Transcript 06-Architecture File

DoD Reference Model
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Network Access (or Host-to-Network)
Internet
Host-to-Host
Process/Application
The DoD Model has only 4 layers. The OSI Physical and
Data Link Layers are combined in the Network Access Layer
and the OSI Session and Presentation Layers are combined in
the Process/Application Layer
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Hybrid Reference Model
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Physical Layer
Data Link Layer
Network Layer
Transport Layer
Application Layer
This model is the same as the OSI model for layers 1-4 but the
Session and Presentation Layers which are very thin layers in the
OSI model are combined with the Application Layer
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Comparison of Reference Models
OSI
DoD
Hybrid
Process/
Application
Application
Transport
Host-to-Host
Transport
Network
Internet
Network
Data Link
Network
Access
Data Link
Application
Presentation
Session
Physical
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PDU Names
Layer
OSI
Application
APDU
Presentation
PPDU
Session
SPDU
Transport
TPDU
Segment
Network
NPDU
Packet
Data Link
LPDU
Frame
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DoD/Hybrid
Message
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Network Devices
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Amplifiers
Repeaters
Modems
Hubs
Multiplexers
Inverse Multiplexers
Bridges
Routers
Brouters
Switches
Packet Switches
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ATM Switches
Telephone Switches
PABXs
Gateways
PADs
FRADs
DSLAMs
Front End Processors
DTEs
DCEs
NTEs
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Amplifiers
• An amplifier is a physical layer analogue
device that boosts (or amplifies) the
strength of a signal to overcome the effects
of attenuation
• Unfortunately, it boosts the noise as well as
the signal
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Repeaters
• An repeater is a physical layer digital
device that regenerates a digital signal to
overcome the effects of attenuation and
noise
• Provided the signal is still recognisable it
will regenerate a pure digital signal,
eliminating noise
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Hubs
• A hub is a multi-port repeater that repeats
signals it receives on any one port to all of
the other ports (apart from the port it
received the signal from) in a star network
• A hub is also a physical layer device
• Hubs are used to support twisted-pair
(telephone cable) Ethernets
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Modems
• A modem (modulator/demodulator) is a
physical layer device that converts a digital
signal so that it lies within the voice
frequency range and can be carried over the
Public Switched Telephone Network
(PSTN). In the opposite direction it will
convert a modulated signal in the voice
frequency range, back to a digital signal
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Multiplexers
• A multiplexer (or mux) is a physical-layer
device that combines multiple data streams
into one or more output channels at the source.
• Multiplexers demultiplex the channels into
multiple data streams at the remote end and
thus maximize the use of the bandwidth of the
physical medium by enabling it to be shared
by multiple traffic sources.
• Multiplexers are also sometimes called
concentrators
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Primary Multiplexers
Up to 31
64 kbit/s
circuits
Up to 31
64 kbit/s
circuits
1 Shared
2 Mbit/s
circuit
A Primary Multiplexer (or PMUX) is installed on a
customer’s premises by a Telco to provide fractional
(nx64 kbit/s services). It will multiplex up to 31x64
kbit/s circuits onto a single 2.048 Mbit/s circuit
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Inverse Multiplexers
• An inverse multiplexer (or IMUX) is a
physical layer device that combines several,
lower-speed output channels to support a
single input channel of higher capacity
• IMUXs also pull together and synchronize
multiple channels at the receiving end of a
data, voice or video transmission to recreate
the original signal
• E.g. Channel bonding of ISDN circuits
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ISDN IMUX
1 x 128 kbit/s
circuit
1 x 128 kbit/s
circuit
2x Bonded
64 kbit/s
circuits
An ISDN (Integrated Services Digital Network)
Inverse Multiplexer will bond together two 64
kbit/s channels to support a 128 kbit/s data stream
over two ISDN calls
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Bridges
• A bridge is a data link layer device that is
used to connect two LAN segments that
operate the same data link layer protocol
• It operates by filtering (discarding) and
forwarding frames between the two
segments as necessary
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Bridging two Ethernet segments
A1
A2
A3
A4
Segment A
A5
A6
A7
B2
B3
Bridge
B1
Segment B
B4
B5
B6
B7
The bridge learns the addresses of the stations which are
on each segment by looking at the physical addresses of all
the frames originating on each segment. It discards frames
if it knows that the destination is on the same segment as
the source. Otherwise it will forward the frame
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Routers
• A router is a network layer device that
forwards packets from one network to another
(using the same network layer protocol) and
determines the optimal path for forwarding
network traffic
• Routers route traffic by examining destination
network addresses
• Routers were historically called gateways, but
this term now usually has a different meaning
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Brouters
• A Brouter (or Bridge/Router) is a device
that combines the functions of a bridge and
a router
• If a brouter cannot route a packet, it will
bridge the frame
• It can therefore be a data link layer or a
network layer device.
