Transcript Fast Packet Technology

Fast Packet Technology
ISOM 591
March 27, 2000
Global WANs Must Support
Bandwidth Demands
• http://www.specialty.com/highband
• How to Determine Broadband Needs?
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Internet access
video-on-demand and multimedia
teleconferencing
CAD/CAM applications
Broadband in the Analog World
• broadband in the analog world is the ability
to stack frequencies on a single
transmission medium, providing multiple
channels on the same medium like radio or
TV, using multiplexing techniques
Broadband in the Digital World
• in the digital world, broadband loosely
means any data rate greater than T-1 speeds
(1.544 Mbps)
Broadband vs. Narrowband
• narrowband communications channels
provide a single channel for
communications so that only one device can
transmit at a time
– ex: Ethernet LANs
Fast Packet Switching to Support
Broadband
• fast packet-switching
– differs from conventional packet-switching in
these ways paths are established dynamically
bandwidth is dynamically assigned packets are
rates
• modern transmission media is highly
reliable and needs less error-checking
Types of Communication
Connections
• connection-oriented
– a connection path is established before data is
transfered
– this connection path is unavailable to other
users while in use
– ex: telephone calls
Types of Communication
Connections
• connectionless
– no connection is established for a
communication
– packets associated with the communication are
transmitted on the medium as they are
generated
Bursty Data
• traditional data applications are called
“bursty” because the data transported
between the application and the user is not
transmitted at a fixed, predictable rate
• this causes congestion at peak demand
times
• packet-switching serves bursty traffic well
• note that voice and video need packets to
arrive at a highly uniform rate
Protocols
• for two devices to communicate, must agree
on the same set of conventions, or protocols
• protocols include decisions about how to
structure a packet or frame, what is included
in a packet header, how to do error
detection, network management issues
• during transmission, different nodes need to
have different levels of information
OSI and Interconnection Devices
• Application
(gateway)
– user application
• Presentation
– formats and converts data
• Session
– establishes the communications session
these three layers are not used for transport
across the network
OSI (Open Systems
Interconnection)
• Transport
– manages the end to end control of the
communication
• Network (routers)
– routes the data to the correct destination
• Data link (bridges)
– transmits data to and from each node
• Physical (repeater)
– electrical connections
The Life of a Packet: Its Journey
FEP
Network Interface
Card
Global network
The Life of a Packet:
Application Layer
• A spreadsheet is to be sent
from Chicago to London
• the spreadsheet application
program adds some
information to the data
identifying it as a spreadsheet
so that the destination program
knows what to do with it
The Life of a Packet:
Presentation Layer
• The combination of data and
control information flows to
the company gateway computer
or front end processor
• the bit stream is compressed or
encrypted as necessary and a
header is added to identify the
application needed
The Life of a Packet: Session
Layer
• FEP divides the bit stream
into segments, each of
which is encapsulated into a
packet
• the packets are moved to a
network interface device
The Life of a Packet: Transport
Layer
• A header is added that is
needed for end-to-end error
control and network
management
• TCP typically used at this
layer
The Life of a Packet: Network
Layer
• Another header is added by the end node,
which will be used by the intermediate
nodes to determine routing and perform call
setup and teardown
• IP typically used at this layer
The Life of a Packet: Data Link
Layer
• Final header is added which includes
necessary info on error checking in
route, how the packet is constructed,
how to handle time outs
• HDLC typically used at this level
The Life of a Packet: Physical
Layer
• Procedures at this layer handle how the
physical interface is achieved
• hardware and driver software work together
• for example, voltage levels and how the
data pins carry specific signals are
established
Packets and Protocol Data Units
Packet
Data Link Network
Header
Header
Layer 2
Layer 3
Transport Session
Header
Header
Layer 4
Layer 5
Raw data &
control info
Layers 6,7
The Packet Is Ready to Travel
The Packet Travels Through the
Network
• At each intermediate node, bridge, or router,
the lowest two layers of info are stripped off
so the node knows how to
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route the packet to its destination
perform error checking
manage the network
create reports on network conditions
• when the packet arrives at the next
gateway…..
Data Link and Network
• At the data link layer, arrange the packets in
sequence and passed up to the network link
layer
• gateway checks the destination address from
the session layer
• if gateway recognizes the destination
address, layer 4 header information
(transport) is read to determine if network and
session layers need to exchange any info
Transport and Session Layer
• End to end checking is done here
• the PDU is sent on to the session layer
• the receiver follows the instructions which
might include error checking to be done at
the session layer
• corrupt packets are discarded, a NAK
(negative acknowledgement) is sent
requesting the packet be re-sent
Presentation and Application
• Good packets move into the destination
computer for de-encryption and
decompression as needed
• finally, all control information is stripped
away and the destination computer is
informed about the type of application
which has been sent
• this entire process takes less than one
second!
