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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

Comparison of Reference Models OSI DoD Hybrid Application Process/ Presentation Session Transport Host-to-Host Network Internet Data Link Access Physical 26/10/10 06-Architecture

PDU Names Layer OSI DoD/Hybrid Application APDU Message Presentation PPDU Session SPDU Transport TPDU Segment Network NPDU Packet Data Link LPDU Frame 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

Primary Multiplexers 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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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. 26/10/10 06-Architecture

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) 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

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 26/10/10 06-Architecture

Physical Layer Devices Data Link Network Transport Application Physical layer devices include amplifiers, repeaters, hubs, modems, multiplexers and IMUXs 26/10/10 06-Architecture

Data Link Layer Devices Physical Data Link Network Transport Application Data Link layer devices include bridges, brouters, FRADS and switches 26/10/10 06-Architecture

Network Layer Devices Physical Data Link Network Transport Application Network layer devices include routers, brouters, switches, PADs, DSLAMs, gateways and PABXs 26/10/10 06-Architecture

Application Layer Devices Physical Data Link Network Transport Application Application layer devices include gateways, mail relays and Front End Processors 26/10/10 06-Architecture