Post on 14-Apr-2018
7/27/2019 WAN Tech CT
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A WAN is a data communications network thatcovers a relatively broad geographic area andoften uses transmission facilities provided bycommon carriers, such as telephone companies.
WAN technologies function at the lower threelayers of the OSI reference model: the physicallayer, the data link layer, and the network layer.
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There are two prevailing definitions WAN
book definition
A network that
spans largegeographical
locations, usually to
interconnect
multiple Local AreaNetworks (LANs).
practical definition
A network that
traverses a publicnetwork or
commercial carrier,
using one of several
WAN technologies.
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WANs are generally grouped into threeseparate connection types
Point-to-Point technologies
Circuit-switched technologies
Packet-switched technologies
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Often called dedicated or leased lines Usually the most expensive form of WAN
technology.
Leased from a service provider, and provide
guaranteed bandwidth from location to another (hence point-to-point).
Cost is determined by the distance of theconnection, and the amount of bandwidth allocated.
Generally, point-to-point links require no call-setup,and the connection is usually always on. Examples ofpoint-to-point technologies include T1 lines
T3 lines
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Circuit-Switched technologies require call-setupto occur before information can be transferred.
The session is usually torn down once datatransfer is complete (this is identified as an On-
Demand Circuit). Circuit-switched lines are generally low-speed
compared to point-to-point lines.
Examples of circuit-switched technologies
include: Dial-up
ISDN
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Share a common infrastructure between allthe provider’s subscribers. Thus, bandwidth is not
guaranteed, but is instead allocated on a best
effort basis.
Packet-switched technologies are ill-suited
for applications that require consistent bandwidth,
but are considerably less expensive than
dedicated point-to-point lines. Examples of packet-switched technologies
include: Frame-Relay
X25
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8.1
Main Ideas
Circuit-switched networks
use a direct, physicalconnection to transmit
data back and forth.
Message-switched
networks send entire
transmissions between
intermediaries. Packet-
switched networks route
packets through virtual
circuits.
Key Terms
carrier services
remote accessdial-up networking (DUN)
Virtual Private Network
(VPN)
circuit-switched network
message switching
packet switching
Packet Switching
Exchange (PSE)
How a WAN Works
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8.1
After a network growsbeyond the confines of theLAN, a carrier service isrequired.
Carriers provide a varietyof services which areconnected throughremote access.
carrier services Datatransfer services provided bytelecommunicationscompanies.
remote access A directconnection to a networkusing regular dial-up lines. After the connection is made,the user can work on thenetwork as if in the office.
Carriers
How a WAN Works
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8.1
Remote access
connections are
established through either
a dial-up network (DUN)
or virtual privatenetwork (VPN).
The advantages of a VPN
are that users benefit from
faster network
connections, privacy, and
security.
dial-up networking (DUN)
A client service using regular
telephone lines to connect to
a network.
Virtual Private Network
(VPN) An alternative to
using a dial-up network
connection. Is a network
connection between two
computers. With a VPN, a
remote user can securely
access the internal network
from a remote location.
Carriers
How a WAN Works
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8.1
Data in the WAN istransported throughvarious technologies:
• Circuit switching• Message switching
circuit-switched network Anetwork that creates a directphysical connection betweenthe sender and receiver.
message switching Atechnology used to route anentire message from onesystem to another. Thatmessage is routed through
intermediate (go-between)station and does not involvea direct physical connectionbetween the sender andreceiver.
Switching Technologies
How a WAN Works
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8.1
Switching Technologies
Message switching sends the entire message to
intermediary stations that decide which route to use.
How a WAN Works
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8.1
Packet switching iscommonly found in LANs.
Packets are routed
through PacketSwitching Exchanges until they reach their destination.
packet switching Atransmission method inwhich all transmissions arebroken into small units andsent over the network.
Packets are reassembled atthe destination computer.
Packet SwitchingExchange (PSE) Go-betweens in the packetswitching process. The PSEinspects the packet’sdestination address, consultsa routing table, and forwardsthe packet at the highestpossible speed.
Switching Technologies
How a WAN Works
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8.1
Switching Technologies
Packet-switching networks send packets along the best
route available.
