X.25 and Frame Relay

X.25 and FrameRelay Compiled By: Ashish Kr. Jha

X.25 • Designed to provide a low cost alternative for data communication over publicnetwork • ▫ Pay only for bandwidth • Standard provides error detection and correction for reliable data transfer. • Can support speed of 9.6kbps to2Mbps • LAPB(Link Access Procedure, Balanced : version of HDLC) is X.25 DLLprotocol • LAPB manages communication and packet framing between DTE andDCE. • Makes sure frames are delivered in sequence anderror-free Compiled By: Ashish Kr. Jha

X.25Layers • An interface between DTE and DCE for terminal operation at the packet mode on public datanetwork. Compiled By: Ashish Kr. Jha

X.25 • LAPB frameTypes • ▫ I-Frames (InformationFrames) • Carry data as well as Next Send (NS) and Next Receive(NR) • counts • ▫ S-Frames (SupervisoryFrames) • Controls flow of data with Receiver Ready (RR), ReceiverNot • Ready (RNR), and Reject (REJ)frames • ▫ U-Frames (UnnumberedFrames) • Establish and maintain communications with, Unnumbered Acknowledgment (UA), Disconnect (DISC), Disconnect Mode (DM) and Frame Reject(FRMR) Compiled By: Ashish Kr. Jha

X.25 LAPB FrameFormat Flag : 8 bits indicates start and end offrame Address : (8 bit) Data : (sizevaries) Control : (8bit) FCS : (16 bit) Frame CheckSequence • Packet Layer Protocol (PLP) is X.25 network layerprotocol • PLP manages calls between a pair DTE devices using Permanent Virtual Circuit(PVC) or Switched VirtualCircuit(SVC) • PLPhandlessegmentation,reassembly,bitpaddinganderrorand • flowcontrol. Compiled By: Ashish Kr. Jha

PLP Operates in FiveModes • CallSetup • ▫ Used to setup virtual circuit forSVC • Data Transfer • ▫ Used for transferring data with both SVC andPVC • Idle • ▫ Used when SVC call has been established but no datais • currently beingtransferred • CallClearing • ▫ Used to end communication between DTEs for aSVC • Restarting • ▫ Used to synchronize DTE and DCE for all virtual circuits that exist betweenthem Compiled By: Ashish Kr. Jha

X.25Devices • Data Terminal Equipment(DTE) • ▫ Terminals, personal computers, and networkhosts • ▫ Located on premises ofsubscriber • Data Circuit-terminating Equipment(DCE) • ▫ Modems and packetswitches • ▫ Usually located at carrierfacility • Packet Switching Exchange(PSE) • ▫ Switches that make up the carriernetwork Compiled By: Ashish Kr. Jha

Sample X.25Network X.25 WAN PSE PSE Modem DCE Personal Computer DTE Terminal DTE Modem DCE Modem DCE PSE PSE Server DTE Compiled By: Ashish Kr. Jha

Packet Assembler/Disassembler(PAD) • Acts as intermediary device between DTE andDCE • Performs threefunctions • Buffering to store data until a device is ready to processit • PacketAssembly • PacketDisassembly PSE X.25Packet Data PAD Terminal DTE Modem DCE PSE Data Assembly/ Disassembly Buffer Compiled By: Ashish Kr. Jha

FrameRelay • Frame Relay (FR) is a high-performance WAN protocol that operates at the physical and data link layers of the OSI referencemodel. • FR originally was designed for use across Integrated Service Digital Network (ISDN)interfaces. • Today, it is used over a variety of other network interfaces aswell. • FR is an example of a packet-switchedtechnology. • Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth. • Usually considered a replacement for X.25 using more advanced digital and fiber opticconnections Compiled By: Ashish Kr. Jha

FrameRelay • FR is strictly a Layer 2 protocol suite, whereas X.25 provides • services at Layer 3 (the network layer) aswell. • This enables FR to offer higher performance and greatertransmission efficiency than X.25, and makes FR suitable for current WAN applications, such as LANinterconnection. • DevicesattachedtoaFrameRelayWANfallintothe following two generalcategories: • ▫ Data terminal equipment(DTE) • DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. • ExampleofDTEdevicesareterminals,personalcomputers, routers, andbridges. • ▫ Data circuit-terminating equipment(DCE) • DCEs are carrier-owned internetworkingdevices. • The purpose of DCE equipments is to provide clocking and • switching services ina network, whicharethe devicesthat actually transmit data through theWAN. Compiled By: Ashish Kr. Jha

Sample Frame RelayNetwork PacketSwitch DCE PacketSwitch DCE PersonalComputer DTE Terminal DTE FrameRelay WAN PacketSwitch DCE PacketSwitch DCE NetworkHost DTE Compiled By: Ashish Kr. Jha

FrameRelay • Assembler/Disassembler(FRAD) • A FRAD can either be a separate device or part of a router/switch X.25 ATM PPP X.25 ATM PPP FrameRelay FRAD FRAD Compiled By: Ashish Kr. Jha

FrameRelay • Link Access Protocol for Frame Modes Services (LAPF) is the protocol defined for Frame Relay Layer 2services • LAPF is a version ofHDLC • ▫ Does not provide flow or errorcontrol Compiled By: Ashish Kr. Jha

Frame Relay OperatingStates • Original Frame Relay standard only coveredPVC • SVC support was added but does not havewidespread • implementation • PVCStates • ▫ Data Transfer – data is being transmitted between DTEdevices • ▫ Idle – connection is still active but no data is beingtransferred • SVC required the addition of two additionalstates • ▫ Call Setup – virtual circuit between DTE devices isestablished • ▫ Call Termination – virtual circuit between DTE devices isterminated Compiled By: Ashish Kr. Jha

LAPF FrameFormat • 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 andresponses • 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 istraveling • BECN:(1 bit) Backward Explicit Congestion Notification indicates congestion inthe • opposite direction the frame istraveling • DE:(1 bit) Discard Eligibility indicates that a frame is low priority whenset Compiled By: Ashish Kr. Jha

Comparison of X.25 and FrameRelay Compiled By: Ashish Kr. Jha

X.25 and Frame Relay