Bandwidth

Learning Objectives:

explain the importance of bandwidth when transmitting data;

explain how different types of communication media govern the bandwidth available ; cables, wireless, optical

Bandwidth• maximum transmission rate of a communications

channel– measured in bits per second (bps)

– Kbps, Mbps, Gbps

• a measure of the capacity of a communications channel

Why is Bandwidth important?• the greater the bandwidth:

– the more information the channel can carry,

– the faster the information can travel to its destination

– more users connected to the network without loss of speed

• multimedia data require high bandwidth connections – due to the large size of the data files

1 track mp3 = 4MB = 32Mbits HD video requires 4x bandwidth of standard

– low bandwidth = ‘jerky’ & unsynchronised video

– need high bandwidth for streaming or video conferencing• e.g. iPlayer, YouTube, Skype

Bandwidth

Communications Bandwidth

Dial-up (modem/telephone line) 56Kbps

Broadband (ADSL) up to 20Mbps

Wireless (WiFi) 54Mbps

LAN (wired) 100Mbps (1Gbps)

Leased Line (DSL) up to 1Gbps

ADSL BroadbandAdvantages:• uses existing copper telephone line for connection to the Internet

– no extra installation of cable is required (good for domestic installation)

• ADSL provides an ‘always on' Internet access – telephone can be used at the same time.

Disadvantages:• needs to be connected to a telephone exchange within a workable

distance– no use for remote locations

• downstream bandwidth is different to the upstream bandwidth. – downstream bandwidth is greater than upstream

• majority of ADSL connections used to download from the Internet,– takes longer to upload an image than to download – takes longer send an email with attachments than to receive

ADSL BroadbandFactors which affect the bandwidth on ADSL:

• distance from the telephone exchange

• electrical interference on the telephone line

• local AM radio stations causing interference

• other devices connected to the telephone line– e.g. fax machines

Learning Objectives:

explain how different types of communication media govern the bandwidth available ; cables, wireless, optical

Bandwidth

Communications Bandwidth

Copper cable (LAN) 100Mbps (1Gbps)

Fibre Optic (LAN/WAN) 20Mbps

Wireless (WiFi, WLAN) 54Mbps

Mobile (GPRS) ~100Kbps

Mobile (3G) 7Mbps

Copper Cable• used to cable LANs• data is sent via electrical pulses

– using copper wire

• 100Mbps bandwidth most common– 1Gbps & 10Gbps possible

• relatively cheap to install – compared to optical cables

• limited to ~100m maximum length– beyond this the data signal becomes too weak

• prone to electrical interference– results in corrupted or lost data packets

– reducing the overall data transmission rate

Optical Cable• used to cable LANs over long distances

– >100m 100+km

• uses infra-red lasers to convert data into light pulses– electrical 0s & 1s converted into light on/light off pulses

• uses optical cable (fibre optic cable) to carry the data• cable consists of multiple thin glass strands

Optical Cable

Optical Cable• used to cable LANs

– over long distances (>100m 100+km)

• uses infra-red lasers to convert data into light pulses– electrical 0s & 1s converted into light on/light off pulses

• uses optical cable (fibre optic cable) to carry the data• cable consists of multiple thin glass strands• not susceptible to electrical interference

– very little data loss

– higher transmission rates than copper cables

Optical Cable• used to cable LANs

– over long distances (>100m 100+km)

• uses infra-red lasers to convert data into light pulses– electrical 0s & 1s converted into light on/light off pulses

• uses optical cable (fibre optic cable) to carry the data• cable consists of multiple thin glass strands• not susceptible to electrical interference

– very little data loss

– higher transmission rates than copper cables

• >10Tbps bandwidth possible– domestic cable ~20Mbps (Virgin Media cable ADSL)

Wireless• used to connect LANs

– over short distances (<100m)

• bandwidth less than copper cable or fibre optics– 54Mbps 802.11n most common (2010)

– ~100Mbps now possible with 802.11n

• distance limited to ~50m indoors (100m outdoors)– signal attenuated by walls, steel ….

• prone to electrical interference– microwave ovens, mobile ‘phones, X-ray machines …

– results in corrupted data packets & reduced data rate