How a WAN Works
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Setup, data transfer, and teardown phases as in a circuit-switched network (CSN)
Resource allocated during setup phase, as in a CSN, or ondemand as in a datagram network (DN)
As in DN, data are packetized and each packet carries an
address in the header. The address has local jurisdiction,not end-to-end jurisdiction.
As in CSN, all packets follow the same path establishedduring the connection
VCN is normally implemented in the data link layer, while
CSN is in physical layer and DN in the network layer
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Two types of addressing in a virtual-circuit network: globaland local (virtual-circuit identifier: VCI)
Global address is used only to create a VCI
Virtual Circuit Identifier
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Setup phase, Data transfer phase, Teardown phase Switch and tables in a virtual-circuit network
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Source-to-destination data transfer in a virtual-circuitnetwork
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In virtual-circuit switching, all packets belonging to thesame source and destination travel the same path; but thepackets may arrive at the destination with different delaysif resource allocation is on demand.
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8.2
Main Ideas
X.25 is a packet-switchingprotocol originally designed
for analog telephone lines.
Frame relay usespermanent, virtual circuitsand reliable, digital lines totransmit data.
ATM transmits data, voice,and video over many typesof lines.
Key Terms
X.25
Data Terminal Equipment
(DTE)packet assembler/disassembler (PAD)
Data Circuit-terminatingEquipment (DCE)
frame relay
Asynchronous Transfer Mode (ATM)
cell relay
broadband ISDN (B-ISDN)
Sending Data across a WAN
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8.2
Computers on an X.25
network can receive and
transmit data at the same
time. This is also called
full-duplexcommunication.
X.25 A connection-oriented,
packet-switching protocol
designed for use on analog
telephone lines. Computers
on an X.25 network canreceive and transmit data at
the same time.
X.25 Packet-Switching Protocol
Sending Data across a WAN
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8.2
The X.52 protocolconcentrates on theinterface through whichtransmission flows. The
interface is made up of the following devices:
• Data Terminal Equipment(DTE)
• Packet
assembler/dissembler (PAD)
Data Terminal Equipment(DTE) X.25 deviceterminology. There are nocomputers, hosts, or nodes.Instead, sending and
receiving computers arereferred to simply as this.
packetassembler/disassembler
(PAD) A device in X.25transmissions that preparespackets for transmission anddisassembles the packetsthat come in.
X.25 Packet-Switching Protocol
Sending Data across a WAN
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8.2
• Data Circuit-terminating
Equipment (DCE)
• Packet switching
exchanges (PSEs)
Data Circuit-Terminating
Equipment (DCE) Packets
are sent here from the DTE
or PAD. A DCE might be a
modem or packet switch. ADCE is located at each end
of the connection.
X.25 Packet-Switching Protocol
Sending Data across a WAN
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Designed to provide a low cost alternative for data communication over public networks Pay only for bandwidth actually used
Ideal for “bursty” communication over low
quality circuits Standard provides error detection and
correction for reliable data transfer X.25 standard approved in 1976 by CCITT (now
known as ITU) Can support speeds of 9.6 Kbps to 2 Mbps Can provide multiplexing of up to 4095 virtual
circuits over on DTE-DCE link
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Data Terminal Equipment (DTE) Terminals, personal computers, and network hosts
Located on premises of subscriber
Data Circuit-terminating Equipment (DCE) Modems and packet switches
Usually located at carrier facility
Packet Switching Exchange (PSE) Switches that make up the carrier network
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X.25
WAN
Personal Computer
DTE
Terminal
DTE
Server
DTE
Modem
DCE
Modem
DCE
Modem
DCE
PSE
PSE
PSE
PSE
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Used for DTE devices that are too simple toimplement X.25 (such as character-mode terminals)
Acts as intermediary device between DTE and DCE
Performs three functions Buffering to store data until a device is ready to process
