Why does ATM use small fixed size
cell? Justify your answer
Ans
ATM is not contention-based or shared like FDDI, ethernet or token ring. A full 155 Mbps is continuously available to every endstation. ATM transmits fixed-length 53 bytes cells. It consists of a header field (5 bytes) and information field of 48 bytes. The header field contains routing info and the information field carries the service data. Small fixed length cells facilitate efficient multiplexing and a traffic prioritization scheme ensures delay-sensitive traffic receives special consideration. Multiplexing techniques allow more than one service to share a particular ATM link at any given time. Services are organized into virtual channels(VC) and virtual paths(VP). A VC identifies the service and a VP identifies a collection of VC's. ATM can be switched at both the VP and VC level.
ATM uses cell switches, not bridges or routers. Switching is done by connecting one port to another directly. All this is done in hardware which makes it more efficient and faster. Bridges and routers also introduce latency/delays. This is because bridges and routers normally process large/variable length packets. These large packets introduce noticeable application delay to a user.
In Ehternet or FDDI rings, media is shared. As more stations are added, available bandwidth to each station decreases. When this happens, we normally segment the network into smaller peices by adding bridges and routers. However, these now increases latency since packets must cross more devices between endstations. Additionally, adding more routers/bridges means adding more subnets and additional complexity. So traditonal networks scales poorly. As we add more bandwidth by adding more devices like bridges and routers, we suffer from higher latency, worse performance end to end and added complexity to a network.
ATM is scalable and inherently exhibits low latentcy. ATM networks are not contention-based. There are built on point-to-point connections between endstations and switches. Each connection runs at a full guaranteed 155Mbps even through a switch. Stations are added by plugging in to an unused port or additional port modules. Also good ATM switches can tie switches/modules together to act like a bigger switch. Also, switch-to-switch rates normally can run faster than 155 Mbps.
Price of ATM technology is dropping rapidly. Price of an ATM adapter card is about the cost of an Ethernet NIC card in 1983.
ATM is not contention-based or shared like FDDI, ethernet or token ring. A full 155 Mbps is continuously available to every endstation. ATM transmits fixed-length 53 bytes cells. It consists of a header field (5 bytes) and information field of 48 bytes. The header field contains routing info and the information field carries the service data. Small fixed length cells facilitate efficient multiplexing and a traffic prioritization scheme ensures delay-sensitive traffic receives special consideration. Multiplexing techniques allow more than one service to share a particular ATM link at any given time. Services are organized into virtual channels(VC) and virtual paths(VP). A VC identifies the service and a VP identifies a collection of VC's. ATM can be switched at both the VP and VC level.
ATM uses cell switches, not bridges or routers. Switching is done by connecting one port to another directly. All this is done in hardware which makes it more efficient and faster. Bridges and routers also introduce latency/delays. This is because bridges and routers normally process large/variable length packets. These large packets introduce noticeable application delay to a user.
In Ehternet or FDDI rings, media is shared. As more stations are added, available bandwidth to each station decreases. When this happens, we normally segment the network into smaller peices by adding bridges and routers. However, these now increases latency since packets must cross more devices between endstations. Additionally, adding more routers/bridges means adding more subnets and additional complexity. So traditonal networks scales poorly. As we add more bandwidth by adding more devices like bridges and routers, we suffer from higher latency, worse performance end to end and added complexity to a network.
ATM is scalable and inherently exhibits low latentcy. ATM networks are not contention-based. There are built on point-to-point connections between endstations and switches. Each connection runs at a full guaranteed 155Mbps even through a switch. Stations are added by plugging in to an unused port or additional port modules. Also good ATM switches can tie switches/modules together to act like a bigger switch. Also, switch-to-switch rates normally can run faster than 155 Mbps.
Price of ATM technology is dropping rapidly. Price of an ATM adapter card is about the cost of an Ethernet NIC card in 1983.
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