High Speed Network Transport

WAN and Enterprise Network Communications

• Several technologies provide alternatives for enterprise and WAN requirements

• Fast Ethernet
• FDDI
• X.25
• ISDN
• Frame Relay
• Cell Relay
• ATM
• SMDS
• SONET

Fast Ethernet

• The need for high speed solutions has created rapid development of Ethernet products capable of packet transfer rates of 100Mbps.
• At 100Mbps a multiple volume encyclopedia set can be transmitted over a network segment in well under a minute.
• Hewlett Packard led the way in developing 100BASE-VG also called 100VG-AnyLAN, while a group of vendors developed 100BASE-X.
• Only the 100BASE-X is accepted by the IEEE as the fast Ethernet standard.
• 100BASE-VG is endorsed by the IEEE for fast network communications, but not as true Ethernet using CSMA/CD transport.
• Although both 100BASE-VG/100VG-AnyLAN and 100BASE-X are explained here, you should plan to implement 100Mbps communications using 100BASE-X on your Ethernet network because it is accepted by the IEEE as truly Ethernet based.
• There are more network equipment options for 100BASE-X and more assured compatibility with other network options.

The IEEE 802.12 Standard

• Adopted by the IEEE as the 802.12 standard, the 100BASE-VG/100VG-AnyLAN and 100BASE-X approach abandons the CSMA/CD transmission technique for one called demand priority.
• Demand priority ensures that the transmitted signal travels in only one direction.
• It is used in star networks, in which workstations are linked by a central hub. In that scheme, each node sends the hub a request to transmit.
• Requests are granted one by one. Incoming packets are examined for their destination address and sent directly to the recipient node on the star.
• Because of the physical star configuration, none of the other nodes sees the packet, since it never travels past other nodes. Each packet is moved from the transmitting node through the hub directly to the recipient node.
• Demand priority enables packets to travel up to 100Mbps by eliminating the possibility of collisions.
• Besides fast transmission, demand priority has two other important benefits:

• Security—Only the receiving node sees the transmitted packet.
• Ability to handle multimedia and time-sensitive transmission—The highest priority can be given to such transmissions, so that voice and video are transmitted in appropriate time sequences to prevent interruptions.

The IEEE 802.3u Standard

• 100BASE-X using the CSMA/CD media access method for transmission of signals.
• The signal is sent in more than one direction.
• To work, the 100BASE-X algorithm requires that the total length of the network be limited to 250 meters and that the signal cannot go through more than two repeaters.
• Although it is possible to use cable other than category 5 for Fast Ethernet communications, category 5 cable provides the best assurance of reliable high-speed communications.

FDDI

• Developed in the mid 1980s to provide higher-speed data communications than those offered by Ethernet (10Mbps at the time).
• The FDDI standard is defined by the ANSI X3T9.5 standards committee, providing an access method to enable high-capacity data transmission on busy networks.
• FDDI uses Fiber-Optic cable as the communications media.
• With a rate of 100Mbps, FDDI is an improvement over 10Mbps Ethernet and token ring, but it is used less and less since the development of Fast Ethernet.
• FDDI supports up to 500 nodes on a single fiber-optic cable segment.

The Timed Token Method

• FDDI is similar to the token ring access method because it uses token passing for network communications.
• It differs from standard token ring in that it uses a timed token access method.
• Because FDDI uses a timed token method, it is possible for several frames from several nodes to be on the network at a given time, providing high-capacity communications.
• One a node transmits a frame, the frame goes to the next node on the network ring. Each node determines if the frame is intended for it, and each node checks the frame for errors.
• The path of a packet on a FDDI ring:

• If the node is the intended target, it marks the frame as having been read.
• If any node detects an error, it marks a status bit in the frame to indicate an error condition.
• When the frame arrives back at the originating node, it is read to determine whether the target node received it.
• The frame also is checked for errors.
• If an error is detected, the frame is retransmitted.
• If no errors are found, the frame is removed from the ring by the originating node.

