Previous Table of Contents Next


CPE (Customer Premise Equipment)

CPE comprises Data Terminal Equipment (DTE), Data Communications Equipment (DCE), and true CPE.

DTE
includes Mainframe, Mid-range and PC-server host computers connected through the UNI.
DCE
comprises ATM-equipped bridges, routers, brouters and gateways connected through the DXI UNI. ATM premise switches are offered by a number of vendors. Such switches are used for LAN interconnection, employing a highly redundant cell-switching fabric capable of switching speeds which can reach as high as OC-3 (155 Mbps).
CPE
includes ATM-based PBXs and Video Servers. While these devices currently do not exist, they will be added to this definition. First, ATM proves itself as being truly voice- and video-capable, and that capability must be standardized. Such devices will be connected via the UNI or the DXI.

BSSs (Broadband Switching Systems)

BSSs are carrier exchange switches capable of broadband switching and transport. They employ highly redundant cell-switching fabrics that currently can operate at less than 80 to 100 Gbps. ATM switches are also highly intelligent, providing buffering, routing and flow control, as well as segmentation and reassembly. The switches are highly redundant and fault-tolerant. They fall into two categories, core switches and edge switches.

Edge Switches
also are known as Access Nodes or Service Nodes. They are distributed in close proximity to ATM users and are connected to the core switches via fiber facilities, much as is a CO in the voice world. In fact, they often are collocated with a CO switch.
Core Switches
also are known as Backbone Switches. They generally are housed in a wire center, along with a traditional CO circuit switch or Tandem.

Transmission Facilities

Transmission facilities at the Network-to-Network (NNI) level and between network nodes are SDH/SONET fiber optic in nature; other media can be supported, but are not ideal. Local loop facilities (UNI and DXI) can be any medium capable of supporting transmission speeds of > DS-1 (1.544/2.048 Mbps), although fiber is always preferable. For very short distances in a premise-based ATM environment, Category 5 UTP will support ATM speeds up to 155 Mbps; Cat 3 UTP has been proven at 25.6 Mbps for distances < 90 meters.

ATM Protocols and Cell Structure

ATM is based on a 53-octet cell structure, comprising 48 octets of payload and 5 octets of overhead (Figure 11.8). Unlike the SMDS cell, all 5 octets of overhead precede the payload. The choice of 48 octets was a compromise between the U.S. ECSA (Exchange Carriers Standards Association) T1S1 committee and ETSI (European Telecommunications Standards Institute). The ECSA promoted a 64-octet cell, while ETSI was in favor of 32 octets, each reflecting the bandwidth required for the parochial PCM voice encoding technique. The choice of 48 octets was a perfect mathematical compromise [11-39]. It is worth noting that the cell size and composition is very overhead-intensive, at about 10%. Standard PCM-encoded voice, at 8 bits per sample can deal effectively with a small cell payload. Data, on the other hand, generally are presented in much larger packets, blocks, and frames. Hence, the data world would have preferred a much larger cell, while many in the voice world actually would have preferred a smaller cell.


Figure 11.8  ATM cell structure. Source: TA-NWT-00113, Copyright © 1993, Bellcore. Reprinted with Permission.

While this level of overhead might seem wasteful of bandwidth, the advantages far outweigh the drawbacks. The small size offers the ability to handle any type of data. Additionally, the fixed cell size offers the advantage of predictability, unlike the variable-length frames of Frame Relay. Additionally, bandwidth is cheap over SONET pipes.

In any event, the 48-octet payload was set as a standard for the ATM cell. The cell is divided into header and information segments (Figure 11.9), with the 5-octet header structured as follows:


Figure 11.9  ATM protocol reference mode. Reprinted with Permission by Network VAR.

  Generic Flow Control (GFC): 4 bits that provide local flow control, but which field has no significance on an end-to-end basis. Intermediate ATM switches overwrite this field with additional VPI information [11-9]. In other words and for example, GFC is significant in order to control data flow across a UNI, but is unnecessary at a NNI (Network-to-Network Interface) level.
  Virtual Path Identifier (VPI): 8 bits identifying the virtual path.
  Virtual Channel Identifier (VCI): 16 bits identifying the virtual channel.
  Payload Type Indicator (PTI): 3 bits, distinguishing between cells carrying user information and those carrying service information.
  Cell Loss Priority (CLP): 1 bit, identifying the priority level of the cell in order to determine the eligibility of that cell for discard in the event of network congestion.
  Header Error Control (HEC): 8 bits, providing error checking of the header.

The ATM reference model is multidimensional, with three planes and four layers, as illustrated in Figure 11.9. The bottom two layers of this reference model loosely compare to the Physical Layer of the OSI Reference Model. As in the OSI model, each layer of the ATM model performs its designated functions independently, all layers are tightly linked and the functions are highly coordinated. The layers of the ATM Reference Model are the Physical Layer, the ATM Layer, ATM Adaptation Layer, and higher layers and functions. The planes include the Control Plane, User Plane, and Management Plane.


Previous Table of Contents Next