Transmission Methods and
Procedures
• data communications standards
• ex: X.25 is a standard protocol for packet
switching
• orderly rules for transferring information
throughout the network
• what is a protocol?
– exchange of information to initiate a
conversation and set up rules for conducting the
conversation
Transmission Protocol: What
Does It Do?
• initiation of communication
• character identification and grouping
• message or conversation identification to
determine where one ends and another
begins
• link control to regulate the flow of data
• error detection and correction
• termination of the communication
Communication Services
• packet-switching services
– data is transmitted, a packet at a time
– each packet may take a different path through
the network
– no predefined virtual circuit
– can route packets around broken or congested
lines
– handles bursty communications well
– ex: X.25, frame relay
Communication Services
• cell-switched services
– smallest unit of data switched is a fixed-size
cell rather than a variable sized packet
– switching can be done in hardware
– ex: ATM (asynchronous transfer mode), SMDS
(switched multimegabit data service)
Network Connections
• need to consider hardware devices,
protocols supported, speed, cost, function
• switching techniques
– on-the-fly forwards each frame before it
receives the entire packet
– store-and-forward (buffered) places the data in
storage until the entire packet is received
Switches
• switches receive electronic signals and
redirect them through the network to their
final destination
• switches route your telephone call
• multiplexing (combining many slower
speed messages into single messages to take
advantage of high speed capacity lines)
• intelligent switches route both voice and
data traffic
Intelligence in Switches
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error handling
accounting and control
message format conversion
analog-to-digital conversion
prevention of unauthorized
access
• fault detection and diagnosis
X.25
• designed in the mid 1970s as a type of
packet-switching for the public data
network over copper twisted-pair wire
• divides user data into packets sent from
node to node through a network
• extensive addressing and error checking
between intermediate nodes
• needed because of inherent unreliability of
the communication lines in the 70s
X.25
• works at the physical, data link, and
network layers
• requires special interface at subscriber
premises
• PBX or PAD (packet
assembly/disassembly) device
• bridges and routers contain PAD to support
X.25
• still an economical choice for business
Frame Relay
• global WAN technology offered by most public and private
carriers
– AT&T, MCI, RBOCs
• supports speeds of 2.048 Mbps (E-1 speed)
• virtual circuit packet switching
• packet size is variable, up to 9000 bytes
• uses twisted pair or above
• single point of network access so that multiple LANs can
have access to the frame relay network over a T-1 line to a
central office
Frame Relay
• lower overhead and higher throughput than
X.25 because less error checking is done
• standards set by CCITT, ANSI
• world wide use
• can transport other types of packets
• major limitation is inability to deliver
quality video since the variable length
frames create variable delays between
frames
Advantages of Frame Relay
• higher performance than X.25 packet
switching
• simple software upgrade from most X.25
devices
• supports applications like file transfer, high
resolution graphics, CAD/CAM
Disadvantages of Frame Relay
• time to correct errors may be higher
• could be difficult to manage
• may not work well for delay sensitive data
like voice and vide
Web Sites for Frame Relay
• http://www.frforum.com/
• http://www.mot.com/MIMS/ISG/tech/
frame-relay/resources.html
Cell Relay: ATM
• ATM is an example of this switching
technology
• breaks all data into cells and transmits them
from one location to another on the network
connected by switches
• transfers data using fixed length packets of
53 bytes called cells
– cells are always the same size and take up only
as much bandwidth as they need
• a string of cells with a destination stamp
ATM
• high speed, cell-based transmission scheme
which offers bandwidth-on-demand
forvoice, data, and video applications
• low latency making it good for video and
voice which are time-sensitive
– latency is the amount of delay experienced with
a connection
• fast transmission and switching over great
distances
ATM
• grown out of the need for a worldwide
standard to allow interoperability
• it is not based on a physical transport and
ATM cells can be sent over twisted pair,
coaxial cable, or fiber optic networks
(different physical media)
• the goal is one international standard
• ATM equipment is available for purchase
• expensive by the box, but cheaper by the
byte!
Advantages of ATM
• supports the integration of LANs to WANs
• evolving to a standard technology for local,
campus backbone, and public and private
wide area services (scalability)
• many powerful players are making it
happen: manufacturers, telcos
Advantages of ATM
• private wide area services (scalability)
• many powerful players are making it
happen: manufacturers, telcos
• industry standardization is occuring at a fast
pace
• http://www.atmforum.com
• http://cell-relay.indiana.edu/cell-relay