it
Packet Assembly
Packet Disassembly
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Terminal
DTEModem
DCE
PAD
Buffer
Data
Assembly/
Disassembly
Data
X.25 Packet
PSE
PSE
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Application
Presentation
Session
Transport
Network
Data Link
Physical
PLP
LAPB
x.21 bis, EIA/TIA-232, EIA/TIA-449,
EIA-530, G.703
Other Services
X.25
ProtocolSuite
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Several well-known standards are used for X.25 networks X.21bis – supports up to 2 Mbps
15-pin connector
RS-232 (EIA/TIA-232) – supports up to 19.2
Kbps 25-pin connector
RS-449 (EIA/TIA-449) – supports up to 64Kbps 37-pin connector
V.35 – supports up to 2 Mbps 34-pin connector
Uses serial communications in either asynchronous or synchronous modes
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Link Access Procedure, Balanced (LAPB) is theprotocol used for this layer
LAPB is a version of HDLC
HDLC in Asynchronous Balanced Mode (ABM)
DTE and DCE are peers and can both perform allfunctions
LAPB manages communication and packetframing between DTE and DCE devices
Makes sure that frames are delivered in sequenceand error-free
Uses sliding window of 8 or 128 frames
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Packet Layer Protocol (PLP) is the X.25 networklayer protocol
PLP manages calls between a pair DTE devicesusing a Permanent Virtual Circuit (PVC) or aSwitched Virtual Circuit (SVC)
PLP handles segmentation, reassembly, bitpadding and error and flow control
PLP uses X.121 Addressing Scheme to setup avirtual circuit
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Call Setup Used to setup virtual circuit for SVC
Data Transfer Used for transferring data with both SVC and PVC
Idle Used when SVC call has been established but no data
is currently being transferred
Call Clearing Used to end communication between DTEs for a SVC
Restarting Used to synchronize DTE and DCE for all virtual circuits
that exist between them
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Call
Setup
Phase
Data
Transfer
Phase
Call
Clearing
Phase
DTE to DCE
Interface
DCE to DTE
Interface
Call Request
Call Connected
Data Packet
Incoming Data
Clear Request
Clear Confirm
Incoming Call
Call Accepted
Incoming Data
Data Packet
Clear Indication
Clear Response
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Frame Relay was originally designed for use onIntegrated Services Digital Network (ISDN)
Usually considered a replacement for X.25 usingmore advanced digital and fiber optic connections
Does not perform error correction at intermediatenodes making it faster than X.25
When an error is detected (FCS) the frame is discardedand correction is left up to higher layer protocols
Original standard proposed in 1984 but widespread
acceptance did not occur until the late 1980’s
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Data Terminal Equipment (DTE) Terminals, Personal Computers, routers, and bridges
typically at the customer location
Data Circuit-terminating Equipment (DCE)
Typically packet switches owned by the carrier thattransmit data through the WAN
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Frame Relay
WAN
Personal Computer
DTETerminal
DTE
Network Host
DTE
Packet Switch
DCE
Packet Switch
DCE
Packet Switch
DCE
Packet Switch
DCE
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To handle frames from other protocols a FRAD isused to provide conversion to Frame Relaypackets
A FRAD can either be a separate device or part ofa router/switch
Frame RelayFRAD FRAD
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Application
Presentation
Session
Transport
Network
Data Link
Physical
LAPF
Other Services
FrameRelay
ProtocolAny Standard
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No specific protocol is defined Any protocol recognized by ANSI can be
implemented
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Link Access Protocol for Frame Modes Services(LAPF) is the protocol defined for Frame RelayLayer 2 services
LAPF is a version of HDLC
Does not provide flow or error control Uses Address field for DLCI (addressing) as well as for
congestion control
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Flag FlagAddress Information FCS
DLCI: (10 bits) Data Link Connection Identifier is used to identify the Virtual
Circuit number
C/R: (1 bit) Provided for up layers to determine commands and responses
