Communication Circuits

Question 1

Communications Media

Answer 1

Communications media for LANs use either wired or wireless approaches in their topologies

Wiring approaches consist of:
. Twisted-pair cabling
. Coaxial cable media
. Fibre-optic cabling

Wireless approaches consist of
. Satellite Communication.
. Radar.
. Mobile Telephone System (Cellular Communication)
.Global Positioning System (GPS)
. Infrared Communication.
. WLAN (Wi-Fi)
. Bluetooth

Question 2

The Wired Approach

Answer 2

Modern Wiring plans normally follow standard structured cabling methods:
. Wired cabinets on each floor of a building
. An orderly cabling installation

Utilizing cabling racks:
. Patch Panel
. Wiring distribution
. Network access devices

Question 3

Cabling Media Choices

Answer 3

The network designer has a number of alternative cabling media choices
. Twisted pair, coaxial cable, and fibre-optic cabling

The cabling selection issues include
. The required data rate
. Including network growth considerations
.The level of electrical interference
. Would your AM radio pick up a lot of interference in this environment?
. The maximum cabling length that must be considered.
. Cost

Question 4

Unshielded Twisted-Pair Cables

Answer 4

The least expensive media (unshielded)

Capable of handling up to 100m

Unshielded twisted pair (UTP)
. Data capacity grades defined by EIA/TIA 568 (ISO 11801)
. Category 3 (to 10 Mbit/s or more)
. Category 4 (to 20 Mbit/s or more)
. Category 5 (to 100 Mbit/s or more)

Category 5e, 6 and 6A are used extensively today (to 1,000Mbit/s and above)

Question 5

Multiplexing

Answer 5

Combining a number of different data/telephone channels

On the same cable-copper and fibre

Question 6

Category 6 Cabling

Answer 6

The latest form of UTP cabling is category 6

It adds additional quality assurances beyond Category 5e
. Labour costs is by far the largest cost component in cabling

It comes in 2 forms
. UTP and screened Twisted Pair (ScTP)
. ScTP has a layer of metallic foil to improve its interference rejection

May utilise larger-diameter copper wires
. Helps for ‘power over Ethernet’ situations

Both Cat 7 & 8 are screen shielded twisted pair (SSTP) or screened foiled twisted pair (SFTP)

Question 7

Shielded Twisted-Pair

Answer 7

Shielded twisted pair (STP)
. Primary used by IBM
. Should be better then UTP
. Shields help prevent interface from outside signals
. Also help prevent interference to outside signals

Token ring environments includes a mix of UTP and STP cabling

Question 8

Coaxial Cable - Legacy

Answer 8

Low noise (low bit-error rate)

Used in a variety of networking applications
. In IBM network (e.g. Cluster Controllers)
. In earlier Ethernet (10 Mbit/s)

The shielding may include multiple layers of foil and/or braid as shown below
. This is a Thickwire Ethernet cable

Question 9

Fibre-Optic Cables

Answer 9

Extremely high Data Rates:
. More than 100 mbit/s for LAN uses
. More than 10 times that for telephone company links

Usage is typically with two unidirectional links, with one fibre in each direction

Convert electrical signals to light and back to electrical

Very small size:
. Hair-like fibre-optical strand (125- micron outer diameter
. Light-conducting core size may be 62.5 microns
. Called 62.5/125 - micron fibre
. May use 50/125 - micron fibre (Europe)
. Single mode 9/125

Question 10

Fibre- Optic Cable Transmission Characteristics

Answer 10

LAN usage is usually Multimode, graded index:
. Multimode supports different light transmission paths
. Pulses spread out in time
. Graded index resists pulse spreading due to different transmission speeds.

Use single-mode fibre for very high speed and/or long distances

Question 11

Fibre-Optic Characteristics

Answer 11

Approximately the same cost as good-quality Ethernet cable:

. Optical interfaces are the most expensive component
. Transmission by light-emitting diodes (LEDS) or laser diodes
. The receiver devices convert light pulses back into electrical pulses

Best available communications medium:
. Excellent electrical noise immunity
. Difficult to tap (security)
. Lightweight
. Small size

Question 12

Fibre-Optic issues

Answer 12

A single fibre may support multiple light beams:
. Dense wave division multiplexing (DWDM)
. Up to 25,000 or more simultaneous transmissions
. Only used with single-mode fibre

