Brief Introduction to ATM

An ATM cell is 5 bytes of header and 48 bytes of data. Look at Figure 5-1 to see the ATM UNI cell format.

Figure 5-1 ATM UNI Cell Format

GFC

VPI

VPI

VCI

VCI

VCI

VCI

The cell format depicted in Figure 5-1 is the User-Network Interface (UNI) cell. The Network-Node Interface (NNI) header is almost identical to this one, except for the GFC field, which has been omitted. Instead, the VPI field occupies the first 12 bits and is thus 4 bits longer, which allows the ATM switches to assign a larger number of virtual path identifiers (VPI).

Table 5-1 shows the name and meaning of each field of the ATM cell header. Table 5-1 ATM Cell Header Fields

Field

Name

Length (bits)

Meaning

GFC

Generic Flow Control

4

Provides local functions

VPI

Virtual path identifier

8

Identifies the next destination of the cell

VCI

Virtual channel identifier

16

Identifies the next destination of the cell

PT

Payload type

3

Indicates user data or control data

CLP

Cell Loss Priority

1

Indicates whether the cell should be discarded in the event of congestion

HEC

Header Error Control

8

Provides a checksum calculated on the header

The GFC field provides local functions for the ATM cell. Local means that it is not end to end, and the intermediate switches override the field. Local functions might mean flow control and identification of multiple stations on a single ATM interface.

The VPI and VCI fields are used together and identify the next destination of the ATM cell. The three bits of the PT field are defined as follows:

■ The first bit indicates whether the cell contains user data or control data.

■ The second bit indicates whether congestion is present.

■ The third bit indicates whether the cell is the last cell of an AAL5 frame (PDU).

ATM can have statically defined PVCs, or private network-network interface (PNNI) can assign the virtual circuits dynamically. PNNI is a hierarchical link-state routing protocol that lays out the virtual circuits throughout the ATM network. For the cells to be interpreted correctly and used by upper layer protocols, ITU-T specified a layer between the ATM layer and the upper layer protocols. This layer is called AAL, and it has five categories. AAL1 is connection-oriented and used for delay-sensitive services and circuit emulation. AAL2 is also connection-oriented, but it is used for variable rate services. AAL3/4 is connectionless and used mainly for the older SMDS. AAL5 can be connection-oriented or connectionless and is used for varying bit rate demands. It is used mostly for IP and LANE.

To carry IP traffic across the ATM cloud, the routers on the edge of the ATM WAN cloud are interconnected across ATM PVCs. To connect the routers in the most efficient way, you need to connect them directly to each other across PVCs. This is needed so that the IP traffic does not cross the ATM cloud twice. Therefore, the routers need to be interconnected in a fully meshed way. This is called the overlay model because all the routers have an Interior Gateway Protocol (IGP) adjacency (a peering) with each other across the ATM cloud. Look at Figure 5-2 to see an overlay network of routers across the ATM cloud.

Figure 5-2 ATM Overlay Network

Figure 5-2 ATM Overlay Network

■ IGP Adjacency

The result is that there is (n-1)/2 number of virtual circuits (VCs) needed for n routers that are connected to the ATM cloud. MPLS solves this problem. When the ATM switches are made aware of routing, they can form an IGP adjacency among themselves and toward the routers. No longer does each router need to form an IGP adjacency to all other routers, but just to the nearest ATM switch(es). Look at Figure 5-3 to see the ATM network where the ATM switches have become label switching routers (LSRs); this means they have become aware of MPLS. This is called the peer model because the routers—which are now edge LSRs—only peer to the nearest ATM switches—which are now LSRs.

Figure 5-3 ATM LSR Peer Network

Figure 5-3 ATM LSR Peer Network

■ IGP Adjacency

For the traffic to be forwarded correctly through the ATM LSRs, the traffic must be MPLS encapsulated, and the MPLS label value must be mapped to VPI/VCI values. That is because the ATM switches are still switching ATM cells on virtual circuits. Because the ATM switches need to be able to map the MPLS label value to a VC, they must first learn those label values. Hence, the ATM switches must run a label distribution protocol. An ATM LSR consists of the following:

■ A routing protocol in the control plane

■ A label distribution protocol in the control plane

■ Switching ATM cells in the data plane

The Cisco ATM switches support Open Shortest Path First (OSPF) as the routing protocol and LDP as the label distribution protocol. The Cisco ATM LSRs distribute the routes in OSPF and the label bindings associated with the routes with LDP. The incoming and outgoing labels are mapped to incoming and outgoing VPI/VCI pairs. The result is that in the data plane, the ATM switch just needs to switch cells from the incoming virtual circuit to the outgoing virtual circuit, just like regular ATM forwarding. The ATM switch never forwards IP packets. If this is needed, the ATM switch would need to reassemble all incoming ATM cells into frames first. Every ATM switch along the path would need to do this. This is undesirable for performance reasons.

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