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This chapter describes Frame Relay Switched Virtual Circuits as they are implemented by the Cisco-StrataCom BPX Service Node. This chapter includes:
Frame Relay switched virtual circuits (SVCs) allow access through the Cisco StrataCom Service Node network for Frame Relay connections only when the need arises. Although SVCs entail overhead for setting up and tearing down links, the virtual connection is only established when data must be transferred. So the number of virtual connections is proportional to the number of actual data transfers between sites rather than to the number of sites.
Frame Relay SVCs offer cost savings via usage-base pricing instead of fixed pricing for a PVC connection, dynamic network bandwidth allocation or bandwidth-on-demand for large data transfers such as FTP traffic, backup for PVC backbones, and conservation of resources in private networks.
Frame Relay SVCs share many common software functions with ATM SVCs described in Chapter 4.
This signaling channel between the Frame Relay CPE, across the Frame Relay UNI, and the BPX Service Node is again a PVC. This PVC is used to provide signaling for the set up and tear down of Frame Relay SVCs across the network.
Signaling messages from the Frame Relay CPE must be routed to the ESP through the BPX. These messages contain information elements, defined by the Frame Relay Forum (FRF.4), with the link layer following CCITT Recommendation Q.922. These signaling messages dynamically establish, maintain, and clear Frame Relay connections at the Frame Relay UNI. For Frame Relay SVCs, a signaling channel is created between the ASC in the AXIS and the ESP. There will only be one signaling channel between the ASC and the ESP. Signaling messages (with DLCI = 0) from all the FRSMs in the AXIS will be routed to the ASC, where they multiplexed together and passed to the ESP. This signaling channel will terminate on the ESP ATM NIC card as shown in Figure 3-1. The dotted lines in Figure 3-1 are used to point out that multiple signaling channels are multiplexed at the ASC.
As shown in Figure 3-2, this signaling channel PVC will use DLCI 0 at the Frame Relay UNI, the remote end. The local end of this PVC is configured by the ESP when the Frame Relay UNI port is configured with the ESP Configuration Interface. The local end of the signaling PVC is specified as an ATM VPI/VCI and the Frame Relay parameters have to be converted to ATM cells. This Frame Relay to ATM Network Interworking is described in the following section. Note that the local and remote ends of the signaling PVC are defined from the ESP viewpoint.
Because the BPX Service Node is an ATM switch, Frame Relay messages and data from the Frame Relay CPE (end user) have to be converted to ATM cells. In other words, Q.933 messages have to be converted to ATM Forum UNI 3.0/3.1 messages, and vice versa. These ATM cells are transported across the Cisco StrataCom wide area network and converted back into Frame Relay format at the remote BPX Service Node (that is, AXIS FRSM) before being passed to the remote Frame Relay CPE. This Frame Relay-to-ATM-to-Frame Relay conversion is referred to as Frame Relay to ATM network interworking.
The Frame Relay FRF.4 messages are mapped to ATM messages and Frame Relay information elements are mapped to ATM information elements or transported across ATM Trunks.
The Committed Information Rate (CIR) will be specified as a minimum and a maximum in the Setup message from Frame Relay CPE. Since the BPX Service Node tries to set up all connection segments simultaneously between nodes, it has no way to guarantee that all BPX Service Nodes will provide the requested CIR. The BPX Service Node will guarantee, however, that the CIR for the routed Frame Relay SVC will be within the requested minimum and maximum range. The CIR could vary between hops along the route.
The BPX Service Node supports Frame Relay UNI addresses in E.164 format, a telephone number-like format. Native E.164 addresses are defined by CCITT E.164, and provide the numbering plan for Integrated Services Digital Network (ISDN). The BPX Service Node provides a default E.164 address based on the default ESP MAC address as shown in Figure 3-2. This is a 15-digit address, which will use 8-bytes when it is encapsulated in a private ATM address, as described in Chapter 2 in the section ATM Addresses.
All routing by the PNNI Route Agent is done using NSAP-formatted numbers. Therefore before a source route request can be made to the PNNI Route Agent at the originating node an incoming Called Number in native E.164 format must first be converted to NSAP E.164 format. This is done by padding native E.164 address digits with leading semi-octets in the IDI field of the NSAP E.164 number as described in ATM Forum UNI 3.0/3.1 specification. The DSP part of the NSAP E.164 address is set to zero. Figure 3-3 illustrates the mapping of a native E.164 address into an NSAP E.164 address format.
The BPX Service Node performs the same address processing and screening for Frame Relay SVCs as it does for ATM SVCs. For a description of address processing and screening, refer to the section in Chapter 2 on Address Processing.
Figure 3-4 illustrates a simple Frame Relay SVC point-to-point switched channel connection. The dotted line indicates the signaling connection, and the solid line indicates the actual data transfer that occurs after the call is set up. Routing across the Cisco StrataCom network will be discussed in Chapter 4, Networking.
A typical Frame Relay SVC l call setup would follow this sequence:
Step 3 The ESP processor processes this message by:
Step 4 When the next hop BPX Service Node receives the message, if the destination is not local, it builds the ATM connection on the local BPX and passes the same message to the next adjacent BPX Service Node. If this BPX Service Node is the destination node, the ESP converts the received ATM parameters and messages back to Q.933 format and passes them along to Frame Relay CPE 2. Upon receiving Q.933 connect messages from CPE-2, the BPX Service Node builds the Frame Relay segment of the call to Frame Relay CPE 2; it also instructs the BPX to build the ATM segment of the call with the received ATM parameters.
Step 5 A Call Detail Record is opened at the originating BPX Service Node.
And a typical call disconnect would follow this sequence:
Step 2 The Frame Relay SVC processor instructs the BPX Service Node to delete the local Frame Relay segment of the call on the AXIS and the local ATM segment of the call on the BPX.
Step 3 The Q.933 message is converted to ATM UNI 3.0/3.1 message and passed to the adjacent BPX Service Node.
Step 4 The adjacent (or far end) BPX Service Node receives the message. If the destination is not in the local node, the BPX Service Node deletes the ATM connection from the BPX and passes the message to the next adjacent BPX Service Node. If this BPX Service Node is the destination node, the ESP receives the Release message and deletes the Frame Relay and ATM segments of the call on the local AXIS and BPX. The BPX Service Node converts the ATM UNI 3.0/3.1 message to Q.933 format and sends it to Frame Relay CPE 2.
Step 5 Frame Relay CPE 2 responds with the Release Complete message.
Step 6 The Release Complete message is routed back to Frame Relay CPE 1 through the originating end BPX Service Node. The Call Clearing procedure is complete.
Step 7 The BPX Service Node closes the Call Detail Record with call duration.
The BPX Service Node at the originating or terminating ends of the call (depending on the Billing configuration) can generate a Call Detail Record (CDR). The CDR includes the call parameters, timing information, and Cell counts. Call Detail Records are written to files on the ESP.
Each BPX Service Node also keeps other statistics for maintenance and diagnostic purposes.
The BPX Service Node supports the Frame Relay standards listed in Appendix D, Specifications and Compliance.
Posted: Fri Jan 19 20:19:49 PST 2001
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