SYSTEM ADMINISTRATIONChapter 11 WAN Technologies
Wide Area Network (WAN) Technologies • A wide area network (WAN) is a data communication network that spans a large geographic area and usually makes use of transmission facilities of third-party vendors. • Third-party vendors are usually referred to as common carriers. • WANs are used to interconnect remote locations of a business operation. • The functions of WAN technologies are clustered at the three lowest layers of the OSI Model.
Switching Technologies • Two types of switching technologies are used with WAN connections: • circuit switching • packet switching
Circuit Switching • A circuit is a path between two or more communicating entities and usually refers to the physical path the data takes. • Circuits may be pre-reserved, as with a dedicated connection between two end points.
Virtual Circuits • Logical circuits represent a path between two points and are not fixed on one physical path. This is called a virtual circuit. • Permanent virtual circuits (PVCs) guarantee that a certain amount of bandwidth will always be available when needed. • The PVC eliminates the need to reserve a specific path in advance. • Switched virtual circuits (SVCs) are dynamically created when a request for transmission is made and ended when the transmission is complete. These are good for sporatic traffic. • Circuit switching is used by the public telephone network (POTS or PSTN). • Circuit switching involves some setup time to establish the circuit, but the time delay is usually not apparent to the user.
Packet Switching • Packet switching is commonly used for WAN connections. • A message will be broken into smaller units called packets. • Each packet has a header that contains the source address, destination address, and a sequence number. The sequence number is used to reassemble the message once it reaches the destination. • With packet switching, the pieces of the message (the packets) often take different paths from the source to the destination. (continued)
Packet Switching(continued) • Once the packets reach the destination, the message is reassembled using the sequence numbers in the headers of the packets so the message can be read by the destination. • Packet switching involves no circuit setup time. The transmission is connectionless, so there is no guarantee of delivery. • Routers make the decisions about the path a series of packets will take.
Which Switching Method is Better? • Every business must evaluate the types of traffic that will be transmitted in order to determine which type of switching method is most suitable. • Benefits of circuit switching: • A guaranteed pathway for the data. • A guaranteed portion of the bandwidth available for transmission. • Smaller packets because there is no need to place source and destination information in the header, only the circuit number so intervening devices will recognize which message the packet belongs to. (continued)
Which Switching Method is Better?(continued) • Benefits of packet switching: • The public infrastructure can be used for data transmission instead of incurring the expense for dedicated links between locations. • Resources, such as bandwidth, are used more fairly and efficiently in packet switching. • Pipelining, the ability to simultaneously use a communication link for 2 or more transmissions, increases the efficiency of use for the available bandwidth.
Integrated Services Digital Network (ISDN) • ISDN is a cost-effective WAN connection that uses digital transmission over traditional telephone networks. • ISDN uses two types of channels to carry data: • Bearer Channels (B channels) carry the payload, which may be voice or data. • Data channels (D channels) carry the control information for connection setup, timing, and disconnection. (continued)
ISDN(continued) • ISDN is offered in two types of interfaces: • Basic rate interface (BRI) provides two B channels and one D channel for a total transfer rate of 128 Kbps. • Primary rate interface (PRI) allocates twenty-three B channels for data and one 64 Kbps D channel for control information for a total transfer rate of 1.544 Mbps. (continued)
ISDN • The benefits of ISDN include: • Data capacity to service many users at the same time • Voice and data transmission over the same physical media at the same time because one of the two channels of the BRI can be switched to voice if a call comes in • Video conferencing • Widespread availability • Cost-effective solution for small businesses and small office/home office (SOHO) operations • To implement ISDN on the network, contact the local telephone service provider.
Fiber Distributed Data Interface • FDDI is a set of standards developed by the ISO and ANSI that provides the guidelines for data transmission over fiber-optic cable. • FDDI uses a dual counter-rotating ring formation capable of 100 Mbps token passing. • FDDI internetworks have extremely high capacity, and so can service thousands of users over great distances because the data is transmitted over fiber-optic cable. • The two FDDI rings provide data flow in opposite directions. (continued)
FDDI(continued) • When a portion of the FDDI network fails (a station or a cable segment), the FDDI network can heal itself by closing the ring at the gap and allowing data to be transmitted. This results in little or no network down time. • FDDI and fiber-optic networks are usually used for backbone deployments because of the expense of constructing a fiber network. Professionals must install fiber-optic cable to minimize damage to the cable and to the installer.
Synchronous Optical Networks/Synchronous Digital Hierarchy (SONet/SDH) and Optical Carrier (OC-x) • These three technologies have as their common ground the fiber-optic cable. Because of this commonality, the three cannot be easily separated when discussing the technology.
SONet • SONet defines the standards for optical carrier levels and synchronous transport signals for the infrastructure. • SONet uses multiplexing to use all bandwidth efficiently for the transmission of data. • This permits SONet to use low-level digital signals and a synchronous structure. (continued)
SONet • Synchronous structure refers to the transitions of the digital signals so that they occur at exactly the same rate. This tells the media and the receiving station where the 1s and 0s are in the signal. • A clocking signal provides the constant and even pulse to keep traffic in line. • Asynchronous Transfer Mode (ATM) uses SONet because of its capacity and its ability to use multiplexing and synchronization.