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Switches
Switch has two different meanings, depending on
context:
– A switch is a data link layer (or sometimes a network
layer) device that performs the functions of a hub, but
does not forward frames to all ports. It forwards
frames to the correct port on the basis of learnt
physical addresses (or sometimes network layer
addresses)
– A switch is also a device that allows connections to
be established when needed and terminated when no
longer needed (E.g. a telephone switch or an ATM
switch)
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Switches
• A Packet Switch is a network layer device used on
X.25 packet switched networks that sets up virtual
circuits and relays packets along these virtual
circuits
• An ATM Switch is a network layer device that sets
up virtual circuits and relays fixed length ATM
cells from along these virtual circuits
• A Telephone Switch is a network layer device that
is used to switch 64 kbit/s time division
multiplexed channels, mainly used for voice
communications
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PABXs
• A Private Automatic Branch eXchange
(PABX) is a telephone switch operating at
the network layer but privately owned by an
organisation and is not part of the Public
Switched Telephone Network It is used to
connect internal telephones with each other
and with the PSTN via external exchange
lines
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Gateways
• A gateway is an application or network
layer device that connects two networks
translating between different application
layer and/or network layer protocols
• E.g. A gateway can be used to connect a
LAN running TCP/IP with a mainframe
running IBM’s Systems Network
Architecture protocols
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Packet Assembler/Disassemblers (PADs)
• A Packet Assembler/Disassembler (PAD) is a
device that receives a character stream (usually
asynchronous) from one or more terminals,
assembles the character stream into packets, and
sends the data packets over a network to a host.
• A PAD can also do the reverse. It can take data
packets from a network host and translate them
into a character stream that can be understood by
the terminals
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Frame Relay Access Devices (FRADs)
• A Frame Relay Access Device (FRAD) is
a data link layer device with the primary
function of assembling and disassembling
various data protocols into frames for
transfer over a public or private wide area
frame relay network
• FRADs also provide support for timesensitive applications such as legacy data
traffic or voice traffic
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Digital Subscriber Line Access
Multiplexer (DSLAM)
• A DSLAM is a network layer device normally
located at a telco’s local exchange that multiplexes
signals from multiple customer ADSL
(Asymmetric Digital Subscriber Line) connections
onto a very high speed ATM line to a carrier’s IP
network
• It can also support non-IP services such as video
conferencing, broadcast TV and video on demand
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Front End Processors
• A Front End Processor (FEP) is a
communications computer under the control
of a larger host computer that manages all
aspects of communications and concentrates
data traffic onto a single channel for the
host
• It can operate at all layers
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Data Terminal Equipment (DTEs)
• A DTE is a device that is located at the end of a
user-network interface serving as a destination, a
source or both
• Hosts (including PCs), dumb terminals, printers
and customer premises based routers are all DTEs.
• In synchronous communications the DTE receives
its timing from a clocking signal
• DTE interfaces often use male D-type connectors
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Data Circuit-terminating Equipment (DCEs)
• A DCE is a device that is located at the network end
of a user-network interface and supplies the physical
connection to the network
• Also known by the EIA as Data Communications
Equipment. (The ITU use the former term)
• Modems, Multiplexers, hubs, switches and network
based routers are DCEs.
• In synchronous communications the DCE generates
and transmits the clocking signal to the DTE
• DCE interfaces often use female D-type connectors
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Network Terminating Equipment (NTE)
• An NTE is a DCE installed on a customer’s
premises by a network operator to provide
access to a network service (private circuit,
ISDN circuit or a public data network)
• It is the demarcation point that defines the
the responsibility of the network operator
and that of the customer
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Physical Layer Devices
Application
Application
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Physical
Physical layer devices include amplifiers,
repeaters, hubs, modems, multiplexers and IMUXs
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Data Link Layer Devices
Application
Application
Transport
Transport
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Data Link layer devices include bridges, brouters,
FRADS and switches
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Network Layer Devices
Application
Application
Transport
Transport
Network
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Network layer devices include routers, brouters,
switches, PADs, DSLAMs, gateways and PABXs
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Application Layer Devices
Application
Application
Application
Transport
Transport
Transport
Network
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Application layer devices include gateways, mail
relays and Front End Processors
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