EA: (1 bit) Determines if this byte is last byte of address (0=more, 1=last)FECN: (1 bit) Forward Explicit Congestion Notification indicates congestion in
the direction the frame is traveling
BECN: (1 bit) Backward Explicit Congestion Notification indicates congestion
in the opposite direction the frame is traveling
DE: (1 bit) Discard Eligibility indicates that a frame is low priority when set
C/R EA DLCIDLCI EADEBECNFECN
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To increase the number of virtual circuits theDLCI can be expanded from 10 bits to 16 bits and23 bits
The EA field is set to 0 to indicate that additional
address bytes are present. The last address bytewill have a 1 in the EA field
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C/R 0
DLCI
DLCI
1DEBECNFECN
C/R 0
DLCI
DLCI
0DEBECNFECN
DLCI 10
C/R 0
DLCI
DLCI
0DEBECNFECN
DLCI 0
DLCI 10
Two-byte Address
(10 bit DLCI)
Three-byte Address
(16 bit DLCI)
Four-byte Address
(23 bit DLCI)
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Two Byte Address Format0 In-channel signaling
1-15 Reserved
16-991 Assigned using Frame Relay connection procedures
992-1001 Layer 2 management of Frame Relay service1002-1022 Reserved
1023 In channel layer management
Three Byte Address Format0 In-channel signaling
1-1023 Reserved1024-63,487 Assigned using Frame Relay connection procedures
63,488-64,511 Layer 2 management of Frame Relay service
64,512-65,534 Reserved
65,535 In channel layer management
Four Byte Address Format0 In-channel signaling
1-131,071 Reserved
131,072-8,126,463 Assigned using Frame Relay connection procedures
8,126,464-8,257,535 Layer 2 management of Frame Relay service
8,257,536-8,388,606 Reserved
8,288,607 In channel layer management
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Original Frame Relay standard only covered PVC
SVC support was added but does not have widespreadimplementation
PVC States Data Transfer – data is being transmitted between DTE devices
Idle – connection is still active but no data is being transferred
SVC required the addition of two additional states Call Setup – virtual circuit between DTE devices is established
Call Termination – virtual circuit between DTE devices isterminated
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Because of the shared resources of a virtual circuit,congestion can cause the loss of packets as buffersbecome full
Frame Relay defines a congestion control mechanismusing the FECN and BECN bits in the address field
When a switch determines that congestion has occurredit will set the FECN bit on packets traveling in thedirection of the congestion to alert the receiver to slowdown requests for data. The BECN bit will be set for packets going in the opposite direction of thecongestion to let the sender know to send data more
slowly The FECN and BECN bits will allow higher layer protocols
to manage flow. Discard Eligible bit is used to identify frames that are low
priority and can be discarded in the event of congestion
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LMI is a set of extensions to Frame Relaydeveloped in 1990 by Cisco Systems, StrataCom,Northern Telecom, and Digital EquipmentCorporation
LMI provides global addressing which allowsadditional management capability such asstandard address resolution and discovery
LMI allows status messages to be passedbetween DCE and DTE devices to providecommunication and synchronization (uses DLCI1023 on a 2-byte address)
LMI specifies multicast capability to allowcreation of multicast groups to limit bandwidth
use
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X.25 and Frame Relay
X.25 Frame Relay
Layer 1 Specification Yes None
Layer 2 Protocol Family HDLC HDLC
Layer 3 Support PLP None
Error Correction Node to Node None
Propagation Delay High Low
Ease of Implementation Difficult Easy
Good for Interactive Applications Too Slow Yes
Good for Voice No Yes
Good for LAN File Transfer Slow Yes
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Many X.25 networks have been replaced byFrame Relay or X.25 over Frame Relay Networks
X.25 still in use for low bandwidth applicationssuch as credit card verification
It is likely that ATM Networks will ultimatelyreplace Frame Relay and X.25 Networks
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8.2
Frame relay is faster thanthe X.52 protocol. Framerelay networks connectingLANs to a WAN rely onrouters and switchingequipment.
frame relay A newer form of
packet switching that relies
on digital technologies, such
as fiber optic and ISDN.