Synchronous Digital Hierarchy (SDH) • SDH was the result of several standards organizations defining a global synchronization standard for transmission. SDH unified the various existing standards for international communication.
The Optical Carrier (OC) • The optical carrier standards are based on the synchronous transport signals (STSs) used by SONet. The STS standards have an equivalent OC (optical carrier) standard that is expressed by a numeric indicator. • The basic building block of OC is based on the STS-1, which has a capacity of 51.84 Mbps transmission speed. (continued)
The Optical Carrier (OC)(continued) • Other examples of OC levels are: • OC-3: Also called STS-3, this signal transmits at a line rate of 155.52 Mbps • OC-12: Also referred to as STS-12, transmission rates of 622.08 Mbps can be achieved • OC-48: This is STS-48, with a 2488.32 Mbps rate • OC-192: At the top of the line, this STS-192 rate is 9953.28 Mbps
Asynchronous Transfer Mode (ATM) • Asynchronous transfer mode (ATM) is a WAN transmission technology that is capable of speeds ranging from 1.54 Mbps to 622 Mbps. • ATM uses a switching technology to move high volumes of data, voice, video, and audio transmissions between end points. • ATM is expensive, so it is seldom used for LAN transmission. • ATM uses cells instead of packets. • Each cell is 53 bytes in length. The fixed-sized cell reduces the overhead for processing the package, reduces the number of bits needed for error control, and functions much more efficiently because of the reduced overhead. (continued)
ATM(continued) • ATM uses virtual circuits between defined end points and routes, but does not allocate bandwidth ahead of time (does not pre-reserve bandwidth). This allows ATM to support several classes of service and to support transmission of traffic like multimedia files. • ATM can be used over fiber-optic cable or some of the newer, high-capacity copper media.
The Layered Technology of ATM • ATM functions with several layers: • At the Physical layer, there are specifications for transmission media, signal-encoding schemes, data rates, and compatibility. • At the ATM layer, provisions are made to access services in the upper layers, packet transfer capabilities, cell size definition, and logical connection specifications. • The ATM Adaptation layer (AAL) changes depending on the service being used. AAL maps higher layer information into the cells and passes them down to the ATM layer, or it assembles information from the ATM cells and passes it up to higher layer technologies.
Virtual Channel Connections (VCCs) and Virtual Path Connections (VPCs) • A VCC is a virtual circuit that carries a sequenced, single flow of cells from end to end. • VCCs can be statically configured permanent virtual circuits or dynamically configured switched virtual circuits. • A set of VCCs can be bundled together to form a VPC for transmission. All VCCs that are made part of a VPC will be transmitted from end to end across the circuit as a single entity, resulting in reduced overhead and easier recovery from a failure in the route. • A VPC acts like a virtual trunk between two sites.
Frame Relay • Frame relay is a packet-switching technology that supports data transport at reasonable cost. Frame relay uses the public infrastructure of common carriers to transmit at rates between 56 Kbps and 1.544 Kbps. • Frame relay uses the “cloud” of common carriers to span large distances. • To create a frame relay network, a connection to the provider must be established. Circuits are purchased that allow the transmission between each end of each circuit. • Frame relay offers both PVCs and SVCs. (continued)
Frame Relay • Frame relay requires that a committed information rate (CIR) be established. This number is the minimum amount of bandwidth that will always be available to the circuit. • Vendors also establish the committed burst rate (Bc), which identifies how much excess bandwidth a circuit may use. • A third rate, called the burst excess rate (Be), indicates the maximum burst transmission bandwidth available to the circuit. • If the network is heavily congested, the vendor has the option of dropping packets if the transmission rate exceeds the CIR. • Frame relay networks are easily scalable because they require minimal amounts of hardware.
The T-Carrier Connection • The T-carrier connections refer to the telecommunications links that provide remote access for business using the public telephone infrastructure as the physical media. • T-carrier solutions are leased line solutions, billed from the local telephone company. • T-carrier connections are digital in nature, eliminating analog to digital conversions and making them a good choice for data network interconnection. (continued)
T-Carrier Connections(continued) • T-carrier connections are based on the same building block as ISDN. A T1 leased line uses 24 channels for a transmission rate of 1.544 Mbps. T3 lines transmit at 44.376 Mbps. • The signal level determines the speed of the channel, which is the OSI Model Physical layer characteristic. The unit of measurement is the data signal x, or DSx. One data channel is a DS0; twenty-four channels are called a DS1. • T-carrier connections are always on, making internetwork communications simple for the users. (continued)
T-Carrier Connections(continued) • T-carrier lines can be purchased in increments. These increments are called fractional T1 lines. Each channel transmits at 64 Kbps. • The European equivalent of the T1 carrier is the E1, which has a capacity of 2.048 Mbps. The European equivalent of the T3 line is the E3 line, transmitting at 34.368 Mbps. • T-carrier connections require the following hardware: • CSU/DSU • Multiplexer