Media converters are required between different media types

Question 13

Modulation

Answer 13

Modulation converts a digital signal into an analogue signal:
. This converted signal is sent across the analogue line

Demodulation converts the signal back to digital

MODEM
Modulation-Demodulation

Modems are standardised by ITU-T, V-series recommendations

Typical modems include:
. V.34 at 28.8kbit/s and 33.6kbit/s
. V.90 at somewhat less than 56kbit/s
. V.92 for higher-speed uplink, faster connection time, and the ability to accept an incoming call

Question 14

Modems

Answer 14

The data rate may fall back to lower rates:
. The modem will operate at the highest available dial-up line data accepted on an incoming call

May perform V.42 error correction,
V.42bis (4:1) or V.44 (6:1) data compression
V.54 loopback testing
V.250 command set

Question 15

Reasons for going Digital

Answer 15

Computer data is inherently digital:
. Adapt more easily using digital transmission

Higher data rates are available

Easier to switch

Better error rate:
. Noise is not cumulative, because repeaters can reject induced noise
. Amplifiers also amply the noise

Question 16

Digital Telephone Channels

Answer 16

Digital telephone communications channels are also available:
. 56 or 64 kb/ps channels (or a multiples of then)
. 1.544mbit/s
(US, Canada and Japan or 2.048 mbit/s channels)

Instead of modems, Data Services Unit/ Channel Services Unit (DSU/CSU) adapter devices may be needed:
. The DSU adapts the digital signal (transmit and receive voltages and timing)
. The CSU normalizes voltage levels, provides maintenance capabilities, and protects the public network

Question 17

The V.35 Interface

Answer 17

V .35 has two-wire circuits for data and timing:
. Balance lines

Contrast this with RS 232 one-wire circuits:
. Unbalanced (used on CISCO routers)

The problem is that the V.35 interface can be put in the wrong way round unlike the RS 232.

Question 18

X.21 and Other Serial I/O Interfaces

Answer 18

A popular serial I/O interface is X.21 (European):
. Reduced number of pins (15-pin connector)
. Has transmit and receive pairs (for data and encoded commands)
. Input and output control pairs (to indicate whether the transmit and receive are currently handling data or control)
. Timing signal pair

Networking devices typically support many different serial I/O Standards:
. Usually with a common connector on the I/O module
. Separate adapter cable for each different type of serial I/O standard (eg RS 232, RS 449, V.24 or X.21)

Question 19

Problem with T1/E1 and T3/E3 Systems:

Answer 19

There are several problems with traditional T1/E1 systems:
. T1 (North America and Japan) and the E1 (the rest of the world) are incompatible
. It is very complicated to add or drop one 64 kbit/s channel
. There is little problem isolation information in these systems
. There is a need for higher bandwidth

A new system is needed:
. This is SONET/SDH

Question 20

Fractional and Full T1/E1 and T3/E3 Links

Answer 20

Fractional T1/E1 links are multiples of 64 kbit/s:
. A common example is 384kbp/s 6X64kpb/s
. Used with Video conferencing

Fractional T1 may also be in multiples of 56 kbit/s

Full T1/E1 is one of the most common types of WAN links:
. T1 =1.544Mbit/s (24 slots @364kbit/s + 1-bit framing)
. E1 = 2.048Mbit/s (32 slots @364kbit/s including framing)

Sharing a communication circuit in this manner is called time-division multiplexing

T3 channel is 28 T1 channels multiplexed together:
. T3 is approximately 45Mbit/s

An E3 channel is 16 channels:
. E3 is approximately 34 Mbit/s
. An E4 channel is 64 channels
. E4 is approximately 144Mbps

Question 21

High-Speed Synchronous Optical Networking Standards

Answer 21

Two different forms have been developed:
. Synchronous Optical Network (SONET) is a North American standard
. Multiples of 51.84 Mbit/s for SONET
. STS-3 support triple the bandwidth 155.52 Mbps
. Multiples of 4 currently upto 40Gbps
. Synchronous Digital Hierarchy (SDH) is an international standard
. Multiples of 155 Mbit/s for SDH
. STM-1 155mbps STM-4 622Mbps; STM-64 10Gps

Question 22

SONET/SDH Rings

Answer 22

A very resilient form of SONET/SDH is a dual ring:
. Automatically wraps to use both rings when cut
. Recovery is within 50msec