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How To Configure ATM

ATM (Asynronous transfer mode) is non-broadcast, high-speed layer 2 technology that is somewhat similar to frame relay. It supports high bandwidth and integrates with QOS mechanism. It is considered flexible technology that supports voice, video and data traffic. Some other layer 2 technologies like Token Ring, Eathernet support transmission on variable packet based technology.  

For example, Eather packet contains 1514 bytes but MTU has a change number. While ATM uses fixed packets that is known as cells. Each cell is consisting of 53 bytes, as the size of cell always remains consistent. Due to consistent size of cell, traffic routes more efficiently.


There are two types of ATM interface:

User-Network Interface (UNI):

 It connects end-user device of ATM to a switch or router.
Network-Network Interface (NNI):

It connects ATM switch to other switch of ATM such as Fram Relay and builds circuits or virtual channels between devices of ATM. These virtual channels are permanent (PVC) or switched (SVC). However, these channels possess one-way mode of communication.


Virtual channel is recognized with the use of VCI (virtual channel identifier) and frame relay DLCI. Virtual channel identifier is useful for local router or switch. Multiple virtual channel can be grouped together into virtual path. The main task of VCI is to identify possible virtual path and responds to transmission medium. It identifies a route between two devices of ATM.

ATM Layers:
ATM is composed of AAL (ATM Adaptation Layer) that dictates information like TCP ports or IP addresses to upper layer into ATM cell.
Various sorts of AAL dictate different class of services for traffic.
AAL-1: It is used for video, voice, and delay sensitive traffic. It uses constant bit rate to deliver consistant timing and bandwidth.
AAL-2: It is used for video and voice traffic and Variable Bit Rate in place of constant bit rate that makes a difference between the rate of traffic and bandwidth. VBR (Variable Bit Rate) is either (VBR-RT) real time or (VBR-NRT) non real time.
AAL-3/4: It is called legacy layer that is used for the traffic of SMDS (MultiMegabit Data Service).

AAL-5: It is used for data traffic and use UBR (Unspecified Bit Rate
) and ABR (Available Bit Rate) but traffic and bandwidth rate are not guaranteed. It also supports VBR and CBR in case of any need. Here is order of priority for ATM service classes (high to low).

  • Constant Bit Rate (CBR)
  • Variable Bit Rate – Real Time
  • Variable Bit Rate – Non Real Time
  • Available Bit Rate
  • Unspecified Bit Rate
  • Unspecified Bit Rate +

Basic ATM Configuration:
It is not difficult to make a basic configuration of ATM interface.

RouterA (config) # interface atm0
RouterA (config-if) # ip address

In the next step, you need to identify PVC with the combination of VPI/VCI and the type of encapsulation is identified. You can get all this information from ATM provider.

RouterA(config)# interface atm0
RouterA(config-if)# pvc 0/200
RouterA(config-if-vc)# encapsulation aal5mux ip

The command of PVC creates permanent virtual channel starting from 0 of VPI and VCI of 200. You should remember that VPI is supposed to identify the path and various channels that are present in that path. The command encapsulation is configured as aal5mux that dedicates the presence of virtual channel in the single layer 3 protocol. At last, you need to map virtual circuit in layer 3 address to communicate with router existing at remote side.

RouterA (config)# interface atm0
RouterA (config-if)# pvc 0/200
RouterA (config-if-vc)# protocol ip broadcast
RouterA (config-if-vc)# protocol ip

In this configuration command, mapped protocol is IP that points to the IP address to the remote side.  ATM is regarded as non-broadcast multi-access medium where you need to specify the parameters of broadcast to establish connection with remote router IP. ATM is known as non-broadcast multi-access medium. In that status, the broadcast parameter should be mentioned to broadcast directly the IP of remote router. In other case, routing protocols like EIGRP and OSPF will fail to establish relation with neighbor routers. The final command of protocol IP points to the local interface of ATM that allows router to ping itself.

These some basic steps can assist you in the process of basic atm configuration in better way and save your time and money because you do not neet to hire any network technician for it.


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