New network applications that involve video, high-resolution graphics, and other rich media data type is driving the growth of networks. Applications such as these, require higher bandwidth networking equipment to handle increasing file sizes and increasing number of users. Table 1 summarizes the applications that require higher bandwidths and it's impact on the network. At this point, the issue for
orangizations is which new emerging network technology will be the best choice to implement. Currently, the debate between Gigabit Ethernet and ATM (asynchronous transfer mode) networks as a possible solution to high bandwidth requirements has taken center stage.
Gigabit Ethernet versus ATM is a highly debate issue. Supporters of Gigabit Ethernet claim that it will be the "...dominant networking topology because it is inexpensive, simple to install, and easy to manage." (Pcweek?) However, proponents of ATM argue that ATM will offer more bandwidth and greater performance when multiple connections established. This paper will describe Gigabit Ethernet and ATM and analyze the pros and cons of both technologies by comparing the physical aspects.
Gigabit Ethernet is one of the new emerging network technologies that plans to offer one gigabit per second data transfers. Its initial goals are to support three key areas:
Aggregating traffic between Ethernet clients and
centralized file or compute servers
Connecting multiple 100Base-T Fast Ethernet
switches through 100/1000 Mbs switches
Connecting both workstations and servers with
Gigabit Ethernet to run high-bandwidth
applications, such as CAD/CAM, medical imaging,
Figure 1 shows an initial deployment of Gigabit Ethernet.
Initially, Gigabit Ethernet will be a campus backbone technology placed between routers, switches, and hubs. It will connect power workstations, servers, and server farms to the high-speed backbone and use a full and half-duplex flow control. Physically, Gigabit Ethernet will initially operate over optical fiber and will require four hardware types:
Gigabit Ethernet Network Interface Cards(NICs)
Aggregating switches that connect 100Mbps Fast
Ethernet and 1000Mbps Gigabit Ethernet segments
All-Gigabit Ethernet switches
All-Gigabit Ethernet Repeaters
This will allow Gigabit Ethernet to merge into existing Ethernet and Fast Ethernet networks.
Gigabit Ethernet's hardware consists of network interface cards, switches, and repeaters. The network interface cards (NICs) are attached to the hosts and servers. The NICs allow the hosts and servers to interface with the Ethernet switches. The Ethernet switches are used for driving high bandwidth applications and establishing connections between the backbone and server and servers and clients. The switches increase the bandwidth by dividing the network into segments and insulating each from the other's local traffic. Finally, the Ethernet repeaters are devices that read an Ethernet signal on one cable and re-transmit it on another cable.
As mentioned above, Gigabit Ethernet will initially operate over optical fiber. Single-mode and multimode optical fiber will be used. The single-mode fiber will be used for distances up to 2 kilometers and the multimode fiber for distances up to at least 500 meters. Since optical fiber is available and optimized for high performance, this will minimize the time-to-market for Gigabit Ethernet products. However, the plan for the future is to operate Gigabit Ethernet over Category 5 unshielded twisted-pair cabling. Category 5 UTP cabling will primarily be used for distances up to 100 meters for computer rooms and wiring closet applications.
Gigabit Ethernet will use full-duplex transmission to handle point-to-point connections. Full-duplex transmission allows signals travel in both directions on the same connection. This mode doubles the aggregate data rate. "Full-duplex can be used between a single workstation and a switch port, between two switch ports, or between two workstations" (Alteon). Full-duplex transmission does not need to invoke CSMA/CD, so it cannot use shared port connections. Figure 2 illustrates a full-duplex configuration.
On the other hand, half-duplex transmission works with shared port connections. Shared port connections are repeaters or hub port connecting to multiple workstations. In half-duplex mode, the signal can travel in both directions on the same connection, but not at the same time.
In order to employ half-duplex, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is used as the standard access method. This means that the station has to wait for a clear channel before it can transmit. If a collision occurs, each station must retransmit its data at so random time interval. Figure 3 shows the configuration for half-duplex mode.
ATM (asynchronous transfer mode) is another newly emerging network technology that plans to offer the high bandwidth requirements of multi-media communications. "ATM technology is based on small, constant-sized cells that permit sufficiently rapid switching that multiple isochronous data can be statistically multiplexed together, along with computer network traffic" (Koester). Initially, the goals of ATM are as follow:
High transfer rate.
Ability to transfer all data types.
Use of the same technology for LAN and WAN
Figure 4 show a typical ATM configuration. ATM is a
connection oriented system that will use fiber optic cabling and will require the following hardware:
Interfaces to the public network.
Internetworking devices, such as routers.
This will allow ATM to handle emerging multi-media applications.
The hardware requirements of ATM are ATM switches, routers, and interfaces. The network interface cards (NICs) are attached to the hosts and servers and allows the hosts and servers to interface with the ATM switches. The ATM switches establish, maintain, and re-route paths across the ATM network. Finally, routers are protocol dependent devices that connect subnetworks together and forward packets between them.
ATM will also use single-mode and multi-mode optical fiber as its cabling type. In the same manner as Ethernet, the single-mode fiber will be used for distances up to 2 kilometers and the multimode fiber for distances up to at least 500 meters. In the future, ATM also plans to operate over Category 5 unshielded twisted-pair for distances up to 100 meters.
ATM uses point-to-point and point-to-multipoint connections. Point-to-point connections use full-duplex transmission mode to control signals. This means that the one user can send information to another user and receive information from that same user at the same time. Figure 5 shows the configuration for a point-to-point connection. However, the point-to-mutipoint connection is a unidirectional connection using half-duplex transmission.
In this connection type, there are several called users and one calling user. The calling user transfers information to
the called users and each calling user receives one copy of the information.
Ethernet and ATM: Differences
Gigabit Ethernet and ATM networks are similar in some aspects and different in others. For example, the physical aspects of Ethernet and ATM are similar for the most part. However, there are some differences in their applications. One of the biggest factors is that the Ethernet network does not provide QoS (Quality of Service) for mutlimedia applications. In other words, a lack of QoS may adversely affect time-sensitive voice and video applications. On the other hand, ATM offer QoS for time-sensitive voice and video applications. Secondly, Gigabit Ethernet is a technology that only exist in local area networks. Whereas, ATM will interwork with the WAN network and continue to offer QoS. Thirdly, ATM appears to be more scalable than Gigabit Ethernet. For example, OC-3 at 155 Mbps can be scaled to OC-12 at 622 Mbps by merely putting in a new interface module. Finally, one of the major short-comings of ATM for LANs is that it more costly. This is due to that fact that offers QoS and advanced congestion-control features.
The physical aspects of Gigabit Ethernet and ATM are very similar. They both use fiber optical cabling to transmit data and full and half-duplex transmission modes. They are also closely matched when it comes to their hardware requirments. However, the major differences in Gigabit Ethernet and ATM laid in their applications and cost. ATM out rank Gigabit Ethernet when it comes to quality of service, scalability, and the ability to interwork in WAN networks. On the other hand, Gigabit Ethernet is more cost-effective than ATM. It appears that ATM and Gigabit Ethernet are complementary rather than competing technologies. The choice between the technology is based on the applications designed.