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As shown in Exhibit 4, LANE provides a mechanism for tying legacy networks to ATM networks and using them without modification to software. It allows ATM’s connection-oriented fabric to mimic a connectionless system and makes two separate LANs appear as one big LAN. LANE does this by emulating the MAC layer protocol so that the ATM network looks like just another MAC sublayer similar to either Ethernet or Token Ring. Because of this, it is not possible to mix Token Ring and Ethernet medias on the same LANE. However, they can be emulated separately and bridged. LANE provides mechanisms to map the media access address to the ATM address and vice versa and to multicast the same information to different systems on the network.8

The ATM switch itself does not emulate. It basically sets up a virtual connection and switches 53-byte cells as usual. By separating the physical and logical infrastructure into multiple segments, this virtual connection provides significant benefits in terms of increased security and scalability.9

Suitability for Multimedia Traffic. Although LANE is attractive, it lacks robustness as defined currently, because a single broadcast server is used to do all multicasts and broadcast. Hence, the system is exposed to a single point of failure. The current LAN emulation service does not specify the interaction between LAN emulation service components LNNI, a specification necessary to ensure interoperability. The ATM Forum is working on this aspect and a standard is expected shortly.

Because different types of networks (i.e., Ethernet or Token Ring) must still be bridged or routed, LANE is only useful at the workgroup level and cannot be used at the WAN level. In addition, LANE does not allow applications to take advantage of the QOS characteristics that are the greatest benefit of ATM. LANE’s greatest benefit lies in the protection of investments in the existing infrastructure; as such, it serves as an important step in the ATM migration strategy for many organizations.

WAN TECHNOLOGIES

As is the case for LANs, several technology options are available for supporting multimedia communication on the WAN. Chief among these transport technologies are the IP, frame relay, SMDS, fiber channel, and ATM.

Frame Relay

Frame relay typically connects at speeds of T-1 lines and is an evolution of standard X.25 networks. Like X.25, it combines packets into frames and allocates bandwidth to multiple data streams. Frame relay uses the LAP-D frame structure with a data link connection identifier to route the data. Its payload is as high as 4K bytes and it handles Ethernet data without segmentation. It does not provide for error checking or flow-control mechanisms.

Frame relay connections are either PVC or SVC. PVC establishes a fixed path through the network for each source/destination node and remains defined for long periods of time. SVCs are defined and used only for the specific sessions. Multicasting services can be added over PVC lines, and the bandwidth assigned to each PVC is the committed information rate.

Frame relay over ATM lets a frame relay site communicate directly with an ATM site so that frames can be sent through the ATM backbone. Edge devices convert frame relays to ATM cells and vice versa. This allows LANs to be interconnected through ATM switches or IP to be transmitted over frame relay.

Suitability for Multimedia Traffic. Frame relay is not suitable for voice or video traffic because it does not provide for latency or constant bit rate transmission. The protocol does not add substantially to the delay itself, but it cannot guarantee a set delay. Frame relay provides for committed information rate on PVC lines and has been tested for videoconferencing with limited success. It is a popular mechanism for providing wide area data interconnections. Frame relay and ATM together enable a company to handle both low-speed and high-speed multimedia networking over the WAN. Frame relay is currently more affordable than ATM because it has been available longer.

Internet Protocol

The TCP/IP has become the backbone of the WAN for carrying data services. The problem for carrying multimedia data with IP is the delay associated with the routing of the packets from one node to another. IP does not allocate a specific path nor guarantee a specific bandwidth to the multimedia application. Several technology options are being developed to resolve these problems, including Classic IP over ATM, RSVP, MPOA, and NHRP.

Classic IP over ATM. Classic IP over ATM uses the bandwidth of ATM to provide IP services without changing the fundamental nature of the protocol. The technology uses 9,000-byte frames at the higher layers and reduces packet overhead, which is advantageous for bulk data transfers. Classic IP over ATM also uses a modified version of IP’s address resolution protocol ARP to find ATM’s VCI/VPI connectionless correspondence to the IP network address.

Suitability for Multimedia Traffic. Classic IP over ATM aims to expand and use the huge bandwidth of ATM switches, but it fails to provide for QOS considerations. Because the approach shifts a lot of work to the router feeding the ATM switch, the TCP/IP network may be congested and slow regardless of the speed of the switch. In addition, the fact that two separate routing/switching structures are trying to find the best path adds to a lot of confusion.10

RSVP

A better option for carrying multimedia using IP is to use the IETF’s RSVP, which should be available in mid-1997. To support a mix of voice, data, and video, RSVP confirms QOS parameters with the networking devices using vendor-specific APIs.

RSVP operates through allocation of the following types of resources:11

  Active resources (such as a CPU that is a service provider).
  Passive resources, which are the system capabilities used by the active resources. They include main memory or bandwidth (i.e., link throughput).

In addition to reserving and allocating resources so that traffic flows follow QOS specifications, RSVP uses proper service disciplines and allocation strategies for multimedia delivery and adapts to resource changes.

Suitability for Multimedia Traffic. RSVP essentially allows a router-based network to mimic a circuit-switched network on a best-effort basis. Best effort suffices if there is adequate bandwidth; otherwise quality suffers. RSVP tries to ensure that the bandwidth is reserved and available. It has the advantages of working over both physical network architectures (Ethernet and Token Ring) and matching well with the new IPV6, which allows applications to label packets with traffic patterns. Packets belonging to a particular traffic flow from an application are easily switched.

RSVP thus allows the user to provide QOS to the network workstation without extending ATM to the workstation. Multicast extensions such as the MBone are separate extensions from RSVP. Because the Internet is so widely available, however, and despite the presence of delay and jitter, RSVP is being used as an experimental multimedia communications platform.

Multiprotocol over ATM. MPOA allows ATM switches to route LAN-based traffic between subnetworks, eliminating the router bottlenecks. In addition, IP switching is becoming available. It removes all ATM overhead and uses just the switching feature of ATM, enabling routing to be bypassed. IP switching is still an experimental technology and should be viewed with caution.


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