Frame Relay is a technology data transmission and voice, high-speed packet switching. Notable for its low cost compared to leased lines.
is classified as a non-broadcast multi-access network (NBMA), ie does not send broadcast packets. Comes from X.25 and inherits many of its features.
Frame Relay provides a switched network to different clients at the same time, but is based on that they never need to transmit data constantly. FR works by providing a portion of bandwidth to each user, allowing even exceed their guaranteed bandwidth if resources are available. FR
When hiring, we hired a bandwidth or average since and can transmit data. This is known as Committed Information Rate (CIR). is possible to overcome the CIR, but then the data is sent as a "best effort" and discard frames in case of congestion.
encapsulation types
There are two types: Cisco and IETP (Internet Engineering Task Force). When connecting equipment other than Cisco will have to opt for the latter.
Virtual Circuits
Frame Relay operates using virtual circuits that connect two DTE devices, making it appear that are connected through a circuit, when in fact do so by a large shared infrastructure.
There are two types of virtual circuits:
- Permanent Virtual Circuit (PVC) : they are the most common. Are permanent because they create mappings between transmission equipment for repeated use. Eliminate the need for establishment and release of the connection. Switched
- Virtual Circuit (SVC) . are generated dynamically for each connection, and removed when done.
PVCs are identified by a DTE Data Link Connection Identifiers or DLCIs. These interfaces are used in to distinguish between different virtual circuits. Inverse ARP allows us to map DLCIs to IP addresses, ie translates addresses from level 2 to level 3, the reverse ARP.
The DLCIs are considered locally significant. When a router1 want to send a frame to Router2, uses IARP or manual mapping to translate the DLCI to an IP address. Then sends the frame with DLCI in the header of the RF. The RF switch supplier receives it, and use the port corresponding to the DLCI. It is significant that locally, between him and the next switch.
Local Management Interface LMI
is a standard used between the router and the first RF switch. Can transmit information, the virtual circuit state. Communicates information about:
- keepalive: checked that need to stream data.
- Multicasting: extension that allows you to distribute routing information protocol and ARP over Frame Relay network (use reserved for multicast DLCI). Addressing global
- : provides global significance to DLCIs (unique address on the WAN). This causes it to behave like a LAN.
- virtual circuit state: allows packets to be sent through a circuit nonexistent.
There are three types of messages LMI: Cisco, ANSI and Q.933A.
congestion control
The switch notifies the DTE FR congestion problems. There are three bits with a different meaning:
- Discard Eligibility (DE): when transmitting packets beyond the CIR of a PVC, any package can be discarded if congestion occurs. The excess bits are marked with the assets in the head and the FR switch will discard if the network is congested.
- Forward Explicit Congestion Notification (FECN): FR when the switch detects congestion in the network, activates the FECN bit in the header of FR and the destination DTE knows that the road was congested.
- Backward Explicit Congestion Notification (BECN): when the switch detects congestion in the network enables the BECN in a frame for the home router. So notice that there is congestion.
Subinterfaces
is possible to have multiple virtual circuits for a single interface and treat each one as different interfaces, using the concept of subinterface. Logical interfaces are treated as defined by the IOS.
Several subinterfaces will share a hardware interface, and configuration level will operate as if they were physically different (multiplexing). There are two types:
- Point-to-point: virtual circuit connects one router another. Each pair of routers is point to point on its own subnet and each point to point subinterface has a single DLCI. Multipoint
- : using a single multipoint subinterface to establish multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. All interfaces involved are on the same subnet.
0 comments:
Post a Comment