Linkkikerros kahden pisteen välillä Kirja 102-128, 135-143, 144-151
Linkkikerros Tietokoneet käsittelevät tavuja, IP-kerros paketteja, fyysinen kerros bittejä Linkkikerros yhdistää nämä kehystämällä ylemmän tason datan siirrettäväksi fyysisen median ylitse Verkkokerroksella on tavuista koostuvia paketteja Fyysinen kerros siirtää bittejä tai tavuja Tänään kahden pisteen väliset verkot, seuraavalla luennolla lähiverkot
Tietoverkkojen topologiat Verkko voi olla rakenteeltaan rengas, väylä, tähti tai hybridi Lähiverkot usein tähti- tai väylämäisiä Kampus- ja alueelliset verkot usein kaksoisrenkaita (vikasietoinen ratkaisu) Eri kerroksilla saattaa olla erilainen topologia Koaksaali-Ethernet on fyysinen ja looginen väylä Keskitin-Ethernet on fyysinen tähti, looginen väylä Käytetty media voi olla usean osapuolen jakama tai kahden olion välinen
The Data Link Layer Traditionally data communications are provided using Local Area Networks Ethernet, Token Ring, ATM over physical media Point to point connections between LANs HDLC, PPP, SLIP, ATM etc. over telecoms infrastructure Telecoms infra: Sonet, SDH, PDH Also Campus networks FDDI, HIPPI, ATM, optical rings Mobile wireless networks GPRS, UMTS, WLAN Plenty of other solutions Cable modems, Bluetooth
HDLC High-level Data Link Control Data link layer protocol, bit oriented Point-to-point and point-to-multipoint connections Unbalanced (master/slave) point-to-point and multipoint Balanced point-to-point (each side has dual roles) Encapsulates network layer packets (SDUs) to frames (PDUs) Provides both connection oriented and connectionless service
HDLC Frame Format Flag Address Control Information (data) CRC Flag Flag is 01111110 Bit stuffing is used for other data, every 11111 is followed by 0 Note that HDLC is bit-limited protocol, not byte limited Address is the receiver address Control is 8 bits, 16 in extended mode for longer sequences Information is payload data CRC is the checksum in ITU-CRC, 16 or 32 bits
HDLC Control Byte for Information Frame 1 2-4 5 6-8 0 Send sequence Poll/Final Receive Sequence Starts with bit 0 Sequence numbers can be extended with an additional byte to 7 bits from 3 Receive ACKs may be sent piggypacked in information frames or in control frames Poll/Final bit is used for master/slave communications In unbalanced mode the secondaries (slaves) may not send without receiving first a frame with the P/F bit on form the primary (master)
HDLC Control Byte for Supervisory Frame 1-2 3-4 5 6-8 10 Control field Poll/Final Receive Sequence Starts with bits 10 Control 00 is RR, Receive Ready (ACK) Control 01 is REJ, Reject (NAK) Control 10 is RNR, Receive Not Ready Sent for e.g.. buffers full, used for flow control Control 11 is SREJ, Selective Reject Used to request retransmission of a single frame
HDLC Control Byte for Unnumbered Frame 1-2 3-4 5 6-8 11 Message Poll/Final Message Starts with bits 11 Used to set up connections and master/slave relationships
Pääteyhteys ja modeemi Halpojen mikrotietokoneiden yleistyessä 1980-luvulla niiden ensisijainen tietoliikennevarustus oli RS-232 - sarjaliikenneportti Suunniteltu siirtämään ASCII-merkkejä päätteille/päätteiltä Kehystää 7 tai 8 bittiä alku- ja loppubiteillä, mahd. pariteetti lisäksi Modeemin avulla sarjayhteys voitiin jatkaa etäämmälle käyttäen puhelinverkkoa Tietokoneessa käytettiin tyypillisesti pääteemulaattoriohjelmistoa, joka pysty noudattamaan varsinaisten päätelaitteiden ohjauskoodeja Tavukoodeja, esim. ASCII 10 rivinvaihto, ASCII 13 vaununpalautus Modeemi ja sarjayhteys tarjoavat siis tavan siirtää tavuja tietokoneelta toiselle
Serial Point-to-point Links Need to connect 2 networks or computers with a dedicated link Dial-up hosts and modem pools, inter-office routing Endpoints might be single computers, routers or bridges Dial-up connections, on-demand routing Dial-up access to networks with telephone/isdn lines Routing between offices over leased lines Encapsulating of network routes over different networks (tunneling)
SLIP overview Simple packet framing protocol, byte oriented Framing is done with END (octal 300) and ESC (octal 333) special characters. Sender just transfers data packet and END Occurrences of END and ESC characters in data bytes are escaped with two byte sequences (ESC+octal 334 and ESC+octal 335, respectively) Static IP addresses for endpoints Every dial-up client needs own IP address No type field for packets only one protocol over one SLIP connection Old de facto-standard, description in RFC 1055