• Two types of packets can be sent by FDDI:

• Synchronous—Continuous Bursts
• Asynchronous—Discrete units with a start bit and a stop bit at the end.

FDDI Packet Format

• It is similar, but not identical to that used by standard token ring.
• The synchronous is important because, unlike token ring, FDDI permits more than one token on the network at a given moment.

FDDI Error Detection

• FDDI nodes monitor the network for two types of error conditions:

• long periods of no activity
• long periods in which the token is not present

• If either error condition is present, the following occurs:

• The node that detects the error sends a stream of specialized frames called claim frames.
• The claim frames contain a proposed TTRT value.
• The first node stops transmitting, and the next node on the ring compares its proposed TTRT value with the value sent by the previous node.
• After the comparison, the second node sends the lower of the TTRT values in its claim frames to the next node.
• By the time the last node is reached, the smallest TTRT value has been selected.
• At that point, the ring is initialized by transmitting the token and the new TTRT value to each node until the last node is reached.

FDDI Communications Media

• FDDI employs single-mode or multi-mode fiber-optic cable.
• A mode is like a bundle of light entering the fiber at a particular angle.
• Single-mode cable allows one bundle of light to enter the fiber.
• Multi-mode cable allows many bundles of light to enter at a given time.
• Single-mode fiber is used for network backbones in which data must travel over long distances.
• Multi-mode fiber is used for desktop workgroup applications that involve shorter transmission distances.
• FDDI networks have data transmission redundancy, making them highly reliable.
• Redundancy is accomplished by using two network rings:

• One ring is defined as the primary cable run for information transmission.
• The secondary ring provides a backup route for transmitted information, should the primary ring be broken.

• Data on the secondary ring travels in the opposite direction from data on the primary ring.
• If there is a failure in the FDDI primary ring, the logic of the cable architecture provides wrapping.
• That means the signal is directed onto the cable route so it doubles back to become a single ring.
• Two classes of nodes connect to FDDI:

• Class A nodes are attached to both network rings and consist of network equipment such as hubs. They have the ability to reconfigure the ring to enable wrapping in the event on network failure.
• Class B nodes connect to the FDDI network through Class A devices and attach to the primary ring only. Class B are servers or workstations.

X.25

• A packet-switching protocol developed by the CCITT to ensure reliable data communications, even over networks with low-quality transmission equipment.
• With a transmission rate up to 64 Kbps, it is suitable for connecting older networks with low transmission rates.
• Its strength is that it is a well-established dependable technology.

Switching Techniques

• A technique for sending an information-carrying signal from one point on a network to another but along different paths, such as trains are switched over multiple tracks.
• There are three ways to perform switching:

• Circuit switching—

• Creating a dedicated physical circuit between the sending node and the receiving node.
• Circuit switching acts as a straight channel on which to send data back and forth without interruption, similar to making a phone call between two parties.
• The transmission channel remains in service until the two nodes disconnect.

• Message Switching—

• A store and forward communication method to transmit data from the sending node to the receiving node.
• Data is sent from one node to another one that stores it temporarily until a route is open on the next leg of the journey so that the data can be forwarded.
• Several nodes along the route store and forward the data until it reaches the destination node.

• Packet Switching—

• A combination of circuit and message switching.
• Establishes a dedicated circuit between the two transmitting nodes, but the circuit is a logical connection, not a physical one.
• Several different physical routes may be used during the session, but each node is aware only that there is a dedicated channel.
• The advantages of packet switching are that the route can be established for the type and the amount of data sent, creating an opportunity to make the transmissions high speed.

X.25 Transmission Modes

• An X.25 network can transmit data packets using one of three modes:

• Switched virtual circuits
• Permanent virtual circuits
• Datagrams

• Switched virtual circuits are a two-way channel established from station to station. The circuit is a logical connection that is established only for the duration of the data transmission. Once the transmission is completed, the channel can be made available for other nodes.
• Permanent virtual circuits are logical communication channels that remain connected at all times. The connection remains in place even when data transmission stops.
• Both switched and virtual circuits are examples of packet switching.
• Datagrams are packaged data sent without the establishment of a communication channel and are a form of message switching. The packets are addressed to a given destination and may arrive at different times, depending on which path is selected.
• Datagrams are not used on international networks, but were included in the CCITT specifications for the internet.

