Institute of Technology, Sligo Dept of Computing WANs Semester 4 Chapter 2.
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Institute of Technology, Sligo Dept of Computing WANs Semester 4 Chapter 2
Table of Contents 2.1 WAN Service Providers 2.2 WAN Devices 2.3 How WANs relate to the OSI Model 2.4 WAN Encapsulation Formats 2.5 WAN Link Options
WAN Services A WAN is a Data Communications Network operating beyond a LAN's geographic scope. You must subscribe to a WAN service provider, such as a regional Bell operating company (RBOC) to use WAN carrier network services. A WAN connects the locations of an organization to: each other locations of other organizations external services (such as databases) remote user
WAN Services A WAN uses data links, such as Integrated Services Digital Network (ISDN) and Frame wide-area geographies WANs carry many types of traffic, such as: voice data video WAN technologies function at three layers of the OSI model: physical data link Network
WAN Services Telephone and data services are the most commonly used WAN services. Telephone and data services are connected from the building POP to the WAN provider's central office (CO). The CO is the local telephone company office to which all local loops in that area connect The services offered by the WAN provider are of 3 main types: Call setup (also called signalling) Time Division Multiplexing (TDM) Frame Relay
WAN Services Call setup (also called signalling): Sets up and clears calls between telephone users. Most commonly used call setup is Signaling System 7 (SS7) Time Division Multiplexing (TDM): Information from many sources has bandwidth allocation on a single medium. Basic telephone service and ISDN use TDM circuits. Frame Relay: Data contained in frames shares bandwidth with other WAN Frame Relay subscribers. Uses statistical multiplexing (STDM)
CPE, demarc, "last mile", CO switch, toll network The most commonly used terms associated with the main parts of WAN services: Customer premises equipment ( CPE ) -- Devices physically located on the subscriber's premises. Demarcation (or demarc ) -- The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the POP of a building. Local loop (or "last-mile") -- Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office. CO switch -- A switching facility that provides the nearest point of presence for the provider's WAN service. Toll network -- The collective switches and facilities (called trunks) inside the WAN provider's cloud.
CPE, demarc, "last mile", CO switch, toll network
A key interface is between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DTE is the router, and the DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility (eg. MODEM, CSU/DSU, TA/NT1). The WAN path between the DTEs is called the link circuit channel line The DTE/DCE interface describes the physical layer standards. It uses various protocols (such as HSSI and V.35) that establish the codes that DTEs and DCEs use to communicate with each other.
WAN Virtual Circuits A virtual circuit is a pathway through a packet-switched network that appears to be a dedicated, physically-connected circuit (as opposed to an actual, physical, circuit-switched pathway through a network). Virtual circuits are connection oriented Two types of virtual circuits exist: switched virtual circuits (SVCs) permanent virtual circuits (PVCs). In a PVC, the customer and the carrier have negotiated the endpoints and characteristics of the virtual circuit ahead of time, and they are constantly available. The end points and a stated bandwidth called a Committed Information Rate (CIR) constitute a PVC, which is defined to the frame relay network devices In a SVC, the virtual circuit is available only "on-demand". circuit establishment, data transfer, and circuit termination.
PVC and SVC SVCs are used in situations where data transmission between devices is sporadic. Disadvantage = increased bandwidth due to the circuit establishment and termination phases Advantage = decreased costs compare with constant virtual-circuit availability. A PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations where data transfer between devices is constant. Advantage = PVCs decreased bandwidth use because there are no establishment and termination phases Disadvantage = increased costs due to constant availability.
WAN Line Types OC-768 now exists running at 40Gbps
Fundamental WAN Devices The router is the essential WAN device, but it is also considered a LAN device. It offers many services, including LAN and WAN interface ports WAN switches connect to WAN bandwidth for voice, data, and video communication. Modems interface voice-grade services (telephone lines). They include: CSUs/ DSUs devices that interface T1/E1 lines TA/NT1 devices that interface ISDN services. Communication servers concentrate dial-in and dial-out user communication.
