1. DoD Reference Model 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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2. Hybrid Reference Model
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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3. Comparison of Reference Models
OSI
DoD
Hybrid
Application
Process/
Presentation
Session
Transport
Host-to-Host
Network
Internet
Data Link
Access
Physical
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4. PDU Names Layer OSI DoD/Hybrid Application APDU Message Presentation
PPDU
Session
SPDU
Transport
TPDU
Segment
Network
NPDU
Packet
Data Link
LPDU
Frame
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5. Network Devices Amplifiers Repeaters Modems Hubs Multiplexers
Inverse Multiplexers
Bridges
Routers
Brouters
Switches
Packet Switches
ATM Switches
Telephone Switches
PABXs
Gateways
PADs
FRADs
DSLAMs
Front End Processors
DTEs
DCEs
NTEs
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6. 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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7. 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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8. 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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9. 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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10. 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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11. Primary Multiplexers
Up to 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 Mbit/s circuit
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12. 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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13. 2x Bonded 64 kbit/s circuits
ISDN IMUX
2x Bonded 64 kbit/s circuits
1 x 128 kbit/s
circuit
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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14. 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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15. Bridging two Ethernet segments
Bridge B1 B2 B3 B4 B5 B6 B7 A1 A2 A3 A4 A5 A6 A7
Segment A
Segment B
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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16. 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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17. 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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18. 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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19. 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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20. 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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21. 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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22. 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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23. 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 time-sensitive applications such as legacy data traffic or voice traffic
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24. 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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25. 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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26. 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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27. 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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28. 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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29. Physical Layer Devices
Data Link
Network
Transport
Application
Physical layer devices include amplifiers, repeaters, hubs, modems, multiplexers and IMUXs
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30. Data Link Layer Devices
Physical
Data Link
Network
Transport
Application
Data Link layer devices include bridges, brouters, FRADS and switches
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31. Network Layer Devices
Physical
Data Link
Network
Transport
Application
Network layer devices include routers, brouters, switches, PADs, DSLAMs, gateways and PABXs
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32. Application Layer Devices
Physical
Data Link
Network
Transport
Application
Application layer devices include gateways, mail relays and Front End Processors
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