OECG – Chapter 17


This chapter covers some of the admittedly tedious basics of T1/E1 technology, plus PPP. It’s a short chapter, about 12 pages of text. I’ll do 2 blog posts, this one on T1/E1, and the next on PPP. This is almost all concepts, little IOS application. So it may be only 12 pages, but it’s 12 packed pages. Off we go then…

Some groundwork. Let’s remember that synchronous digital signals are intended to ride over copper twisted pair media at 64Kbps rate, where a single 64K channel is called a Digital Signal Level 0, or DS0. Why 64K? To support the pulse code modulation voice codec developed by AT&T that used 8000 voice samples per second, with 1 byte per sample. So to make a bigger pipe, you can use the 64Kbps lines as building blocks and use a time-division multiplexer to combine multiple DS0s. The TDM process is governed by different standards in different areas, T1 in North America, E1 in Europe and J1 in Japan.

T1 Framing and Encoding

  • You need to frame a signal so that the CSU/DSUs on either end of the link. This allows the CSU/DSUs to distinguish the DS0s and know what bits are for overhead processes such as synchronization, framing, management and CRC check. T1s use Superframe (SF or D4) or Extended Superframe (ESF) frame format. E1 uses ITU G.704 or G.706.
  • The devices on the T1 link have to use the same framing and also line-coding standards to be compatible with one another. Line coding is getting down to layer 1 – the electrical signal properties that represent a 0 or 1. In the case of both B8ZS and AMI, a 3 millivolt signal is used to represent a 1, and 0 voltage for a 0.
    • Bipolar 8 Zero Substitution (B8ZS) – represents 8 consecutive 0s using Bipolar Violations (BPVs). A BPV normally occurs when the 3mV signal is of the same polarity as the last 3mV signal. To help maintain clock synchronization (which happens when the signal transitions), B8ZS will a BPV, a normal signal, another BPV and then another normal signal. The B8ZS knows this custom sequence of pulses represents 8 consecutive 0s.
    • Alternate Mark Inversion (AMI) – does not have the BPV process to represent 8 consecutive 0s.
  • Comparing T1 and E1 standards
    • DS1 line rate
      • T1=1.544Mbps
      • E1=2.048Mbps
    • DS0 channels
      • T1=24
      • E1=30
    • Overhead
      • T1=8Kbps
      • E1=128Kbps – 1 DS0 is overhead; 1 DS0 is reserved for signaling
    • Framing options
      • T1=Superframe or Extended Superframe
      • E1=ITU G.704 or G.706
    • Encoding options
      • T1=B8ZS or AMI
      • E1=High Density Binary 3 (HDB3)
    • DS3 thoughts
      • T1=T3, 44.736Mbps, 28 T1s muxed
      • E1=E3, 34.368Mbps, 16 E1s muxed

T1 Alarms

  • Operation, administration and maintenance (OAM) is needed in T1 signaling. ESF framing reserves 4Kbps of the 8Kbps available for overhead for OAM.
  • OAM can transmit data about a circuit such as error conditions. Some common error conditions follow. (If you’ve ever installed a mux or T1 card with integrated CSU into a Cisco router, you’ve seen these.)
    • Out of Frame (OOF) or Loss of Fram (LOF) – the frame boundaries can no longer be clearly identified.
    • Loss of Signal (LOS) – no pulses of either polarity for a specified amount of time.
    • Alarm Indication Signal (AIS) – sending all binary 1s down the line as a response to framing problems. This allows for signal transitions and should aid in recovering framing and synchronization.
    • Red Alarm – a device experiencing a LOF/LOS/AIS conditions is considered to be in a Red Alarm state. The device in Red Alarm will send a Yellow Alarm signal to the other end.
    • Yellow Alarm – a device has received a Yellow Alarm notice from the other end of the link, indicating that the other end is in a Red Alarm state.

Carrier Detect and Interface Resets

  • Realize that a router doesn’t know about the alarms above, unless the wan interface card has an integrated CSU/DSU. If all you’ve got is an external CSU/DSU with a serial card in the router, the router can still give you some info, however.
  • A couple of definitions:
    • DCE – data communications equipment. In the context of a router with a serial interface and an external CSU, this is the CSU.
    • DTE – data terminal equipment. In the context of a router with a serial and an external CSU, this is the router’s serial interface.
  • 5 important signaling pins in a serial cable:
    • Data Carrier Detect (DCD) – set by the DCE; implies a working link.
    • Data Set Ready (DSR) – set by the DCE; implies that the DCE is ready to signal using pin leads.
    • Data Terminal Ready (DTR) – set by the DTE; implies that the DTE is ready to signal using pin leads.
    • Ready to Send (RTS) – set by the DTE; makes the DCE aware that the DTE would like to send data, “Mother, may I?”.
    • Clear to Send (CTS) – set by the DCE; tells the DTE that the DTE can send data “Yes, you may.”
  • IOS “show interfaces” for a serial interface will show you the settings above, as well as interface resets and carrier transitions.
    • The router will reset the interface for several reasons – loss of DCD, interface congestion, or trying to force the data-link to come back up.
    • The router will increments the carrier transitions counter when DCD changes. For instance, if the CSU (DCE) detects LOF, then DCD is going to be switched off by the CSU. The router detects the DCD state change, and increments the carrier transition counter. If the frame comes back, then DCD will come back on, and the router will increment the carrier transition counter again.
By Ethan Banks

Ethan Banks is a podcaster and writer with a BSCS and 20+ years in enterprise IT. He's operated data centers with a special focus on infrastructure — especially networking. He's been a CNE, MCSE, CEH, CCNA, CCNP, CCSP, and CCIE R&S #20655. He's the co-founder of Packet Pushers Interactive, LLC where he creates content for humans in the hot aisle.