Routers and WAN Switches Routers provide interfaces for a wide range of links and subnetworks at a wide range of speeds. Routers are active and intelligent network devices and can participate in managing the network by: providing dynamic control over resources supporting the tasks and goals for networks such as: Connectivity Reliable performance Management control Flexibility Security A WAN switch is a multiport networking device that operates at the data link layer of the OSI reference model. A WAN switch typically switches traffic such as: Frame Relay X.25 Switched Multimegabit Data Service (SMDS)
Describe modems on a WAN A device that interprets digital and analog signals by modulating and demodulating the signal, which enables data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. That is digital signals are converted into analog signals. At the destination, these analog signals are returned to their digital form.
CSU/DSUs on a WAN A CSU/DSU is : A digital-interface device that connects a T1/E1 Sometimes two separate digital devices Sometimes its integrated into the router Adapts the physical interface on a DTE device (such as a terminal or router) to the interface of a DCE device (such as a switch) in a switched- carrier network
ISDN Terminal Adapters on a WAN An ISDN Terminal Adapter (TA) is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces Its is an adapter that allows non ISDN devices (terminals or routers) to communicate on an ISDN network The ISDN TA can be either a standalone device or a board inside the TE2.
Organizations that deal with WAN Standards WANs use the OSI model layered approach to encapsulation, just as LANs do WANs mainly focus on the physical and data-link layers Physical layer includes delivery of data Data-link requirements include addressing, flow control, encapsulations WAN standards are defined and managed by: International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) International Organization for Standardization (ISO) Internet Engineering Task Force (IETF) Electronic Industries Association (EIA)/ Telecommunications Industries Association (TIA)
WAN Physical Layer Standards WAN physical-layer protocols describe how to provide electrical, mechanical, operational, and functional connections for WAN services. Also describes the interface between the DTE and the DCE
WAN Physical Layer Standards Physical layer standards define the rules for the DTE/DCE interface: EIA/TIA-232: developed by TIA/EIA, supports signal speeds of up to 64kbps, formerly called RS-232 EIA/TIA-449: a faster version of EIA/TIA-232 (up to 2 Mbps), capable of longer cable runs. EIA/TIA-612/613: A standard describing HSSI, which provides access to services at T3 (45 Mbps), E3 (34 Mbps), SONET (51.84 Mbps) rates. V.35 : An ITU-T standard describing a synchronous, physical- layer protocol used for communications between a network access device and a packet network. For speeds up to 48kbps X.21 -- An ITU-T standard for serial communications over synchronous digital lines.
Name and describe 6 data-link encapsulations The WAN data link layer defines how data is encapsulated for transmission to remote sites Frame Relay: uses simplified encapsulation with no error correction over high-quality digital facilities. A very fast protocol compared to the other WAN protocols. Point-to-Point Protocol (PPP): developed by the IETF. PPP contains a protocol field to identify the network-layer protocol. ISDN: a set of digital services that transmits voice and data over existing phone lines. Link Access Procedure, Balanced (LAPB): For packet-switched networks used to encapsulate packets at Layer 2 of the X.25 stack. Provides reliability and flow control on a point-to-point basis. Cisco/IETF: Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Cisco routers. High-Level Data Link Control (HDLC): an ISO standard, HDLC not compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations.
Serial line frame fields The two most common point-to-point WAN encapsulations are HDLC and PPP All the serial line encapsulations share a common frame format, which has the following fields The choice of encapsulation protocol depends on the WAN technology and the communicating equipment.
