
Verifier
Claimant
address
Random
number
Random number
Claimant
challenge
Kinit
+
Claimant
address
Random
number
response
accepted
PIN
Kinit
Link Manager Protocol Summary
Device 1
Device 2
L2CAP
L2CAP
LMP
Data link
LMP
Baseband
Baseband
Physical
 Piconet management
 Link configuration
 Low power modes
 QoS
 Packet type selection
 Security: authentication and encryption
L2CAP
Applications
IP
SDP
Logical Link Control and
Adaptation Protocol
RFCOMM
Data
Audio
L2CAP
Link Manager
Baseband
RF
L2CAP provides
• Protocol multiplexing
• Segmentation and Re-assembly
• Quality of service negotiation
Why baseband isn’t sufficient
IP
MTU
Baseband
RFCOMM
IP
RFCOMM
Multiplexing
demultiplexing
reliable*, flow controlled
in-sequence, asynchronous link
• Baseband packet size is very small (17min, 339 max)
• No protocol-id field in the baseband header
Need a multiprotocol encapsulation layer
IP
IP
RFCOMM
unreliable, no integrity
reliable*, in-order,
flow controlled, ACL link
Desired features
• Protocol multiplexing
• Segmentation and re-assembly
• Quality of service
What about
• Reliability?
• Connection oriented or
connectionless?
• integrity checks?
RFCOMM
Segmentation and reassembly
Payload
Length
Baseband
packets
start of
L2CAP
CRC
CRC
CRC
continuation
of L2CAP
continuation
of L2CAP
• cannot cope with re-ordering or loss
• mixing of multiple L2CAP fragments not allowed
• If the start of L2CAP packet is not acked, the rest
should be discarded
min MTU = 48
672 default
Serial Port Emulation using RFCOMM
Applications
IP
SDP
RFCOMM
Data
Audio
L2CAP
Link Manager
Baseband
RF
Serial Port emulation on top of a
packet oriented link
• Similar to HDLC
• For supporting legacy apps
Serial line emulation over packet based MAC
RFCOMM
L2CAP
 Design considerations
 framing: assemble bit stream into
bytes and, subsequently, into packets
 transport: in-sequence, reliable
delivery of serial stream
 control signals: RTS, CTS, DTR
RFCOMM
L2CAP
IP over Bluetooth V 1.0
Applications
IP
SDP
RFCOMM
Data
GOALS
 Internet access using cell phones
Audio
L2CAP
Link Manager
Baseband
RF
 Connect PDA devices & laptop
computers to the Internet via LAN
access points
LAN access point profile
IP
Access Point
PPP
Why use PPP?
Security
Authentication
Access control
Efficiency
header and data compression
Auto-configuration
Lower barrier for deployment
RFCOMM
L2CAP
Baseband
Inefficiency of layering
Palmtop
LAN access point
IP
IP
PPP
rfc 1662
packet oriented
byte oriented
rfc 1662
RFCOMM
RFCOMM
L2CAP
PPP
packet oriented
L2CAP
 Emulation of RS-232 over the Bluetooth radio link could be
eliminated
Terminate PPP at LAN access point
Palmtop
Access Point
IP
IP
PPP
PPP
RFCOMM
RFCOMM
Bluetooth
ethernet
Bluetooth
 PPP server function at each access point
 management of user name/password is an issue
 roaming is not seamless
L2TP tunneling
Palmtop
Access Point
PPP server
IP
IP
PPP
PPP
UDP
UDP
RFCOMM
RFCOMM
IP
IP
Bluetooth
Bluetooth
ethernet
ethernet
 Tunneling PPP traffic from access points to the PPP server
 1) centralized management of user name/password
 2) reduction of processing and state maintenance at each access
point
 3) seamless roaming
Seamless roaming with PPP
PPP
Server
1
REQ
2
RPL
5
CLR
AP1
MAC level registration
PPP
palmtop
4
RPL
3
REQ
AP2
MAC level handoff
PPP
Bluetooth
Current Market
Outlook
Market Forcasts for year 2005
Cahners In-stat (2000 forcast)
revised (2001 forcast)
Merrill Lynch (2000 forcast)
$ 5.4 bn
2.1 bn
$ 4.4 bn
$ 4.3 bn
revised (2001 forcast)
1.4 bn
$ 2.2 bn
$ 4.4
1.5 bn
995 m
$ 3.6
$ 2.02
Units sold annually
Revenue
Chip price
Value to carriers: Synchronization and Push
 More bits over the air
 Utilization of unused capacity
during non-busy periods
 Higher barrier for switching
service providers
Value to carriers: Cell phone as an IP gateway
Will Pilot and cell phone eventually merge?
