Existing TechnologiesFrame Relay, ATM, and IP-Based Networks: What Can They Solve?

Having looked at the case for building VPNs, we are now going to shift gears and examine how to use existing technologies, such as networks based on Frame Relay, ATM, and IP to solve problems. We'll start with Frame Relay.

Frame Relay

Frame Relay is designed as a telecommunications service for cost-efficient data transmissions in which traffic can be intermittent between an enterprise LAN and distributed between WAN locations. As a packet-switching protocol, Frame Relay was developed as a result of WAN requirements for speed and consequently for LAN-LAN, LAN-WAN interworking. Additionally, Frame Relay circuits rarely experience outages.

Frame Relay inserts data in a variable-size unit referred to as a frame where the error correction function (retransmission of data) is the responsibility of the end-points. The service provider typically provides a permanent virtual circuit (PVC) for most services, and this results in the customer possessing a dedicated virtual connection without being charged for a full leased line.

An enterprise can select a level of service quality by prioritizing some frames to primarily transport data over Frame Relay. Service providers offer committed information rate (CIR) as an option to a customer that permits an allocated minimum capacity and allows for traffic burst when required. Voice and video applications are not really suited for Frame Relay because these applications require a steady flow of transmission. When transmitting voice traffic over Frame Relay, however, the voice traffic is fragmented and encapsulated (via FRF.12 encapsulation) for transit across the Frame Relay network. The hub and spoke configuration is a common topology used for Frame Relay deployments. Although full mesh implementations are supported, they are rare because of the high price of their individual circuits. Furthermore, operational complexities associated with the maintenance of the connections are in the order of magnitude of N+(N-1)/2 connections; such a configuration also poses significant challenges, as stated earlier in this chapter. Typical applications for Frame Relay include LAN interconnections, client/server, e-mail, terminal-to-host, and host-to-host (such as file transfers between mainframe computers).

Asynchronous Transfer Mode

ATM defines cell switching with a packet adaptation layer that permits the high-speed transmission of data through the use of small, fixed-length packets (cells) rather than frames as used in Frame Relay.

ATM was originally developed to be a key component of Broadband ISDN (B-ISDN) and is a derivative of Frame Relay. ATM was designed to integrate voice, data, and video services by transporting these multiple channels over the same physical connection. A customer can order a private virtual circuit with a specific ATM QoS characteristic, such as voice via constant bit rate (CBR) or transactional applications via variable bit rate (VBR). Available bit rate (ABR) adjusts bandwidth according to the congestion levels in the network, but it is not used for time-critical data, such as real-time voice and video. Unspecified bit rate (UBR) can be used for noncritical applications and is the lowest ATM class.

The class of service elements of ATM provide QoS assurance for the various service types, such as voice and data. ATM benefits include dynamic bandwidth capability and CoS support for multimedia service classes. Typical business applications include videoconferencing, voice, real-time audio, and high-bandwidth data such as medical imagery. Frame Relay and ATM offer connection-oriented services, whereas IP is connectionless.

It should be said that ATM today faces its most serious competition from private leased lines. This is due to the fact that ATM is a point-to-point service and customers with point-to-point requirements often compare the cost of a private leased line with ATM or IP/MPLS and find private leased lines less expensive than ATM and IP/MPLS. However, customers often fail to consider the lack of redundancy in a leased line as compared to a resilient ATM or IP/MPLS service. Therefore, for network requirements that include resiliency or more than a few point-to-point connections, scaling and resiliency factors usually render this option unviable.

The Internet Protocol

IP has become the world's most popular open system because it is used to communicate across any set of interconnected networks. The IP suite consists of IP and the Transmission Control Protocol (TCP) and specifies common applications, such as e-mail, file transfer, and terminal emulation. The User Data Protocol (UDP) is a lightweight transport built on top of IP, and it squeezes extra performance from IP by not implementing some of the features that a more heavyweight protocol such as TCP offers. Specifically, UDP allows individual packets to be dropped (with no retries) and UDP packets to be received in a different order than the one in which they were sent. UDP is often used in videoconferencing applications or games where optimal performance is preferred over guaranteed message delivery.

Data traverses an IP-based network in the form of packets, where each packet consists of a header that specifies the source, the destination, and the message itself. The IP addressing scheme uses either IPv4 or IPv6 to address computers on the Internet. IPv4 uses 32 bits for addressing, whereas IPv6 has a 128-bit source and destination address scheme that provides more addresses than IPv4. IP permits connectivity via a variety of physical media and provides a best-effort datagram service. Therefore, no hard packet delivery guarantees exist. TCP is often used where reliability is a concern because it guarantees the delivery and ordering of transmitted data. IP provides any-to-any connectivity, as demonstrated by the Internet. Common applications that are used today by companies include e-mail, web hosting, electronic commerce, corporate intranets and extranets, and emerging VoIP. Moreover, enterprise applications, such as enterprise resource planning (ERP) and supply chain management (SCM), use IP as the key transport protocol. Business engineering processes, such as order entry, billing, and reporting, also implement IP-based applications that automate these processes.

Real-Time Transport Protocol (RTP) is one of the IPv6 protocols. It is designed to provide end-to-end network transport functions for applications transmitting real-time data, such as audio, video, or simulation data, over multicast or unicast network services. RTP provides services that include payload-type identification, sequence numbering, time-stamping, and delivery monitoring to real-time applications.

As service providers and enterprise customers converge on IP-based solutions, providing QoS and CoS guarantees such as ATM are requirements for SLA contractual adherence. Further, service interworking between access technologies, such as Frame Relay, ATM, and Ethernet, offer benefits for the customer, including nondisruptive service migration with service enhancements for a cost that is as good or better than what customers are receiving with their existing service. For the service provider, this opportunity translates to customer churn avoidance and economies of scale by eliminating stove-pipe networks, while simultaneously providing service continuity. The main goal for service convergence, therefore, is to provide common Layer 2 transport capabilities, such as QoS; SLA; operations, administration, and maintenance (OAM); and security, while retaining the ubiquity and flexibility of IP-based services. Revenue migration rather than cannibalization is the real benefit for the service provider, and this factor is discussed further in Chapter 15.