Broadband and Last Mile Networks

Transcription

1 Broadband and Last Mile Networks Last-Mile Network Design Concerns Infrastructure Cost Throughput: Upstream and Downstream Packet or Circuit Switching Congestion Control Latency History: Dial-up First public Internet service used telephone lines Telephone lines: broadly available, low infrastructure cost Phone modem encoded/decoded data in voiceband Dial-up: call a phone number, get Internet Result: different networks used in different scenarios 1

2 History: DSL Digital Subscriber Line: improvement on dial-up Used extra bandwidth on telephone line Always on, circuit-switched Allowed concurrent use of phones Faster than dial-up, but slower with distance Still used in rural areas due to low infrastructure cost History: Cable Broadband Growth of Internet required greater throughput Use extra bandwidth in existing cable TV networks Higher throughput, but along shared medium Primary standard is DOCSIS DOCSIS Data Over Cable Service Interface Specifications Formed by industry consortium, maintained by CableLabs Combined 2 earlier, proprietary standards Physical layer from Motorola CDLP MAC layer from LANcity DOCSIS Architecture (DOCSIS 3.0) 2

3 Physical Infrastructure Coaxial cable or hybrid fiber-coaxial (HFC) network Fiber for trunk network, coaxial (RF) for client access Coaxial cables easy to install near metal objects Cable Modem Termination System (CMTS) Evolved from headend for cable TV distribution Handles the MAC layer and content distribution for an area Interface to wide area network Client Cable Modem Performs demodulation of RF signal into usable data. Connects to network via coaxial cable (usually RG-6) Transmission Radio frequency transmission Downstream bandwidth: 108 MHz 1002 MHz Upstream bandwidth: 5 MHz 42 MHz or 85 MHz selectable Bandwidth divided into 6 MHz channels (NTSC) 38 Mbps downstream throughput per channel 9 Mbps (DOCSIS 1.0) or 27 Mbps (2.0) upstream throughput per channel Quadrature amplitude modulation (QAM) 2 carrier signals: phase difference encodes data Channel bonding (3.0): use multiple channels for more throughput CMTS Core Downstream transmitter, upstream receiver Modulation and demodulation of data Packet forwarding between Network Side Interface & upstream RF Interface Timing module: maintains transmission timing requirements EQAM (3.0): Handles downstream RF interfaces for channel bonding Provisions client modems (CM) CM acquires 1 downstream channel, gets init data from CMTS CM contacts CMTS by ranging upstream channel CM authentication, encryption, MAC layer initialization MAC Protocols Cable is shared medium Upstream: FDMA/TDMA or S-CDMA All users transmit upstream on same channels Downstream: Request-Grant model Clients use certain channel(s), request packet from CMTS Packet sequence number to maintain order at receiver Upper layer protocols SNMP for CM management TFTP for configuration DHCP for IP management 3

4 Packet Headers Frame Layout: FC_PARM specifies type of packet: Timing MAC Management Request Frame Fragmentation Concatenation MAC Header Layout: Alternative: Satellite Broadband Wireless broadband: access by satellite or broadcast tower Little infrastructure needed Satellite Satellite dish at client location Issues with latency Dish -> satellite -> headend -> Internet takes time Affected by weather, satellite location Upcoming: Fiber-Optic Cable Last-mile network comparisons Encode data in light Lower latency, less interference, higher throughput than cable Longer transmission distances possible than cable Current providers: Google Fiber, Verizon FiOS Adoption blocker: cost of new infrastructure No pre-existing last-mile fiber-optic network Used mainly for trunk networks Requires planning, permits, construction work Planned to supersede cable DSL: on its way out Lowest infrastructure cost (phone lines) Circuit-switched Low bandwidth Limited effective range Still used in rural areas Cable (DOCSIS): current standard Low infrastructure cost (cable TV lines) Shared medium Higher bandwidth Longer effective range Widely used in urbanized areas 4

5 Last-mile network comparisons Fiber-optic cable: the future High infrastructure cost No pre-existing infrastructure Shared medium Very high bandwidth Very low latency Long range Growing adoption in urban areas Satellite: when wires don t work Low infrastructure cost (satellites) Can offer good bandwidth Significant latency issues Long range Used in remote areas 5