Digital Access Services
T-1 and SONET
Topics

T-Carrier Hierarchy

SONET Hierarchy

Other fiber

Leased Line Details

Pricing and Billing

Router configuration

PPP configuration
Leased Line Service Rates

T1



Fractional T1 (n × 64 kbps)






physical facility: same as T1 (4-wire copper)
STS-1/DS3 (45M bps)


Channelized T1: 24 × DS0 (64k bps)
unchannelized T1: 1.544MP bps
physical facility: DS3X coax (in-building)
physical facility: fiber (out-building)
Fractional DS3 (n x T1)
SONET OC-3 (155M bps) – fiber facility
SONET OC-12 (622M bps) - OC-??
Dark Fiber
T-Carrier Systems
Service
Circuit
DS0
Bit Rate
# Voice
Channels
64 Kbps
1
DS1
T1
1.544 Mbps
24
DS2
T2
6.312 Mbps
96
DS3
T3
44.736 Mbps
672
DS4
T4
274.176 Mbps
2016
T-1 Frame Format
1 bit
8 bits
8 bits
8 bits
F
DS0 #1
DS0 #2
DS0 #3
8 bits
. . .
DS0 #24
Each DS0 called a time slot
8000 frames/sec * 8 bits/slot = 64 Kbps
24 * 8 + 1 = 193 bits/frame
8000 frames/sec * 193 bits/frame = 1.544 Mbps
8000 Framing bits sent per second
Channelized?

Channelized T-1 Circuit



T-1 is utilized as 24 DS0 channels of 64 Kbps
each.
Each DS0 can be allocated to carry any single
service, such as CO trunk, DID trunk, WATS,
FX, 56K data, switched 56K, etc.
Unchannelized T-1

T-1 is utilized as a single 1.536 Mbps data
circuit.
Leased Line – Fractional T1
CH1 CH2 CH3 CH4 CH5 CH6 CH7 CH8
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Physical facility: T-1 (4-wire)
Data rate: 64 kbps × 8 = 512 kbps (for this
example)
The customer pays a monthly charge of
physical facility (data rate independent) + data
services (data rate dependent)
used
unused
Signaling
The basic purpose of signaling is to determine the
status and control of each 64 Kbps channel. There
are two basic types:
nIn-band
nOut-of-band
T1 systems originally provided for signaling using:
In-Band Robbed-Bit Signaling
Source: KnowledgeLink, Inc.
Bit Robbing
Bit robbing “steals” the
least significant bit for
signaling transport
1 0 1 0 1 0 1 1
Least Significant Bit
1 0 1 0 1 0 1 1
Value is determined by
Signaling, NOT Channel Sample
Source: KnowledgeLink, Inc.
T-1 Framing Bits

D4 T-1 lines (1972):




Allow receiver to find the start-of-frame (frame
synchronization).
Group sets of 12 frames into superframes
Indicate that frames 6 and 12 contain signaling
bits (to specify if channel is in use or not)
D5 (ESF) T-1 lines (1983):


Provide error checking (CRC) (ESF T1)
Provide Facilities Data Link channel to transmit
network management messages (ESF T1)
T-1 Super Frame (SF)
(AB Signal)
12 frames
= 1 SF
6th frame
12th frame
Figure 15.8
T1 Extended Super Frame (ESF)
(ABCD Signal)
24 frames
= 1 ESF
Figure 15.9
S: Synchronization (001011) C: error detection F: Facility Data Link control
DS1 Line Coding 
placing 1’s and 0’s
AMI (Alternate Mark Inversion)
AMI sends zero volts for a "0" (space) and alternately sends +V
and -V volts for a "1" (mark). Doesn’t guarantee 0’s density so it
is susceptible to clock drift since clock is imbedded in data.


General Requirements: 12.5% 1’s and no more than 15
consecutive 0’s

B8ZS (Binary 8 Zero Substitution)

Replaces 8 consecutive 0s with 000VB0VB (bi-polar violation)
AMI and B8ZS Signal
1
0
0
0
0
0
0
0
0
0
1
Sender
T1 sent
T1 recv
Receiver
Figure 15.6
Data over T-1
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
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A T-1 carries 24 DS0 channels
Each DS0 may carry a maximum of 56 Kbps
or 64 Kbps of data
A restricted T-1 carries 24 x 56 Kbps =
1.344 Mbps.
A clear-channel T-1 carries 24 x 64 Kbps =
1.536 Mbps.
How do you make sure that you get a clearchannel T1? ASK about it!!
Summary - T1 Configuration

Line Coding



AMI (Alternate Mark Inversion) Coding is the older standard and
only allows restricted T1 data service (i.e. 56 Kbps per DS0)
B8ZS (Binary 8-Zero Substitution) Coding allows clear-channel
service.
Signaling

Robbed Bit Signaling is the older method and only allows restricted
service.


No signaling (i.e. leased line) or Common Channel Signaling
(CCS) or Signaling System 7 (SS7) allows clear-channel service.
Framing


D4 or SuperFrame (SF) Framing is the older standard and does not
allow error detection or automatic failover services from carrier.
Extended SuperFrame (ESF) Framing allows carrier to
automatically detect errors and to perform failover to good circuit when
errors occur.
The E-1 Interface


Similar to T-1, used outside North
America
The E-1 interface provides a 2,048
kbit/s access rate. It can support up to
32 channels (64 Kbps DS0).