X.25 Packet Switching

• The X.25 standard defines communications between two types of entities:

• Data terminal equipment (DTE)
• Data communications equipment (DCE)

• DTE can be a terminal, PC, or a host computer.
• DCE is a network equipment that functions as a packet switching node.
• In most common configurations, the DTE is attached to a packet assembler/disassembler (PAD).
• The PAD translates data from the DTE into X.25 format.
• It also translates the received X.25 formatted data into a format understood by the DTE.
• Software at the PAD formats the data and provides extensive error checking.
• PADs can also send out data from several DTEs at the same time, which they do through packet switching.
• This form of packet switching involves transmitting messages using a store-and-forward technique.
• Data messages are broken into packets by the DTE and sent to the PAD.
• Because X.25 supports multiple channels, several DTEs can transmit at the same time. The switch sequentially switches from channel, transmitting the data from each DTE.

X.25 Communication Layers

• The CCITT specifications have defined three layers in X.25 for communications between DTEs and DCEs.

ISDN

• Integrated Services Digital Network (ISDN) was introduced in the 1970s by the CCITT to provide voice, data, graphics, and video digital services.
• The first CCITT set of standards was made official in 1984 and later refined in 1988.
• The "I" series of standards from 1988 includes the following:

• I.100 This portion of the standards is an introduction to ISDN, and a glossary of terms.
• I.200 The services provided to users include:

• Complete guaranteed end-to-end compatibility.
• Standardized terminals and procedures.
• Listing of ISDN subscribers in an international directory.
• Standard testing and maintenance procedures.
• Charging and account rules.

• I.300 The series focuses on network issues such as numbering and addressing.
• I.400 This portion deals with network interface topics such as equipment configuration, transmission rates, and protocol specifications.
• I.500 This defines the interface between ISDN and dissimilar networks.
• I.600 Subscriber installation, access services, and general architecture are defined.

• Implementation of ISDN has been expensive for long-distance carriers. Because ISDN is entirely digital, old analog and electromechanical switches have to be replaced.
• The major U.S. long-distance carriers are implementing ISDN as they replace aging equipment.

I.200 Services for Networking

• The I.200 portion of the CCITT specifications for ISDN offers various networking capabilities, divided into bearer services, teleservices, and supplementary services.
• The bearer services include circuit-mode and packet-mode options.

Digital Communications Techniques

• Two interfaces are supported in ISDN:

• Basic Rate Interface (BRI) has an aggregate data rate of 144 Kbps. Because many conventional LANs have data rates of 1-16 Mbps, ISDN BRI is not suitable for some network applications such as a file transfer and graphics.
• Primary Rate Interface (PRI) supports faster data rates, particularly on channel H11, which offers switched bandwidth in increments of 1,536 Kbps.

• Broadband ISDN (B-ISDN) is being developed to provide an initial ISDN transfer rate of 155 Mbps. The theoretical limit is 622 Mbps.

• ISDN is compatible with many existing digital networks and telecommunications technologies, such as ATM, X.25, and T1.
• Digital signals are placed on the network in two ways.

• Time-compression multiplexing is a form of circuit switching in which several physical channels are connected to a switch called a multiplexor.
• Echo cancellation transmits data in two directions at the same time. A device called a hybrid connects to the transmitter and the receiver to the subscription line. The two-way simultaneous transmissions often cause reflection (echo) of the transmitted signals. Echo of signals on the line may be three times greater than the power of reflected signals, thus obscuring the data. ISDN uses an echo canceler

T-Carrier

• A dedicated telephone line that can be used for data communications to connect two different locations.
• T-carrier lines offer dependable service over long distances.
• T-carriers use switching techniques to offer high speed data transmission options.