 More bits over the air
 Enhanced user experience
 Palmpilot has a better UI than a cell phone
 Growth into other vertical markets
Value to carriers: Call handoff
Cordless base
Threat
or
opportunity?
 More attractive calling plans
 Alleviate system load during peak periods
 Serve more users with fewer resources
Biggest challenges facing Bluetooth
 Interoperability
 Always a challenge for any new technology
 Hyped up expectations
 Out of the box ease of use
 Cost target $5
 Critical mass
 RF in silicon
 Conflicting interests – business and engineering
References
 [1] IEEE 802.11, “Wireless LAN MAC and Physical
Layer Specification,” June 1997.
 [2] Hirt, W.; Hassner, M.; Heise, N. “IrDA–VFIr (16
Mb/s): modulation code and system design.” IEEE
Personal Communications, vol.8, (no.1), IEEE, Feb.
2001.
 [3] Lansford, J.; Bahl, P. “The design and
implementation of HomeRF: a radio frequency
wireless networking standard for the connected
home.” Proceedings of the IEEE, IEEE, Oct. 2000.
 [4] Specification of Bluetooth System, ver. 1.0, July
1999
References (cnt)
 [5] Haartsen, J.C. “The Bluetooth radio system.”, IEEE Personal


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Communications, IEEE, Feb. 2000.
[6] Haartsen, J.C. ‘Bluetooth towards ubiquitous wireless
connectivity.’, Revue HF, Soc. Belge Ing. Telecommun. &
Electron, 2000. p.8–16.
[7] Rathi, S. “Bluetooth protocol architecture.” Dedicated
Systems Magazine, Dedicated Systems Experts, Oct.–Dec.
2000.
[8] Haartsen, J.C.; Mattisson, S. “Bluetooth–a new low–power
radio interface providing short–range connectivity.” Proceedings
of the IEEE, IEEE, Oct. 2000.
[9] Gilb, J.P.K “Bluetooth radio architectures.” 2000 IEEE Radio
Frequency Integrated Circuits (RFIC) Symposium Digest of
Papers, Boston, MA, USA, 11–13 June 2000.
References (cnt)
 [10] N. Benvenuto, G. Cherubini, “Algoritmi e circuiti per le
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telecomunicazioni”, Ed. Libreria Progetto.
[11] The Bluetooth Special Interest Group, Documentation
available at http://www.bluetooth.com/
[12] IEEE 802.15 Working Group for WPANs™;
http://www.manta.ieee.org/groups/802/15/
[13] Barker, P.; Boucouvalas, A.C.; Vitsas, V. “Performance
modelling of the IrDA infrared wireless communications
protocol.” International Journal of Communication Systems,
vol.13, Wiley, Nov.–Dec. 2000.
[14] Tokarz, K.; Zielinski, B. “Performance evaluation of IrDA
wireless transmission.” 7th Conference on Computer Networks,
Zakopane, Poland, 14–16 June 2000.
[15] ETSI RES, “Digital European Cordless Telecommunications
(DECT), Common interface Part 1: Overview,” ETS 300 175–1,
1996.
Synchronization using a Beacon
(infrastructure)
beacon interval
access
point
medium
B
B
busy
busy
B
busy
B
busy
t
value of the timestamp
B
beacon frame
Synchronization using a Beacon (ad-hoc)
beacon interval
station1
B1
B1
B2
station2
medium
busy
busy
B2
busy
busy
t
value of the timestamp
B
beacon frame
random delay