Cisco built upon the classic hierarchical network model (access, distribution, core) to provide network designers with the Enterprise Composite Network model. The classic hierarchical model always suffered from scalability and manageability weaknesses. Cisco recommends use of the Enterprise Composite Network model to provide the functional components for network design.
This model is aligned with the current Cisco “best practice” for building converged networks that provide security and other network services to support the transport of voice, video, and data.
Figure 5-3 depicts the Enterprise Composite Network model.
Figure 5-3 Enterprise Composite Network Model
It is interesting to note that this model does not abandon the hierarchical model but rather adopts hierarchy separately within the enterprise campus to meet the specialized needs for user access; for access to application servers; and for access to or from remote users, sites, and external networks.
PVC or SVC
DCE DCE
Enterprise Campus Enterprise Edge Service Provider
Edge
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Initial feedback from network designers regarding the new model has been positive. Network architects can focus on a “divide-and-conquer” approach in which they concentrate on each block of the model and the relationships between them. Designers divide the network into functional areas and focus their efforts more effectively to meet the business needs of their organizations.
Where do WAN technologies fit in the Enterprise Composite Network model? They operate between the Enterprise Edge and the SP Edge using the various technologies in this chapter.
Wide-Area Networking Technologies
Today, there are many technologies available for designing WAN infrastructures. Constant research and development in the area of WAN technologies allows designers to implement increasingly cost-effective WAN designs that take advantage of new mechanisms such as metropolitan Ethernet and virtual private networks (VPNs) using cable and DSL for access. You should acquire a solid com-prehension of the technology options available before designing WAN solutions.
Dial-Up
The first of the WAN technologies presented here is one that many readers have used at some point, dial-up. Dial-up refers to the use of a modem connected to the PSTN to carry data. Although the available bandwidth is lowest with this technology (transmissions of just 53 Kbps are possible with current V.90 standards), it is still a popular remote-access choice for home office and telecommuter environments—because of low costs and almost universal availability. Users can easily achieve dial-up wide-area networking through the acquisition of inexpensive customer premises equipment (CPE) such as the digital modem.
Dial-up WAN services have managed a presence in more than just the home-office environment, in part because of dial-on-demand routing (DDR) capabilities built in to most Cisco devices. DDR enables networking equipment to dynamically dial a connection to a remote WAN destination upon the receipt of “interesting” traffic. Of course, it is up to the Cisco administrator to determine and define these interesting traffic patterns.
Chapter 7, “Backup Options and Sample WAN Designs,” details yet another powerful reason why dial-up solutions continue to appear in WAN designs today—WAN backup options.
ISDN
Regional telephone carriers make ISDN a WAN design possibility for many internetwork WAN designs. ISDN permits the digitization of access to the telephone network so that existing phone lines are capable of carrying data in a digital form rather than as analog signals.
It is important that network designers understand the devices and reference points in an ISDN WAN.
We refer to ISDN terminals that are “ISDN ready” as terminal equipment type 1 (TE1s), whereas non-ISDN terminals are TE2s. To connect a TE2 device to the ISDN network, use a terminal adapter (TA). TE1 and TE2 devices connect to network termination type 1 (NT1) or network termination type 2 (NT2) devices. These devices connect the four-wire subscriber wiring to the two-wire local-loop provider network. In North America, the NT1 is a customer-provided CPE device, whereas in most other parts of the world, this device is part of the carrier’s network. NT2s are more complicated devices usually found as components in PBXs. They typically provide Layer 2 and 3 protocol functions and concentration services.
The key reference points within an ISDN network are defined in the ISDN standards produced by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T).
The R reference point identifies the area between non-ISDN equipment and the TA. The S reference point identifies the area between user terminals and the NT2. The T reference point identifies the area between the NT1 and NT2 devices. Finally, the U reference point identifies the area between the NT1 devices and line-termination equipment in the carrier network. Obviously, the U reference point is relevant only to ISDN subscribers in North America. Figure 5-4 depicts TE1 and TE2 devices and their respective connections to the ISDN switch with the respective reference points.