Frame Relay
Sending Data across a WAN
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Two approaches for the VC setup Permanent virtual circuit (PVC):
Switched virtual circuit (SVC): setup, data transfer,teardown
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Frame Relay is a virtual circuit wide area network VCIs in Frame Relay are called DLCIs(Data Link Connection
Identifier)
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Frame relay operates at a higher speed. It can easily be usedinstead of a mesh of T-1 or T-3 lines (1.544 Mbps or 44.376 Mbps)
Frame relay operates just the physical and data link layers. It isgood as a backbone to provide services to protocols that alreadyhave a network layer protocol, such as Internet
It allows bursty data
It allows a frame size of 9000 bytes accommodating all LAN framesizes
It is less expensive than other traditional WANs
It has error detection at the data link layer only. There is no flowcontrol pr error control
X.25 Leased Lines Frame Relay
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Frame relay operates only at the physical anddata link layers
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Frame relay requires congestion control, because Frame Relay does not have a network layer
No flow control at the data link layer
Frame Relay allows the user to transmit bursty data
Congestion avoidance
Two bits in the frame are used BECN(Backward Explicit Congestion Notification)
FECN(Forward Explicit Congestion Notification)
Discard eligibility(DE): Priority level of the frame for traffic control
Discarding frame to avoid the congestion or collapsing
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Asynchronous Transfer Mode
ATM is the cell relay protocol designed by ATM forum andadopted by ITU-T
ATM uses asynchronous TDM
Cells are transmitted along virtual circuits
Design Goals Large bandwidth and less susceptible to noise degradation
Interface with existing systems without lowering theireffectiveness
Inexpensive implementation
Support the existing telecommunications hierarchies
Connection-oriented to ensure accurate and predictabledelivery
Many functions are hardware implementable
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Multi-speed network environment that providesa variety of complex network services
Can carry voice, data, video separately or simultaneously
Can be used in LANs, MANs, or WANs Fixed-lenth packets (cells)
Allows multiple logical connections to bemultiplexed
Minimal error and flow control capabilities Connection-oriented virtual channel
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Similar to frame relay Difference?
Frame relay switches variable length frames withinframe relay cloud from source to destination
ATM switches fixed-length cells (48 byte information
field, 5 byte header) Based on packet switching (connection-oriented)
Cell sequence integrity preserved via virtual channel
VCC – virtual channel connection – is set up betweenend users, variable rate, full duplex
VCC also used for control
Information field is carried transparently through thenetwork, with minimal error control
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So far, ATM has been implemented in : PC, workstation, and server network interface cards
Switched-Ethernet and token-ring workgroup hubs
ATM enterprise network switches
ATM multiplexers
ATM-edge switches
ATM-backbone switches
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Main features of ATM
Service is connection oriented, with data transferred over aVC
A cell-switched network (architecture).
Fixed-size cell (53-Bytes) Uses Asynchronous time-division multiplexing
(Asynchronous TDM)
The Quantity of Service (QofS) enable carriers to transmitvoice, data, and video.
ATM is independent of the transmission medium. ATM cellscan be sent on a wire or fiber, and can also be packagedinside the payload of other carrier system.
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Advantage:
Transmitted with predictability and uniformity.
Easy to be multiplexed with other cells, and routed through thecell network.
With high speed of the links, small and fixed-size cells seem toarrive their respective destinations in an approximation of
continuous stream, despite interleaving. E.g. phone call.
Simpler buffer hardware, avoiding memory fragmentationproblem
Simpler cells scheduling:
- Easier to allocate different bandwidths and delays to different
VCs.- Easier to implement priority
- Fixed sized can be switched in parallel in synchronous fashion.
It’s suitable for time-critical information such as voice or video
Quicker recovery in case of circuit failure.74
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Disadvantage: Processing overhead as messages are segmented
into cells
Segmentation mismatch, as the last cell in afragmented message may not be fully used. Thiseffect will decrease as the message lengthincreases.
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The variety of packet sizes makes trafficunpredictable A cell network uses the cell as thebasic unit of data exchange A cell is defined as a small, fixed sized block of
information
Cells are interleaved so that non suffers a long delay
A cell network can handle real-time transmissions
Network operation is more efficient and cheaper
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UNI: user-to-network interface
NNI: network-to-network interface
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Connection between two endpoints isaccomplished through Transmission path (TP)
Virtual path (VP)
Virtual circuit (VC)
A virtual connection is defined by a pair ofnumbers: VPI and VCI
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ATM Layers
ATM Technology
Reference Model
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Reference Model
Control
responsible for generating and managing signaling request (connection
management).
User
deals with data transport, flow control, error correction, and other user
functions.
Layer Management :
manages layer-specific functions (detection of failures and protocol problems)
Plane Management:
manages and coordinates functions related to the complete system.
ATM Technology
Reference Model
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Physical Medium-Dependent (PMD) – having two functions:
Synchronizes transmission and reception by sending and
receiving a continuous flow of bits with associated timing info.