PPP and HDLC PPP is a standard serial-line encapsulation method This protocol can check for link quality during connection establishment. Provides authentication through Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). HDLC is Cisco's default encapsulation for serial lines No windowing or flow control Address field is 11111111, just like PPP A proprietary type code is inserted in the frame which means that HDLC framing is not interoperable with other vendors' equipment. Used when both ends of a dedicated-line connection are routers running Cisco IOS
Two basic WAN link options Dedicated lines are physical circuits that involve no switching Switched lines are broken up into 2 categories: Circuit-switched: A physical circuit is established for the duration of the data transfer Packet-switched: Packets may take different physical routes through the cloud on the way to the destination
Dedicated Lines Dedicated lines, also called leased lines, provide full-time service. Typically are used to carry: Data Voice Video Generally provide core or backbone connectivity between major sites or campuses, as well as LAN-to-LAN connectivity. The following are required for dedicated line connections: A router port CSU / DSU An actual circuit from the service provider. Most important dedicated lines are T1, E1, and J1 depending on where in the world you are located
Leased Lines Leased lines are a form of dedicated lines. Dedicated lines also are referred to as point-to-point links because their established path is permanent and fixed for each remote network reached through the carrier facilities. Connections are made using the router's synchronous serial ports with typical bandwidth use of up to 2 Mbps (E1) available through the use of a CSU/DSU. Point-to-point links provide a single, pre-established WAN communications path from the subscriber through a carrier network (such as a telephone company), to a remote network The service provider reserves point-to-point links for the private use of the customer. Point-to-point is used for direct physical links or for virtual links consisting of multiple physical links.
Packet-Switched Connections Network devices share a (PVC) permanent virtual circuit to transport packets from a source to a destination across a carrier network, Switched networks can carry variable-size frames or fixed-size cells. The most common packet-switched network type is Frame Relay. Others include X.25 and SMDS
Frame Relay Designed to be used over high-speed, high quality digital facilities Does not offer much error checking or reliability, but expects upper- layer protocols to attend to these issues Connect multiple network devices on a multipoint WAN Frame Relay access is typically at 56 kbps, 64 kbps, or 1.544 Mbps Each site can be connected to every other by a virtual circuit. Frame Relay service is offered through a PVC. A data-link connection identifier (DLCI) identifies a PVC The DLCI number is a local identifier between the DTE and the DCE The DLCI identifies the logical circuit between the source and destination devices.
Frame Relay Two common topologies can be used in a Frame Relay solution: Fully meshed topology: Every Frame Relay network device has a PVC to every other device on the multipoint WAN. Partially meshed topology: also often called a star topology or hub-and-spokes topology. In a partially meshed topology, not every device on the Frame Relay cloud has a PVC to every other device.
Circuit-switched connections A dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session. Operates much like a normal telephone call Connections from one site to another are brought up when needed and generally require low bandwidth ISDN connections are limited to 64 or 128 kbps Used primarily: to connect remote users and mobile users to corporate LANs as backup lines for higher-speed circuits like Frame Relay and T1 lines.
DDR Dial on Demand Routing is a technique that routers use to initiate and close circuit-switched sessions when transmitting end stations need them. DDR means that the connection is brought up only when a specific type of traffic (interesting traffic) initiates the call or when you need a backup link. DDR is a substitute for dedicated lines when full-time circuit availability is not required. DDR can be used to provide backup load sharing and interface backup
Describe ISDN Terminal Equipment 1 (TE1): Designates a device that is compatible with the ISDN network. A TE1 connects to an NT of either Type 1 or Type 2. Terminal Equipment 2 (TE2): Designates a device that is not compatible with ISDN and requires a TA. TA: Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network. NT Type 1 (NT1): Connects four-wire ISDN subscriber wiring to the conventional two-wire local loop facility. NT Type 2 (NT2): Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating.
ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today. ISDN BRI delivers a total bandwidth of a 144 kbps line into three separate channels. Two of the channels, called B (bearer) channels, operate at 64 kbps and are used to carry voice or data traffic. The third channel, the D (delta) channel, is a 16-kbps signalling channel used to carry instructions that tell the telephone network how to handle each of the B channels. ISDN BRI often is referred to as 2B+D. ISDN