Understanding Inverse Multiplexing for ATM

 

Inverse multiplexing for ATM (IMA) is a technique of transporting ATM traffic over a bundle of T1 or E1 interfaces. The following sections explain IMA in detail:

Understanding Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a high-speed networking technology that handles data in fixed-size units called cells. It enables high-speed communication between edge routers and core routers in an ATM network.

ATM is designed to facilitate the simultaneous handling of various types of traffic streams (voice, data, and video) at very high speeds over a dedicated connection. ATM uses asynchronous time-division multiplexing (TDM) and it encodes data into 53-byte cells, thereby simplifying the design of hardware and enabling it to quickly determine the destination address of each cell. ATM operates over either fiber optic cables or twisted-pair cables. Each ATM PIC is assigned an ATM switch ID that displays the switch’s IP address and the local interface names of the adjacent Fore ATM switches. For information about ATM PICs, see the platform-specific Hardware Guide.

ATM relies on the concepts of virtual paths (VPs) and virtual circuits (VCs). A virtual path, represented by a specific virtual path identifier (VPI), establishes a route between two devices in a network. Each VPI can contain multiple VCs, each represented by a virtual circuit identifier (VCI). VPIs and VCIs are local to the router, which means that only the two devices connected by the VCI or VPI need know the details of the connection. In a typical ATM network, user data might traverse multiple connections, using many different VPI and VCI connections. Each end device, just like each device in the network, needs to know only the VCI and VPI information for the path to the next device.

An ATM interface is indicated by the at-fpc/pic/port CLI descriptor.

Understanding Inverse Multiplexing for ATM

Inverse multiplexing is a method where a single data stream is divided into multiple smaller data streams that are transmitted over either fiber optic cables or twisted pair cables and are recombined on the other end to form the original data stream. This concept is useful for attaining high-speed data transmission rates. This concept has been extended to ATM and is called inverse multiplexing for ATM or IMA.

IMA is a technique of transporting ATM traffic over a bundle of T1 or E1 interfaces. IMA divides a single data stream into multiple smaller data streams, that is transmitted at the same time across separate channels (such as T1 or E1 interfaces) and then reconstructed at the other end back into the original data stream.

Two versions of IMA are available–IMA 1.0 (af-phy-0086.000-IMA) and IMA 1.1 (af-phy-0086.001-IMA). You can configure either of these versions, by using the Junos OS CLI. If not specified, IMA 1.1 is selected by default. Note that IMA 1.0 and IMA 1.1 do not interoperate. The IMA v1.1 specification increments the OAM (operations and maintenance) label value used in the IMA OAM cells in order to differentiate v1.1 from v1.0 IMA units.

How Inverse Multiplexing for ATM Works

Figure 1 displays IMA frames on different links. An IMA frame consists of ATM cells, an ICP cell, and filler cells (if required).

Figure 1: IMA Frames on Links
IMA Frames on Links

On the transmission side of the ATM IMA network, the ATM cell stream (received from the ATM layer) is divided across multiple links in an IMA group on a cell by cell basis. On the receiving end of the ATM IMA network, the cells are recombined to form the original ATM cells stream (with the help of ICP cells), and then passed on to the ATM layer. Figure 2 displays an IMA frame being transmitted and received through an IMA group.

Figure 2: IMA Frames Transmitted Through IMA Group
IMA Frames Transmitted Through
IMA Group

IMA Control Protocol (ICP) cells are special cells that are sent over the ATM IMA interface with the ATM cell stream to help align the ATM cells at the receiving end. An ICP cell tracks link differential delay, reduces cell delay variation (CDV), and performs other functions.

When there are no ATM cells to be sent on an IMA frame, the IMA transmitter inserts filler cells to maintain a continuous stream of cells at the physical layer. The filler cells are discarded by the IMA receiver. An OAM cell has codes that define it as an ICP cell or a filler cell.

Supported Platforms

The following are the various Juniper Networks routers and their components that support inverse multiplexing for ATM (IMA):

  • 16-port Channelized E1/T1 Circuit Emulation MIC (MIC-3D-16CHE1-T1-CE) on MX Series routers (from Junos OS Release 13.2R1 onward).

  • 4-port Channelized OC3/STM1 (Multi-Rate) Circuit Emulation MIC with SFP (MIC-3D-4COC3-1COC12-CE) on MX Series routers (from Junos OS Release 13.2R1 onward).

  • 4-port Channelized OC3/STM1 Circuit Emulation PIC with SFP (PB-4CHOC3-CE-SFP) on M7i, M10i, M40e, M120, and M320 routers supports channelized OC3/STM1 (down to T1) ATM IMA.

  • 12-port E1/T1 Circuit Emulation PIC (PB-12T1E1-CE-TELCO) on M7i, M10i, M40e, M120, and M320 routers supports discrete T1 ATM IMA.

Note

Circuit Emulation PICs require firmware version rom-ce-9.3.pbin or rom-ce-10.0.pbin for ATM IMA functionality on M7i, M10i, M40e, M120, and M320 routers running Junos OS Release 10.0R1 or later.