“Framed E1” supports 30 voice channels
Unchannelized E1 gives you the full 2.048
Mbps.
E1 Frame




TS0 is dedicated for synchronization, alarms,
control messages, and future extensions.
TS16 is usually used for signaling. It is known
as Clear Channel Signaling, an example of out of
band signaling.
TS0 and TS16 can carry data as well
(unchannelized E1)
TS1-TS15 and TS17-TS31 are used for carrying
user data.
Leased Line Service (T1)
(example: to Internet)
Customer
router
V.35
Enterprise Network
Internet
ISP
CSU/DSU
T1
4-wire
Local
Carrier
(AT&T)
CSU/DSU
CSU/DSU
“WIC
Module”
CSU card
in router
Cisco
2600
Router
RJ48C
Equipment Cost (example)
10/100BaseTX
LAN
Cisco 2811 Router
$1,779.99
http://www.cdw.com/shop
T1
WIC T1 CSU/DSU
$689.99
WAN
Example: Leased Line Internet Service (T1)
Internet
ISP
(Cogent)
$350
Carrier
(AT&T)
$200
www.cogentco.com
Installation
Service Order
Full T1 Monthly Charge
Circuit (local loop) Monthly Charge
Chicago
N/A
N/A
$350
$200
Leased Line Pt-Pt Service (T1)
$575
St. Louis
Carrier
(AT&T)
$1.85 ×300
Carrier
(AT&T)
$575
Chicago
Leased Line Service Charge (1Y pan)
Installation
Service Order
Circuit (monthly) –
Per Mile (monthly)
$700
one per location
$325
$575
$1.85
Monthly charge = $575*2 + $1.85 ×300 (miles) × (1-25%) = $1,278
.
Synchronous Optical
Network (SONET)
ADM = Add-Drop Multiplexer
ADM
ADM
ADM
SONET Ring
pass frame
ADM
add frame
to the ring
drop frame
out of the ring
SONET Overview


Synchronous Optical NETwork standards
developed by Bellcore.
Advantages:





transmission standards for optical networks allows “mid-span
meet” between different equipment (vendor interoperability!)
Synchronous multiplexing allows add-drop multiplexing of any
low speed signal into any high speed signal (to fully appreciate this
advantage, consider the activities a multiplexer must perform to
add-drop a T1 to/from a T3 signal).
Operations and Maintenance capabilities greatly improved over
previous systems
Deployment began in the late 80’s and
significantly increased each year.
International equivalent to SONET is known as
the Synchronous Digital Hierarchy (SDH)
standards developed by the ITU.
SONET Interfaces
OC
STS
OC-1
OC-3
OC-12
OC-48
STS-1
STS-3
STS-12
STS-48
OC-192
OC-768
STS-192
STS-768
SDH
Line Rate Payload Rate
(Mbps)
(Mbps)
STM-1
STM-4
STM-16
51.84
155.52
622.08
2,488.32
49.54
150.34
601.34
2,405.38
STM-64
STM-256
9,953.28
39,813.12
9,621.50
38,486.02
SDH: Synchronous Digital Hierarchy by ITU-T
SONET: Synchronous Optical Network by ANSI
OC: Optical Carrier
STS: Synchronous Transport Signal
STM: Synchronous Transmission Mode
3-Layer SONET Transport
STS-1 Frame Format
STS-1 Frame




8,000 frames/second
Each frame: 90 col × 9 rows = 810 bytes
Payload: 86 col × 9 rows = 774 bytes
Each byte of payload can be allocated as a
separate 64 Kbps customer channel



STS-1 frame can support up to 774 standard
DS0 channels (digital voice or 64 Kbps data).
Transmission Rate (raw): 51.84M bps
Transmission Rate (payload): 49.54M bps
Virtual Tributaries



SPE (payload) = 7 VT Groups + packing bytes
One VT Group = 12 columns
Each VT Group is split into multiple VTs of the same type

Either 4 x VT1.5 or 3 x VT2 or 2 x VT3 or 1 x VT6
VT Type
Cols
Rate (bps)
Carries
# in STS-1
VT1.5
3
1.728M
T1/DS1
28
VT2
4
2.304M
E1
21
VT3
6
3.456M
DS1-C
14
VT6
12
6.912M
DS2
7
SONET Networks
SONET Transmission Equipment:
 SONET Repeater
 SONET Terminal/Multiplexer
 SONET ADM (Add Drop Multiplexers)
 SONET DCS (Digital Cross-Connect Systems)
SONET Network Topologies:
 Point-to-Point (w/ Automatic Protection Switching)
 Ring Configuration
 Unidirectional Line Switch Rings (ULSR)



a.k.a Unidirectional Path Switch Ring (UPSR)
2-fiber Bidirectional Line Switch Rings (BLSR)
4-fiber Bidirectional Line Switch Rings (BLSR)
SONET Access
Long Haul
(DWDM)
Network
Core Router
ADM
ADM
Metro SONET Ring
Voice Switch
ADM
ADM
ADM
Access Ring
Access Ring
T1
ADM
Access Ring
ADM
T1
PBX
ADM: Add/Drop Multiplexer
SONET (APS)
Point-to-Point (with Automatic Protection Switching (APS))
Working
DS1s, DS3s,
Ethernet, ...
SONET
Terminal
Protect
SONET
Terminal
DS1s, DS3s,
Ethernet, ...
Faults detected and managed using span switching within 50 msec
automatically!
SONET (ULSR)
FIGURE 20-7 2-fiber ULSR (Unidirectional Line Switched Ring) uses
one ring in normal operation. If both fibers are cut, the rings are wrapped
around.
SONET Networks (BLSR)
Ring Topology BLSR (2-fiber)
Ring Topology BLSR (4-fiber)
SONET
ADM
SONET
ADM
SONET
ADM
SONET
ADM
SONET
ADM
SONET
ADM
SONET
ADM
SONET
ADM
S



Traffic broadcast to rotating and
counter-routing paths
dropped traffic selected based on
path level performance by the
selector, S
used extensive in access network



Enhanced protection due to both span
switching and ring switching
4-fiber per node interface (vs 2)
used extensively in “backbone” network