Figure 5-4 ISDN Devices and Reference Points
The two types of ISDN services are Basic Rate Interface (BRI) and Primary Rate Interface (PRI).
BRI
ISDN BRI offers two bearer (B) channels and a single delta (D) channel. BRI B channels transmit data and operate at 64 Kbps. The BRI D channel handles signaling and operates at 16 Kbps. BRI also provides for framing control and other overhead, thus bringing the total bit rate to 192 Kbps.
NT2 NT1
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PRI
In North America and Japan, PRI ISDN over T1 media provides 23 B channels and 1 D channel.
Unlike BRI, the D channel in PRI operates at 64 Kbps as well, bringing the total bit rate to 1.544 Mbps. In Europe, Australia, and other parts of the world, PRI is provisioned over E1 media and features 30 B channels and one 64 Kbps D channel for a total bit rate of 2.048 Mbps.
X.25
X.25 is an ITU-T WAN protocol. Frame Relay has helped make X.25 mainly a legacy WAN tech-nology, especially in North America. X.25 was designed to operate effectively even over links with relatively high bit-error rates. The packet-switched networks of common carriers have used it. X.25 features high overhead due to built-in error detection and recovery mechanisms. Frame Relay, on the other hand, takes advantage of today’s more reliable physical connections and leaves such error recovery to upper layers of the OSI model. The bandwidth saved is available for other uses.
X.25 WAN devices include data terminal equipment (DTE), data circuit-terminating equipment (DCE), and packet-switching exchange (PSE). DTE devices are end systems that communicate across the X.25 network. They might include your actual workstations or other network hosts. DCE devices are communications devices, such as modems and packet switches, that provide the inter-face between DTE devices and a PSE. DCE devices are generally located in the carrier’s facilities.
PSEs are switches that compose the bulk of the carrier’s network. They transfer data from one DTE device to another through the X.25 PSN.
The packet assembler/disassembler (PAD) is a common device in X.25 networks. X.25 relies upon PADs when an end device cannot implement the full X.25 functionality. The PAD is located between the end device and a DCE device. The PAD performs three primary functions: buffering, packet assembly, and packet disassembly.
X.25 WAN technology maps to the lowest three layers of the OSI reference model. X.25 implemen-tations typically use the following protocols: Packet-Layer Protocol (PLP); Link Access Procedure, Balanced (LAPB); and a variety of physical-layer protocols (such as EIA/TIA-232, EIA/TIA-449, EIA/TIA-530, and G.703).
X.25 supports both SVCs and PVCs. The basic operation of an X.25 virtual circuit begins when the source DTE device specifies the virtual circuit to be used (in the packet headers) and then sends the packets to a locally connected DCE device. The local DCE device examines the packet headers to determine which virtual circuit to use and then sends the packets to the closest switch in the path of that virtual circuit. The switches pass the traffic to the next intermediate node in the path, which can be another switch or the remote DCE device. The remote DCE device examines the packet headers and determines the destination address. This DCE device then sends the packets to the destination DTE device. If communication occurs over an SVC and neither device has additional data to transfer, the virtual circuit terminates.
Although PVCs are more common with Frame Relay, SVCs are more common with X.25 imple-mentations. Also, X.25 requires special addressing. Specifically, X.25 uses X.121 addresses in call setup mode to establish SVCs. The X.121 address field includes the International Data Number (IDN), which consists of two fields: the Data Network Identification Code (DNIC) and the National Terminal Number (NTN). The DNIC is an optional field that identifies the exact PSN in which the destination DTE device is located. X.25 sometimes omits the DNIC in calls within the same PSN.
The DNIC has two subfields: country and PSN. The country subfield specifies the country where the destination PSN is located. The PSN field specifies the exact PSN in which the destination DTE device is located. Finally, the NTN identifies the exact DTE device in the PSN for which a packet is destined. This field varies in length.