Inrtoduction to WAN

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Inrtoduction to WAN

• Jacek Ilow
• j.ilow_at_dal.ca

2
Agenda

• General definition
• connection-oriented vs connectionless
• Market drivers for packet switching
• network applications, evolution and demographics
• Switching taxonomy
• advantages and disadvantages
• queueing model
• Some basics
• OSI model
• HDLC

3
Agenda...

• Overview of different forms of packet switching
• connection-oriented
• X25, frame relay, ATM
• connectionless
• IP
• Packet networking
• adaptation standards
• performance
• technology application map
• network engineering challenges

4
Packet Switching Definition

• Subdividing the overall message or bit stream
• into individually addressed packets such that
• access and trunk multiplexing can take place
• a transmission channel is occupied only when
  there is useful information to send
• the full pipe capacity is available for any user
• subject to class of service i.e., no TDM channels
• minimizing protocol processing
• for max throughput and minimum delay
• leveraging improved transmission error
  performance (where available)
• exploit "bursty" nature and tolerance to delays
  of most applications
• optionally, may include
• node by node flow control
• node by node error control

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What is a Packet?
B

• Each packet
• has well-defined start and end
• has a header
• has a destination id
• control info
• is queued for transmission
• is transmitted at line rate
• line rates need not be equal

Packet Network
A
D
C
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Packet Switching-Technology, Standards

• a technology
• switching multiplexing architecture
• based on multi-byte 'packets'
• connectionless or connection-oriented
• can support multiple (including defacto)
  interface standards
• a service capability
• basis for tariffed services
• various service classes
• connectionless (throughput, delay)
• connection-oriented
• multiple types of variable bit rate
• constant bit rate (ATM only)
• point-point and multicast
• a set of standards
• terminal and networking standards
• user and network interface protocols
• service definitions
• performance parameters
• adaptation and encapsulation standards

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Connection-Oriented vs. Connectionless

• Connection-oriented
• In-band or out-of-band Call Setup/Teardown/Status
• Shared State Information between DTE and Network
• Protocol In Addition to Packet Format
• Multiplexing via Connection-ids
• X.25 LCN, D-Channel LTID, Frame Relay DLCI, ATM
  VCI's/VPI'
• Sequence preserving
• Connectionless
• Routing Inband Always
• Must Be Able to Route External Addresses As Fast
  As Data Transfer
• Multiplexing Requires Longer Headers
• NSAPs from OSI, TCP/IP addresses
• Notelayering is possible such that connections
  are built on connectionless subnets. e.g.,
• X25 VC's implemented on datagram
• TCP sessions on top of IP datagrams
• Note connectionless can be encapsulated over
  connections

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Networking Technology Taxonomy
Voice/fax/modem
Targeted at 10s Mbps and up and suited for
multimedia
All data devices today with addres- sing, routing
control
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Packet Switching Advantages/Disadvantages

• bandwidth only consumed when needed
• reduces cost of bandwidth
• reduces cost sensitivity to distance
• host concentration
• host port reduction
• dynamic routing changes possible
• connectionless
• connection when built on connectionles
• but...
• processing requirements during the call
• complexity
• routing algorithms
• congestion control
• protocols
• variable delays

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OSI Model

• layer 1 physical
• standards for transmission of a serial bit stream
• provides communications channel between two
  entities
• applicable to various forms of data comm
• layer 2 data link layer
• uses layer 1
• operates on link between two communications
  entities
• variable time delay, error free
• uses a data protocol (end-end or link by link)
• optional flow control and error recovery
• layer3
• network layer using layer 2
• network addressing
• optional multiplexing, flow control

OSI Stack
Application Presentation Session Transport Network
Link Physical
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HDLC
F A C Information (I) field FCS F

• F Flag 01111110
• A Address (DLC identification)
• C Control
• Information frames (send/receive sequence
  numbers)
• Supervisory frames (RR,RNR conveying N(R), REJ)
• Unnumbered frames (control)
• I Information field (variable length)
• X25 or lP packet, user data
• FCS Frame check sequence (16 or 32 bit
  redundancy)
• HDLC applications
• (I) leased line data networks (ii) X25 and frame
  relay
• inter-LAN connectivity
• Note Bit stuffing is used to prevent flag
  pattern withinframe (sender always inserts 0 bit
  after every five l's receiver always removes 0
  bit after five 1')

