Frame Relay Encapsulation and Topology


The Frame Relay Encapsulation Process
Frame Relay takes data packets from a network layer protocol, such as IP or IPX, encapsulates them as the data portion of a Frame Relay frame, and then passes the frame to the physical layer for delivery on the wire. To understand how this works, it is helpful to understand how it relates to the lower levels of the OSI model.
First, Frame Relay accepts a packet from a network layer protocol such as IP. It then wraps it with an address field that contains the DLCI and a checksum. Flag fields are added to indicate the beginning and end of the frame. The flag fields mark the start and end of the frame and are always the same. The flags are represented either as the hexadecimal number 7E or as the binary number 01111110. After the packet is encapsulated, Frame Relay passes the frame to the physical layer for transport.
The CPE router encapsulates each Layer 3 packet inside a Frame Relay header and trailer before sending it across the VC. The header and trailer are defined by the Link Access Procedure for Frame Relay (LAPF) Bearer Services specification, ITU Q.922-A. Specifically, the Frame Relay header (address field) contains the following:
DLCI - The 10-bit DLCI is the essence of the Frame Relay header. This value represents the virtual connection between the DTE device and the switch. Each virtual connection that is multiplexed onto the physical channel is represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection.
Extended Address (EA) - If the value of the EA field is 1, the current byte is determined to be the last DLCI octet. Although current Frame Relay implementations all use a two-octet DLCI, this capability does allow longer DLCIs in the future. The eighth bit of each byte of the Address field indicates the EA.
C/R - The bit that follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined.
Congestion Control - Contains 3 bits that control the Frame Relay congestion-notification mechanisms. The FECN, BECN, and DE bits are the last three bits in the Address field. Congestion control is discussed in a later topic.
The physical layer is typically EIA/TIA-232, 449 or 530, V.35, or X.21. The Frame Relay frame is a subset of the HDLC frame type. Therefore, it is delimited with flag fields. The 1-byte flag uses the bit pattern 01111110. The FCS determines whether any errors in the Layer 2 address field occurred during transmission. The FCS is calculated prior to transmission by the sending node, and the result is inserted in the FCS field. At the distant end, a second FCS value is calculated and compared to the FCS in the frame. If the results are the same, the frame is processed. If there is a difference, the frame is discarded. Frame Relay does not notify the source when a frame is discarded. Error control is left to the upper layers of the OSI model.
The Frame Relay Topologies
When more than two sites are to be connected, you must consider the topology of the connections between them. A topology is the map or visual layout of the Frame Relay network. You need to consider the topology from several perspectives to understand the network and the equipment used to build the network. Complete topologies for design, implementation, operation, and maintenance include overview maps, logical connection maps, functional maps, and address maps showing the detailed equipment and channel links.
Cost-effective Frame Relay networks link dozens and even hundreds of sites. Considering that a corporate network might span any number of service providers and include networks from acquired businesses differing in basic design, documenting topologies can be a very complicated process. However, every network or network segment can be viewed as being one of three topology types: star, full mesh, or partial mesh.
Star Topology (Hub and Spoke)
The simplest WAN topology is a star, as shown in the figure. In this topology, Span Engineering has a central site in Chicago that acts as a hub and hosts the primary services. Notice that Span has grown and recently opened an office in San Jose. Using Frame Relay made this expansion relatively easy.
Connections to each of the five remote sites act as spokes. In a star topology, the location of the hub is usually chosen by the lowest leased-line cost. When implementing a star topology with Frame Relay, each remote site has an access link to the Frame Relay cloud with a single VC.
This shows the star topology in the context of a Frame Relay cloud. The hub at Chicago has an access link with multiple VCs, one for each remote site. The lines going out from the cloud represent the connections from the Frame Relay service provider and terminate at the customer premises. These are typically lines ranging in speed from 56,000 bps to E-1 (2.048 Mb/s) and faster. One or more DLCI numbers are assigned to each line endpoint. Because Frame Relay costs are not distance related, the hub does not need to be in the geographical center of the network.
Full Mesh Topology
This figure represents a full mesh topology using dedicated lines. A full mesh topology suits a situation in which the services to be accessed are geographically dispersed and highly reliable access to them is required. A full mesh topology connects every site to every other site. Using leased-line interconnections, additional serial interfaces and lines add costs. In this example, 10 dedicated lines are required to interconnect each site in a full mesh topology.
Using Frame Relay, a network designer can build multiple connections simply by configuring additional VCs on each existing link. This software upgrade grows the star topology to a full mesh topology without the expense of additional hardware or dedicated lines. Since VCs use statistical multiplexing, multiple VCs on an access link generally make better use of Frame Relay than single VCs. The figure shows how Span has used four VCs on each link to scale its network without adding new hardware. Service providers will charge for the additional bandwidth, but this solution is usually more cost effective than using dedicated lines.
Partial Mesh Topology
For large networks, a full mesh topology is seldom affordable because the number of links required increases dramatically. The issue is not with the cost of the hardware, but because there is a theoretical limit of less than 1,000 VCs per link. In practice, the limit is less than that.
For this reason, larger networks are generally configured in a partial mesh topology. With partial mesh, there are more interconnections than required for a star arrangement, but not as many as for a full mesh. The actual pattern is dependant on the data flow requirements.

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