Clear-channel/channelized/fractional operation
Multiple ports
Physical interfaces: V.35, V.36, X.21, RS-232
With or without integrated CSU/DSU
RAS option: with or without integrated digital modems (DSPs)
PRI signaling
Internal/external clocking
Almost all synchronous serial NICs support the following Layer 2 encapsulation formats:
LAPB (X.25 Layer2)
Frame Relay
Cisco HDLC
Synchronous PPP
Frame Relay services are deployed by carriers up to T3 bandwidth in the United States and up to E1 bandwidth in Europe and most other countries in 56/64-kbps or sometimes even smaller increments (subrates/derived channels). Configuration of X.25 or Frame Relay is similar to Cisco configurations with regard to virtual/subinterface concepts and topology (point to point, point to multipoint, and so on).
ATM Interfaces
Some vendors sell PCI ATM interface cards for ATM25 DSL interfaces, 155-Mbps STM-1/OC3, as well as "exotic" 622-Mbps STM-4/OC12 NICs, featuring both optical and electrical RJ-45 interfaces.
OpenBSD, Linux, and FreeBSD provide an ATM stack, but only a limited family of adapters is supported. This family unfortunately includes almost no state-of-the-art models. The best support available for ATM adapters is provided for Marconi ForeRunner and Efficient Networks chipsets. Consult the hardware compatibility list of the respective operating systems for further details.
As far as I have researched the matter, it would be interesting to deploy ATM25 adapter cards for UNIX gateway devices. Unfortunately, few vendors supply PCI models; almost all development effort appears to go into embedded systems for deployment in integrated access devices (IADs). ATM25 supports approximately 10.5-Mbps high-speed, 8-Mbps full-rate, and 4-Mbps or G.Lite downstream speeds and can accommodate ADSL, SDSL, VDSL, and G.SHDSL.
Because I do not own ATM-PCI adapters, no lab is provided in this section. The following sections discuss the Linux and FreeBSD ATM stack and configuration tools in detail. If you own two ATM interfaces cards, you can use an optical crossover cable pair for a nice lab or connect them to an ATM switch for ILMI testing. For RJ-45 crossovers, consult the pin assignments of the vendor's adapter manual.
Linux ATM Support
Unfortunately, there appears to be no further development going on with regard to the Linux ATM Project (http://linux-atm.sourceforge.net), which, of course, does not mean that it is not stable or useful. The drivers are included in up-to-date kernels. In addition, you still need to download the ATM support tools from http://linux-atm.sourceforge.net. Linux ATM implements several ATM-related daemons: atmsigd, ilmid, and atmarpd, as well as several ancillary tools.
Example 4-1 presents configuration of ATM PVC/SVC pairs under Linux. Remember, ATM PVCs are point-to-point abstractions.
To configure the atm0 interface as 10.1.1.1/30 and build a PVC on PHY 0, VPI 0, VCI 51 (emphasized by the shaded text) to the far-end 10.1.1.2/30, type the commands in the order presented in Example 4-1.
Example 4-1. Simple Linux ATM Interface and PVC Configuration
[root@callisto~#] atmarp -c atm0
[root@callisto~#] ifconfig atm0 10.1.1.1 netmask 255.255.255.252 mtu 4470
[root@callisto~#] atmarp -s 10.1.1.2 0.0.51
For an in-depth discussion, consult the Linux ATM-on-Linux HOWTO.
The FreeBSD HARP ATM Subsystem
FreeBSD provides mature ATM support via the Host ATM Research Platform (HARP) software. For configuration details, consult the atm(8) man page and the links in the "Recommended Reading" section
Cable Access (Ethernet Interfaces)
Cable access can be deployed easily. The vast majority of providers deliver a CPE device (cable modem) that terminates the coax network frequency bands that carry data, TV, and telephony, and provide a standard Ethernet/POTS/ISDN interface as the demarcation point.
To get telephony out of the RF side, an additional termination unit is needed. In contrast to DSL architectures, no additional software or stack components (PPTP, PPPoA, PPPoE) are required on the attached end system or gateway. The cable modem connects via coaxial drop and trunk cables as well as signal repeaters to a carrier's cable head-end. Mixed architectures featuring optical-electrical converters for optical trunk cables are used, too. In contrast to DSL, this is a shared medium; therefore, VLAN architectures and MAC-based access control are commonly deployed and addresses delivered to the customer via Dynamic Host Configuration Protocol (DHCP)
DSL Access
Historically, DSL has been an asymmetric service (ADSL), evolving into a symmetric one (G.SHDSL) designed to replace E1 TDM circuits and provide voice, ATM, raw IP, and ISDN transport.
DSL copper cables are terminated at a central office (CO) DSLAM port (digital subscriber access line multiplexer). The DSLAM serves two purposes:
One is to physically terminate the subscriber line and separate the voice band from the data bands utilizing an integrated splitter device similar to the one on the customer end; the voice signal is delivered directly to the PSTN network on OSI Layer 1.
The second purpose is to relay the data traffic to an IP backbone, usually based on ATM or Ethernet. Aggregation and service-selection gateways constitute the distribution layer of modern DSL provider architectures.
Almost all open-source UNIX operating systems provide mature PPTP support required for the PPPoA architectures that are popular in some European countries. Linux, OpenBSD, and FreeBSD support native PPPoE. PPPoA or PPPoE support of your favorite operating system usually requires a modified/patched version of the PPP toolset. Discussion goes beyond the scope of this book, but you can find easily several cookbooks for setup via your favorite search engine or Linux repository. Several DSL NICs are also available (ATM25, splitterless operation). Some of their important characteristics are as follows:
DSL modes of operation: PPPoA, PPPoE, bridging mode
DSL flavors: ADSL, HDSL, SDSL, G.SHDSL, G.Lite, VDSL, and so on
Software requirements of DSL access: PPPoE or PPPoA stack support, PPTP (for example, via Netgraph/mpd daemon under FreeBSD)
Lab 4-1: Synchronous Serial Connection Setup
This lab (as shown in Figure 4-1) facilitates two Sangoma synchronous serial S514/ET1 PCI adapter cards, connected via an RJ-45 crossover cable for point-to-point configuration between a Linux (callisto) and FreeBSD (castor) gateway. This lab deals with Layer 1 and Layer 2 issues; later labs in following chapters add scenarios on top of the data link layer. For the pin layout of the RJ-45 crossover cable as well as the installation of the NIC drivers, consult the Sangoma website.
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