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Packets/Frames vs Cells

• Packets and Frames
• Variable Length
• Efficient Bandwidth Utilization
• Variable Delay
• Problem for Constant Bit Rate
• Greater Memory Demands
• Bandwidth, Allocation
• Asynchronous (to other frames)
• Framing Detection/Transparency

• Cells
• Fixed Length
• Efficient For Constant Bit Rate
• Controllable Delay
• Better for Constant Bit Rate
• Lower Memory Demands
• Synchronous (Sometimes)
• Fixed Time Available to Route

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X25

• End to end features
• connection oriented (SVC's and PVC's)
• X121 numbering plan
• sequence preserving and error free
• throughput class end-to-end rate adaptation
• rich service features (closed user groups, fast
  select et
• standard adaptation for Async, POS, BSC, SDLC, IP
  et al
• Physical level
• Bit Rates To 2 Mbps but 9.6 to 64Kbps most common
• Link level
• Node by node HDLC error control
• Network level
• multiplexing via logical channel number
• call establishing and clearing
• window based flow control
• additional protocol features
• segmentation M bitdata type Q bit
• error recovery

OSI Stack
Application Presentation Session Transport Network
Link Physical
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X25 Data Packet Format
Bit

• 0 for user data
• 1 for supervisory control
• call request/confirmation
• clear indication/confirmation
• interrupt/confirmation
• RR/RNR
• Reset request/confirmation
• Restart request/confirmation
• Q Qualifier bit
• D Delivery confirmation bit
• P(R) Receive Sequence Number
• M More data bit for definition of
• packet sequence
• P(S) Send Sequence Number

Octet
8 7 6 5 4 3 2 1
Q D 0 1
Logical Channel Number
4 3 2 1
P(R) M P(S) 0
User Data (up to agreed max)
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X25 Flow Control

• Wide Area Network
• at VC set-up, the following two parameters are
  defined
• maximum user window size (W)
• maximum number of packets (i.e., buffers)
  supported (netW)
• user can transmit w packets but then must wait
  for more credits
• network keeps track of packets in the network and
  delivers credits if
• of packets in transit ltnetW
• receiving terminal has issued credits
• there is no congestion

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Key Features of X.25

• Call control packets, used for setting up and
  clearing VC, are carried on the same channel and
  same virtual circuit as data packets - Inband
  Signalling
• Multiplexing of Virtual Circuits takes place at
  layer 3
• Both layer 2 and layer 3 include flow control and
  error control mechanisms
• CONCLUSION
• Considerable overhead

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Frame Relaying
OSI Stack

• End-to-end features
• connection oriented (PVC's--gtSVC's)
• X121 and E164 numbering plans
• committed information rate (CIR)
• sequence preserving
• some additional service features
• standard adaptation for IP et al., IBM protocols
• Physical Level
• Bit Rates To 10's of Mbit/s but 56 to 1.5 Mbps
  most common
• Link Level
• HDLC Core Framing
• CRC16 but no error recovery
• Multiplexing via Data Link Control Identifier
  (DLCI)
• HDLC-compatible 2-byte Frame Address
• call establishment/clearing via dedicated
  signaling channel

Application Presentation Session Transport Network
Link Physical
Technology Enablers
Low Bit-Error Rates End-End Error
Recovery Higher throughput for data Lower delay
for data
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FR Data Frame Format
Bit
Octet
8 7 6 5 4 3 2 1

• DLCI Data Link Control Identifier
• C/R command/response
• DE discard eligibility
• E Field extension bit

DLCI
C/R E
FECN DE E
2 1
User Data (up to agreed max)
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FR Committed Information Rate
128 kbps CIR
Network
64 kbps CIR
256 kbps CIR
Unchannelized T1/E1

• Measure of available bandwidth (kbps) on PVC
• Maximum sustainable throughput allowed
• Applications
• congestion management
• engineering
• tariffing

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Key Differences between X.25 and Frame Relay

• Call control signalling is carried on a separate
  logical connection from user data. Thus,
  intermediate nodes need not maintain state tables
  or process messages relating to call control on
  an individual per-connection basis.
• Multiplaxing and switching of logical connections
  take place at layer 2 and 3 instead of layer 3,
  eliminating one entire layer of processing.
• There is no hop-by-hop flow control and error
  control. End-to-end flow control and error
  control are responsibility of a higher layer, if
  employed at all.

