Chapter 4: Using DSL to Connect to a Central Site

Terminology

• ATU-C - ADSL Transmission Unit - Central office; a subscriber-facing modem in the CO
• ATU-R - ADSL Transmission Unit - remote; a provider-facing modem at a remote location
• DSL Access Multiplexer (DSLAM) - A group of ATU-C units in a single chassis
• Line code - Technique used to encode a digital signal onto the wire
• Microfilter - Low-pass filter to prevent noise from reaching traditional analog equipment on a DSL line
• Network Interface Device (NID) - The CPE device terminating the local loop

Limitations

ADSL can typically reach only 18,000 feet from the CO.

Load coils placed to extend voice signals disrupt data signals.

Bridge taps (unterminated wire split) introduce additional interference.

Crosstalk occurs between pairs within a cable bundle.

DSL Types

Asymmetric Types

• Asymmetric DSL (ADSL) - Offers 1.5 - 8 Mpbs down, 16 Kbps - 1 Mbps up, up to 18,000 feet
• G.Lite ADSL - Allows for 1.5/512 without the use of filters for analog equipment
• Rate-Adaptive DSL (RADSL) - Nonstandard implementation which automatically adjusts connection speed to match line quality
• Very-high-bitrate DSL (VDSL) - Offers 13 - 55 Mbps down, up to 4500 feet; Cisco LRE is based on VDSL

Symmetric Types

• Symmetric DSL (SDSL) - Provides identical downstream and upstream speeds from 128 Kbps to 2.32 Mbps; 768 Kbps is typical; reaches up to 21,000 feet
• Symmetric High-Data-rate DSL (G.SHDSL) - Defined in ITU-T G.991.2; offers 192 Kbps - 2.3 Mbps up to 26,000 feet
• High-data-rate DSL (HDSL) - Provides T1 or E1 data rates; cannot coexist with analog telephone service
• HDSL2 - Uses only one pair (HDSL uses two pairs)
• ISDN DSL (IDSL) - Up to 144 Kbps (2x 64K + 1x 16K); can be augmented up to 45,000 feet with repeaters

ADSL Modulation

Carrierless Amplitude Phase (CAP)

CAP operates with three frequency bands:

• Voice - 0 to 4 kHz
• Upstream - 25 kHz to 160 kHz
• Downstream - 240 kHz to 1.1 MHz

CAP is based on quadrature amplitude modulation (QAM).

CAP is legacy and nonstandard, giving way to DMT.

Discrete Multi-Tone (DMT)

DMT uses orthogonal frequency-division multiplexing (OFDM) and operates on multiple carriers (channels) within each frequency range.

The available frequency range of 0 to 1.1 MHz is divided into 256 channels of 4.312 kHz each.

Channels are dynamically allocated to find the combination of channels with the least interference.

Data Transmission over ADSL

RFC 1483/2684 Bridging

Simple multi-protocol encapsulation over ATM, defined in the RFCs 1483 and 2684.

PPP over Ethernet (PPPoE)

Defined in RFC 2516.

Connection setup occurs in two phases:

• Discovery
• PPP Session

Discovery Phase

The CPE router performs the discovery phase to determine the MAC address of its peer.

1. The client broadcasts a PPPoE Active Discovery Initiation (PADI) packet requesting service.
2. The aggregation router responds with a PPPoE Active Discovery Offer (PADO).
3. The client sends a unicast PPPoE Active Discovery Response (PADR) requesting to move on to the session phase.
4. The aggregation router sends a PPPoE Active Discovery Session-Confirmation with a session ID.

PPPoE frame structure:

• VER (4 bits) - Version; always 0x1
• TYPE (4 bits) - Type; always 0x1
• CODE (8 bits) - Determines stage of the discovery process; always 0x00 during session phase
• SESSION_ID (16 bits) - Carries the PPP session ID (once established); constant for the duration of the session
• LENGTH (16 bits) - Length of the payload

Session Phase

The session phase consists of normal PPP operation; LCP and NCP negotiation.

RFC 2516 defines a Maximum Receivable Unit (MRU) (MTU) of 1492 (6-byte PPPoE header + a 2-byte PPP protocol ID field).

PPP over ATM (PPPoA)

PPPoA uses ATM Adaptation Layer 5 (AAL5) and Logical Link Control/Subnetwork Access Protocol (LLC/SNAP) for encapsulation on virtual circuits.

Ethernet frames are appended with an 8-byte segmentation and reassembly (SAR) trailer and padding so that their length extends to a multiple of 48, then split into 53-bye ATM cells for transmission.

Virtual circuits are specified as a pairing of a Virtual Path Identifier (VPI) (8 bits) and a Virtual Circuit Identifier (VCI) (16 bits).

PPPoA also performs discovery and session phases similar to PPPoE.