Specifies the physical media for the physical medium used,
including connector type and cable.
ATM Technology
Reference Model
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Transmission Convergence (TC) – having four functions:
Cell delineation, generating cell boundaries.
Header error control (HEC) sequence generation and verification
Cell-rate decoupling, maintaining synchronization and inserting or suppressing idle ATM cells to rate of valid ATM cells to the payload
capacity of transmission system.
Transmission frame adaptation, packaging cells into frame acceptable
to the particular physical layer implementation.
ATM Technology
Reference Model
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88
ATM Layer
Provides
Defining cells layout
Defining header
Routing
Est and release VC.
Switching
Multiplexing
Congestion control.
0
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ATM Technology
Reference Model
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ATM Adaptation Layer (AAL) Enables ATM to accept any type of payload, both data
frames and streams of bits
Fragments data from upper-layer into 48-byte dataunits for the ATM cells
Reassembles Cells Convergence sub layer (CS): Before the payload is
segmented CS prepares data to ensure their integrity, providing standard interface.
Segmentation and Reassembly (SAR): Segmentsthe payload into 48-byte cells, and at thedestination, reassemble them to recreate theoriginal payload.
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ATM defines four versions of the AAL:
AAL1: Support Constant-bit-rate data (CBR) fromupper layer; video and voice.
AAL2: Used for low-bit-rate and short-frame trafficsuch as audio (compressed or uncompressed), video,
or fax. AAL2 allows the multiplexing of short framesinto one cell.
AAL3/4: support connection-oriented and connectionless data services
AAL5: –SEAL (Simple and Efficient Adaptation Layer)No sequencing and error control mechanismsAssumes that all cells belonging to a single messagetravel sequentially and that control functions areincluded in the layers of the sending application.
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Frame relay uses variable length frames ATM fixed length cells
ATM has higher overhead, but faster speed andtraffic management (better suited for video and
voice)
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SONET is a transport mechanism, transportingdata over fiber.
Can act as a transport carrier for ATM (or FDDI, or ISDN, etc.)
ATM is a technology and protocol designed touse SONET as its carrier service
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Replace “network” (e.g., LAN segment) with
ATM network
ATM addresses, IP addresses
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AALATMphyphy
Eth
IP
ATMphy
ATM
phy
app
transportIPAALATMphy
apptransportIPEthphy
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at Source Host: IP layer maps between IP, ATM dest address
(using ARP)
passes datagram to AAL5
AAL5 encapsulates data, segments cells, passes toATM layer
ATM network: moves cell along VC todestination
at Destination Host: AAL5 reassembles cells into original datagram
if CRC OK, datagram is passed to IP
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IP datagrams into ATMAAL5 PDUs
from IP addresses toATM addresses
just like IPaddresses to 802.3MAC addresses!
ATMnetwork
Ethernet
LANs
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Minimal error and flow control Reduces overhead of processing ATM cells
Reduces number of required overhead bits
Fixed size simplified processing at each ATMnode (can be switched more efficiently – moreefficient use of router)
Small cells reduce queuing delay
Minimal addressing info on each cell
Efficient traffic management
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ATM is mainly a wide-area network (WAN ATM);however, the technology can be adapted to local-area networks (ATM LANs). The high data rate ofthe technology has attracted the attention ofdesigners who are looking for greater and
greater speeds in LANs.
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Connectionless versus connection-oriented
Physical addresses versus virtual-circuit identifiers
Multicasting and broadcasting delivery
Interoperability
Client/Server model in a LANE LANE Configuration Server (LECS), LANE Server (LES), LANE
Client (LEC)
Broadcast/Unknown Server (BUS)
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Anticipating user demand for end-to-end digitalservices the world’s telephone companiesagreed in 1984 under CC I TT to build a new,fully digital, circuit -switched telephone system.
This system was called I S DN ( IntegratedServices Digital Net work)
and it’s primary goal was to integrate the voiceand non-voice services .
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ISDN incorporates all communication connections
in a home or building into a single interface
With ISDN all customer services will become digital
rather than analog.
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Buttons for instant call set up t o arbitrarytelephones anywhere in the world.
Displaying the caller’s telephone number, nameand address while ringing.