ISDN and OSI Layered Communications

Frame Relay

• CCITT standards for frame relay were introduced in 1988 to meet the demands of high volume, high bandwidth WANs.
• Additional standards were approved in 1990, 1992, and 1993 to meet with the evolving demand for frame relay.
• Nearly 60% of Fortune 1000 companies use frame relay or plan to do so.
• Frame relay uses packet switching along with virtual circuit techniques.
• Frame relay is designed to interface with modern networks that do their own error checking.
• It achieves high speed data transmission by recognizing that newer network technologies have error checking on intermediate nodes, so it does not incorporate extensive error checking.
• Used with TCP/IP or IPX-based networks, where those protocols handle the end to end error checking.
• Frame relay is a communications protocol that relies on packet searching and virtual circuit technology to transmit data packets.
• One disadvantage to consider is that it discards packets if it detects heavy network congestion.

Switching and Virtual Circuits

• Frame relay uses multiple virtual circuits over a single cable medium.
• Each virtual circuit provides a data path between two communication nodes.
• Permanent Virtual Circuits

• A permanent virtual circuit is a continuously available path between two nodes.
• The path is given a circuit ID that is part of every transmitted packet.
• Once the circuit is defined, it remains open, so communication can occur at any time.

• Switched Virtual Circuits

• A transmission that is based on the need to establish a transmission session.
• A call control signal is sent between the nodes to establish communications.
• Once the communication is finished, the call control signal issues a command for each node to disconnect.

• Frame relay technology is used with fiber-optic or compatible twisted-pair cable, including T1 and T3 fiber-optic media.

Cell Relay

• A developing technology that takes frame relay a step further.
• Cell relay creates large fixed-length data entities called cell.
• Cell relay is the only was suitable for voice and video applications.

ATM

• Asynchronous Transfer Mode (ATM), developed by CCITT, is a standard that has gained wide acceptance for network interoperability.
• The acceptance of ATM is related to several factors:

• It handles data, voice, and video transmissions.
• Because there is flexibility in geographic distance, it can be used for LAN and WAN communications.
• It can accommodate high speed communications.
• It can provide high speed communications between Ethernet, token ring, Fast Ethernet, FDDI, and other kinds of networks.

• ATM switching technology permits the network to handle many types of data transfer need. Compared to other technologies, ATM switching has the following advantages:

• ATM switching enables data to be transmitted at access speeds appropriate to the type of data sent.
• ATM permits use of higher bandwidth.
• Each ATM connection (communication session) has its own dedicated bandwidth.
• Connection processes are more clearly defined with ATM, because they are handled by the switch from point to point.

SMDS

• Developed by Bell Communications, switched megabit data service was first demonstrated in 1990 as a telecommunications based system to link FDDI networks into a MAN.
• SMDS is a cell based data transmission technology capable of speeds up to 155 Mbps over T-carrier lines.

• SMDS was developed to provide high speed data connectivity similar to that of SONET for applications like the following:

• Providing a high speed link for regional networks
• Transmitting large image files, such as medical X-rays
• Transmitting architectural drawings and other CAD graphics
• Providing fast access to library holdings and electronics catalogs

 

SONET

• Synchronous Optical Network is a fiber optic technology that can transmit data at more than 1 Gbps.
• SONET has grown rapidly in popularity as more telephone companies have added this capability to their services.
• The Alliance for Telecommunications Industry Solutions (ATIS) developed the standard, which was endorsed by ANSI and boasts a data transmission rates of 2.488 Gbps, with the promise of 13.271 Gbps in the future.
• One advantage of SONET is that it is nonproprietary, so point to point network equipment can be purchased from a variety of vendors
• Some applications of SONET are as follows:

• Providing very high speed data connectivity between distant networks
• Video conferencing between distant sites
• Long-distance teaching
• High quality sound and video reproduction
• High speed transmission of complex graphics, such as topographic maps and images created through satellite photography

Communications Media and Characteristics

SONET Network Topology and Failure Recovery

• SONET travels in a ring topology capable of providing three options for failure recovery: unidirectional path switching, automatic protection switching, and bi-directional line switching.

SONET Layers and the OSI Model

• For protocol layers are used in SONET.