6X.25 The first public data network deployed in the 1970s A connection-oriented network that supports switched virtual circuit and permanent virtual circuit like Leased LineWere replaced by a new kind of network called Frame Relay in the 1980s
7X.25 - user device (host), called Data Terminal Equipment X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardwareX.25 is a packet switched data network protocoldefines an international recommendation for the exchange of data as well as control information between- user device (host), called Data Terminal Equipment(DTE)- network node, called Data Circuit TerminatingEquipment (DCE)
8X.25 Equipment Terminology DTE (Data-Terminal Equipment.) A computer that uses a network for communications.DCE (Data Circuit-Terminating Equipment.) A device at the point of access to a network.DSE (Data-Switching Equipment.) A switching node in a packet-switched data network.X.25 utilizes a Connection-Oriented service which insures that packets are transmitted in order.
10To use X.25First establish a connection to the remote computer, (i.e. placed a telephone call). This connection was given a connection number to be used in data transfer.Data packet consists of 3-byte header and up to 128 bytes of dataHeader consists of 12-bit connection number, packet sequence number, and acknowledgement number, and a few miscellaneous bits
11X.25 ProtocolX.25 comes with three levels based on the first three layers of the Open Systems Interconnection (OSI) seven layers architecture as defined by the International Standard Organization (ISO).
12Three Levels of X.25The Physical Level describes the interface with the physical environment. It is similar to the Physical Layer in the OSI model.The Link Level responsible for the reliable communication between the DTE and the DCE. It is similar to the Data Link Layer in the OSI model.The Packet Level describes the data transfer protocol in the packet switched network. It is similar to the Network Layer in the OSI model.
13Protocols used in the link level Link Access Protocol, Balanced (LAPB)- the most commonly used- enables to form a logical link connectionLink Access Protocol (LAP)- an earlier version of LAPB and is seldom used todayLink Access Procedure, D Channel (LAPD)- enables data transmission between DTEs through Dchannel, especially between a DTE and an ISDN nodeLogical Link Control (LLC)- an IEEE 802 Local Area Network (LAN) protocol whichenables X.25 packets to be transmitted through a LAN channel
14Obsolescence of X.25With the widespread introduction of "perfect" quality digital phone services and error correction in modems, the overhead of X.25 was no longer worthwhileFrame Relay, essentially the X.25 protocol with the error correction systems removed, and somewhat better throughput as a result
17Frame RelayFrame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference modelwas designed for use across Integrated Services Digital Network (ISDN) interfacesFrame relay has its technical base in the older X.25 packet-switchingframe relay offers a fast packet technology, which means that the protocol does not attempt to correct errors
18Frame Relay DevicesDTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. In fact, they may be owned by the customer. Examples of DTE devices are terminals, personal computers, routers, and bridges.DCEs are carrier-owned internetworking devices
19Frame Relay Virtual Circuits Frame Relay provides connection-oriented data link layer communicationFrame Relay virtual circuits are identified by data-link connection identifiers (DLCIs)A Single Frame Relay Virtual Circuit Can Be Assigned Different DLCIs on Each End of a VCClick for more information
20Simple Frame Relay Network A Simple Frame Relay Network Connects Various Devices to Different Services over a WAN
21Frame Relay versus X.25The elimination of functions and fields, combined with digital links, enables frame relay to operate at speeds 20 times greater than X.25X.25 specifies processing at layers 1, 2 and 3 of the OSI model, while frame relay operates at layers 1 and 2 only. This means that frame relay has significantly less processing to do at each node, which improves throughput by an order of magnitude.X.25 prepares and sends packets, while frame relay prepares and sends frames. X.25 packets contain several fields used for error and flow control, none of which frame relay needsX.25 has a fixed bandwidth available. Frame relay can dynamically allocate bandwidth during call setup negotiation at both the physical and logical channel level.
26เครือข่าย ATMเครือข่าย ATM เป็นเครือข่ายที่ประยุกต์ได้หลายรูปแบบ ทั้งแบบ LAN หรือ WAN ใช้กับตัวกลางได้ทั้งแบบลวดทองแดงหรือเส้นใยแสง แต่ โครงสร้างการเชื่อมโยงข้อมูลระหว่างโหนดเป็นแบบสวิตซ์ที่เรียกว่า ATM Switch การส่งผ่านข้อมูลแต่ละเซลจึงขึ้นกับแอดเดรสที่ กำหนดเปรียบเทียบเครือข่าย ATM กับระบบอีเทอร์เน็ต
27เครือข่าย ATM แบบ WANจากโครงสร้างการผ่านข้อมูลแบบสวิตซ์ด้วยเซลข้อมูลขนาดเล็กของ ATM จึงทำให้เหมาะกับเครือข่ายคอมพิวเตอร์แบบ WAN ด้วย โดยเฉพาะ อย่างยิ่งกับงานที่ต้องการใช้ความเร็วข้อมูลสูง เครือข่าย WAN ก็เป็นอีก รูปแบบหนึ่งที่สามารถใช้เทคนิคของ ATM ได้เช่นกัน
28ATM uses the following features Fixed-size cells, permitting more efficient switching in hardware than is possible with variable-length packetsConnection-oriented service, permitting routing of cells through the ATM network over virtual connections, sometimes called virtual circuits, using simple connection identifiersAsynchronous multiplexing, permitting efficient use of bandwidth and interleaving of data of varying priority and size
29ATM Network DeviceAn ATM network is made up of one or more ATM switches and ATM endpoints. An ATM endpoint (or end system) contains an ATM network interface adapter. Workstations, routers, data service units (DSUs), LAN switches, and video coder-decoders (CODECs) are examples of ATM end systems that can have an ATM interface
30Virtual Paths and Virtual Channels ATM virtual connections- Virtual path connections (VPCs), identified by a VPI.Virtual channel connections (VCCs), identified by the combination of a VPI and a VCI.
31Virtual Path Switching VP switching is often used when transporting traffic across the WAN. VPCs, consisting of aggregated VCCs with the same VPI number, pass through ATM switches that do VP switching
33Service Categories and Characteristics ATM provides five standard service categories that meet these requirements by defining individual performance characteristics, ranging from best effort (Unspecified Bit Rate [UBR]) to highly controlled, full-time bandwidth (Constant Bit Rate [CBR]).
35Frame Relay to ATM Migration Why do businesses migrate from frame relay service to ATM?Two main driversThe need for greater capacity than frame relay can handle.The need to support mixed-media traffic, especially voice and video.