Connecting the telephone to a computer enablingthe caller ’s database record to be displayed onthe screen as the call comes in.
Call forwarding.
Conference calls worldwide. On line medical, burglar, and smoke alarms giving
the address to speed up response.
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A Digital bit pipe between the customer and thecarrier through which bits flow in both directions.
The digital bit pipe can support multipleindependent channels by time division multiplexingof the bit stream.
Two principal standards for the bit pipe havebeen developed: A low bandwidth standard for home use.
A higher bandwidth standard for business use that
support s multiple channels identical to the home usechannels (multiple bit- pipes ).
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U = connection between t he I S DN exchangeand NT1
T = connector on NT1 to the customer ,
S = interface between the I S DN PBX and theISDN terminal,
R = the connection between the terminaladapter and non-I S DN terminal.
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The ISDN bit pipe supports multiple channels
interleaved by time division multiplexing.Several channel types have beenstandardized:
A - 4 kHz analog telephone channel
B - 64 kbps digital PCM channel for voice or data
C - 8 or 16 kbps digital channel for out -of -bandsignalling
D - 16 kbps digital channel for out -of -bandsignalling
E - 64 kbps digital channel for internal I S DNsignalling
H - 384, 1536, or 1920 kbps digital channel.
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It is not allowed to make arbitrary combination ofchannels on the digital pipe.
Three combinations have been standardized so far: Basic rate: 2B + 1D. I t should be viewed as a
replacement f or P OTS ( Plain Old Telephone Service) .
Each of the 64 kbps B channels can handle a singlePCM voice channel with 8 bits samples made 8000times per second. D channel is for signaling ( i.e. , t oinform t he local I S DN exchange of the address of thedestination) . The separate channel for signaling results
in a significantly faster set up time. N-ISDN Primary rate: 23B + 1D ( US and Japan) or 30B + 1D
(Europe) . It is intended for use at the T referencepoint for businesses with a PBX.
Hybrid: 1A + 1C
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Basic
B
B
D
Primary
B
B
D
B
Basic Service:Management rate: 192 kbps
Standard throughput: 144 kbps
Composition: B + B + D
channels, + Synch & framing
Information:Voice, Data
Signaling:Overhead or
telemetry, etc.
PCM voice
channels
Signaling
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Basic Rate Interface (BRI): Used for homeand small office connectivity.
BRI services include two B channels and asingle D channel.
A B channel offers 64 Kbps and carriesuser data. A BRI D channel operates at 16Kbps and carries control and signalinginformation.
Through these two channels, a homeconnection can reach 128 Kbps of datathroughput.
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Primary Rate Interface (PRI): Used for WANs andruns across leased lines.
The PRI service is composed of 23 B channels at64 Kbps each for user data, along with a single Dchannel, also operating at 64 Kbps to handle
control information.
Overall, the PRI service provides a throughputrate of 1.544 Mbps.
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A computer with an ISDN line is able to connectto any other computer that also uses ISDN simplyby dialing its ISDN number.
Terminal adapter (TA): Also called an ISDNmodem, this is either an internal or external
adapter to connect equipment to an ISDN line.
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Digital
reliable connection
Speed 128 kb/s (160 kb/s) for BRI
1920 kb/s (2048 kb/s) for PRI
Fast call setup 2 seconds
Bandwidth on Demand adding new channels to the bundle of channels
Multiple devices phone, fax, PC, videoconferencing system, router,
terminal adapter,.. each with its own sub-address
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N-ISDN was an attempt to replace the analogtelephone system with a digital one.
N-ISDN basic rate is too low so for home as for business today.
N-ISDN may be partly saved, but by an unexpectedapplication: Internet access .
Various companies now sell ISDN adapters thatcombine the 2B+D channels into a single 144 kbpsdigital channel.
Many Internet providers also support theseadapters . So the people can access Internet over a144 kbps digital link, instead of a 28.8 kbps
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When CCITT found that the N-ISDN was not goingto solve the actual communication problems , ittried to think of a new service.
The result was broadband ISDN ( B-ISDN) , basicallya digital virtual circuit for moving fixed-sized
packets (cells ) at 155 Mbps.
Broadband ISDN is based on ATM technology thatis fundamentally a packet -switching technology.