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ATM
OSI Stack

• End-to-end features
• connection oriented (PVC's--gtSVC's)
• E164 and NSAP numbering plans
• delay and loss critical classes of services
• sequence preserving
• standard adaptation for CBR via AAL1 and data via
  ML314 and 5
• Physical Level
• Bit Rates to Gbit/s but 45 150Mbps today
  (in-building and wide area)
• Link Level
• 485 fixed length cells
• no error control and recovery
• multiplexing via VCI/VPI
• call establishment/clearing via dedicated
  signaling channel

Application Presentation Session Transport Network
Link Physical
Technology Enablers
Fiber Transmission High Speed Hardware Multimedia
networking End-to-end broadband
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ATM Cell Format
Bit
Octet
8 7 6 5 4 3 2 1

• 0 - user cell (SDU 0) no congestion
• 1 - user cell (SDU 1) no congestion
• 2 - user cell (SDU 0) congestion
• 3 - user cell (SDU 1) congestion
• 4 - segment OAM VCC F5 flow
• 5 - e-e OAM VCC F5 flow
• 6/7 reserved

Generic Flow Control
Virtual Path Identifier
Virtual Channel Identifier
VPI
VCI
Payload Type
VCI
CLP
Header Error Check
1 2 3 4 5 6 . . . 53
Cell Payload (48 octetes)
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ATM vs Frame Relay

• Both Connection Oriented
• Frame data only
• Cell multimedia
• Access multiplexing for both
• Frame and cell relay sit on physical medium
• Frame relay (kbps to multi Mbps)
• Cell relay (multi Mbps to Gbps)
• Framing
• Frame variable length HDLC
• Cell fixed length cells ATM adaptation
• Service interworking via frame to cell SAR
  mapping

Single service concept from kbps tp Gbps
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Packet Switching Adaptation Standards

• family of X25 Packet Assemblers/Disassemblers
  (PAD's) defined
• X3/X28/X29 for async
• SDLC PAD (IBM's QLLC standard)
• BSC PAD (industry standard)
• Multiprotocol (incl IP) encapsulation over X25
• Industry specific PAD's (e.g., ALC, POS)
• family of FR Assemblers/Disassemblers (FRAD's)
  defined
• Multiprotocol (md IP) encapsulation over FR
• SDLC FRAD (IBM's standard)

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Packet Switching Adaptation Standards...

• ATM adaptation
• AAL1 for CBR
• for voice and video
• uses one byte of ATM payload
• AAL3/4 and 5 for data (see next chart)
• Multiprotocol (incl IP) encapsulation over ATM
  uses AAL5
• Adaptation can be done
• in the terminal,
• at the edge of the network or
• in the network

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Packet Switching Performance Parameters

• transit delay time from transmission to
  reception
• access link delay (q time, emission time,
  propagation time)
• network transit delay ( access switch trunk
  delay)
• throughput
• switch
• trunk
• connection throughput
• measures of efficiency
• processor and trunk utilization
• challenges
• user behaviour
• protecting the network

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Summary

• main advantage of packet switching highest
  performance for bursty traffic for given cost
• reduced facility costs through bandwidth sharing
• increased networking flexibility
• packet switching technology widely used in
  data-driven environments
• packet and router networks
• frame relay networks
• multimedia ATM LAN's and WAN's (emerging)
• many services/capabilities can be supported
• X25, FRS, IP, ATM and other forms of LAN
  networking, IBM's SNA and even SS7
• Main challenge
• meeting end user needs while protecting the
  network against misuse and congestion

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Networking Engineering Challenges

• Distinguish between technology and services
  supported
• Watch performance comparisons
• vendors speak of user packets or user frames or
  cells per second
• throughput may not equal access/trunk speed
• users care about end-to-end delays
• network failure recovery mechanisms effect
  application performance
• Design the network to fit users
  business/application and networking needs
• network configuration including private, virtual
  and virtual tri
• network management distribution