STS−n is an interleaving of multiple (n) STS−1s. The size of the payload and the overhead are multiplied by n. Figure 3.4 displays an STS diagram.

Figure 3.4: The STS−1 framing and STS−n framing. The overhead and payload are proportionate to the n value, with the STS−1 frame as the base.

You may wonder why we’re talking about synchronous transmission when we said it is only used over short distances. Where did the asynchronous transmission go? Well, the asynchronous traffic is encapsulated in the STS payload. The asynchronous serial transmission eliminates the need for the synchronization of the end transmitting equipment. In SONET, most WAN links are a point−to−point connection utilizing light as the signaling source. The time required for the signal to travel the link does not distort the synchronization. The OC−n signal itself is used for the synchronization between equipment. This combination of asynchronous and synchronous serial transmission enables signals to reach across long distances with minimal overhead.

Cisco WAN Switches

The current Cisco WAN product line consists of the following switches:

∙MGX 8200 series

∙IGX 8400 series

∙BPX 8600 series wide−area switches

∙MGX 8800 series wide−area edge switches

MGX 8200 Series

The Cisco MGX 8200 series is designed to function as a WAN edge device. It combines multiple narrowband transmissions into a single broadband trunk. It functions as a standalone unit to connect to the ATM network or it can be used as a feeder device to other WAN switches. The series consists of the MGX 8220 Edge Concentrator, the MGX 8240 Private Line Service Gateway, and the MGX 8260 Media Gateway.

The MGX 8220 Edge Concentrator has 16 slots with the capability for full redundancy. It accepts two classes of modules: common control cards and function modules. Six slots are reserved for common control cards, and 10 slots are reserved for function modules. The common control card consists of an AXIS Shelf Controller (ASC) card, a Service Resource Module (SRM) card, and a service trunk card. Each card can use either of two specific slots. When both slots are occupied, one of them acts as a hot standby. The ASC card provides a user interface for the overall control, configuration, and management of the unit. The SRM controls the flow of traffic from the trunk card to various function modules. The service trunk module is the only broadband interface (OC−3 or T3) that transports the aggregated traffic to the ATM network. The function modules are narrowband interface cards. The narrowband transmission can be T1, high−speed Frame Relay,

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ATM frame user−network interface (UNI), or System Network Architecture (SNA).

The MGX 8240 Private Line Service Gateway is designed to terminate private lease lines (T1, T3, or DS0). It has 16 slots with 1 reserved for a redundant control card. It can support up to 1,260 channelized T1s. It is designed for large Internet service providers (ISPs) to aggregate dial−in traffic, which is delivered by the local central office’s Class 4 or Class 5 switch in a T1 or T3 interface. The combined traffic is delivered to the broadband network via OC−3 trunk ports.

The MGX 8260 Media Gateway is a high−density, carrier−class gateway for voice and data traffic. It is designed to move data traffic from voice line to packet network. It can also function as a Voice over IP (VoIP) gateway. The chassis has 14 slots for interface modules and 2 slots for switch control cards. A fully configured system has over 16,000 VoIP ports. The gateway has advanced voice features: echo cancellation, dynamic de−jitter, Voice Activity Detection (VAD), Comfort Noise Generation (CNG), and announcement play−outs (AU or WAV files). It can connect to the broadband network via six broadband service cards (BSCs). Each BSC has six channelized DS3 interfaces.

IGX 8400 Series

The IGX is the successor to the IPX switch. It was the first commercial implementation of Cisco’s fastpacket cell technology. It employs fixed−length cells for switching all types of traffic (voice, data, and Frame Relay). The IGX adds a higher bus capacity, a higher access rate, and ATM. The series has three models:

∙IGX 8410—Has 8 slots with 2 reserved for redundant processor modules.

∙IGX 8420—Has 16 slots with 2 reserved for redundant processor modules.

∙IGX 8430—Has 32 slots with 2 reserved for redundant processor modules.

One of the major differences between the MGX and IGX series is the trunk ports. The IGX can use any of the module interfaces as the trunk connection to an edge device. The speed ranges from 256Kbps to OC−3. The IGX also has advanced switching and routing capabilities: It uses a distributed intelligence algorithm to route new connections and react to failures in transmission media. It provides full control of network resources with multiple classes of service, and it can provide different QoS to individual applications.

Each of the service modules has a large buffer. The ATM module can buffer 128,000 cells, and the Frame Relay can buffer 100,000 frames. The buffer can be allocated by QoS to each virtual circuit based on the amount of traffic and service−level agreements.

The IGX is marketed to the enterprise as its core WAN switch. The ability of the IGX to switch and route between multiple trunks enables it to connect a large number of sites. The capability to handle voice, data, fax, and video traffic in a single network minimizes the overall expense for the enterprise.

IGX is also marketed to carriers in situations where there is not enough traffic to justify purchasing a high−end WAN switch (such as a BPX). It enables the carrier to gradually increase the capacity of the network.

BPX 8600 Series Wide−Area Switches

The BPX 8600 series, first introduced in 1993, is the flagship of the Cisco WAN switch line. It is designed to function as the core of the WAN ATM network. The series has three models: BPX 8620, BPX 8650, and BPX 8680. All three models have the same chassis type with 15 slots; 2 slots are reserved for redundant control and switch modules, 1 slot is reserved for an alarm status monitor module, and 12 slots are reserved for interface modules.

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