PPP overview Specification in RFCs 1661,1662, 1663 and others A protocol capable of multiplexing different network protocols over a single point-to-point link For example IP, IPX, XNS and AppleTalk concurrently HDLC-like Byte or bit oriented Carefully designed for compatibility with most commonly used hardware Independent protocols for link and network control Link encapsulation options, authentication and link quality control configured with link control protocol, with reasonable default values Extensible with new network protocols: each network protocol has its own network control packets and configuration options
Practical PPP examples Dial-up Internet connections Physical connection with modem or ISDN Dynamic IP addresses and network configuration for clients Easy to install and reliable Supported for almost all client platforms Simple VPN (Virtual private network) over TCP/IP and SSH Secure Shell connection from distant location to intranet over untrusted networks PPP runs over SSH connection with link ends in secure networks Packets from distant location to intranet are routed over PPP to intranet. Cheap, dirty and practical VPN solution
Practical PPP examples PPP over Ethernet Specified in RFC 2516 Used in some DSL and cable modem configurations Makes it possible to decouple providing Integrates with existing authentication/billing systems
Description of a PPP Session A simplified PPP state diagram: Up Opened Dead Established Authenticate Success/ None Fail Fail Down Terminate Closing Network Session up/down events not shown (from hardware or manually) In Network state different network layer protocols may open or close while PPP stays connected: different possible states not shown
Broadband Digital Subscriber Lines Due to tight cabling requirements of PDH/SDH (tle major telecoms transmission system) transmission, it is not possible to transmit PDH/SDH in the local loop. different techniques were developed. Splitters are used to separate voice band and DSL channels both on network side and customer premises, when applicable.
HDSL The HDSL (ITU-T G.991.1) (High Data-Rate Digital Subscriber Line). HDSL can transmit 3 * 784 kbps on three twisted pairs, or 2 * 1168 kbps on two twisted pairs or 2320 kbps on a single twisted pair. HDSL with a single twisted pair is called SHDSL (ITU-T G.992.2). The coding is the same used with ISDN (2B1Q). HDSL does not support simultaneous voice or ISDN channels. HDSL can transport e.g. ATM or E1/T1.
ADSL ADSL (Asymmetric Digital Subscriber Line) uses a single twisted pair, and allows simultaneous transmission of downstream simplex, duplex, base band analogue, ADSL line overhead and framing, error control, operations and maintenance. Uses Discrete Multitone (DMT) modulation, where the frequency spectrum is divided in narrow sub bands, each of which can be configured separately ADSL transmission is possible simultaneously with POTS, analogue modems, ISDN. ADSL has a low speed full-duplex bearer channel and a high speed bearer channel on the downstream direction. ADSL version ITU-T G.992.1 supports 6.144Mbps downstream and 640kbps upstream. ADSL version ITU-T G.992.2 supports up to 1.563Mbps downstream and 512kbps upstream. ADSL provides transport of STM and/or ATM.
VDSL VDSL (Very-High Data Rate Digital Subscriber Line) trades loop length to transmission speed. While ADSL can reach up to 6km, VDSL gives only 300m - 1km. Data rates are up to 51.84Mbps downstream and 6.48Mbps upstream. VDSL supports several different transmission rates, corresponding to different container sizes in SDH.
Cable modems Cable modems are systems to provide high speed data access. They use the cable television network, and thus are not part of the telecommunications network. Splitters separate TV and data channels, and allow normal TV viewing. Cable modem installations are normally heavily asymmetric; the downstream data rates to customers are much higher than the upstream rates. Sometimes CATV is unidirectional and the return channel is implemented using the telephone network.
Digital Video Broadcasting Television broadcasting is becoming digital Several DVB standards DVB-S Satellite DVB-T Terrestrial DVB-C Cable DVB-H Handheld Supports sending of any kind of data If allowed by regulations Uni-directional A "return channel" is needed The return channel is usually an usual bi-directional connection to the Internet
Some Other Data Link Protocols ISDN Frame Relay FDDI GPRS + UMTS WLAN Bluetooth
Yhteenveto Protokollat ja teknologiat eivät noudata OSI-mallia kovinkaan siististi HDLC esittää miten linkkikerros kehystää datan fyysiseen kerrokseen Kehyksen liput alussa ja lopussa (tarvitaan) Yksinkertainen kättely (ei välttämättä tarpeen) SLIP on pelkkä kehys PPP on liki sama kuin HDLC ADSL- ja kaapelimodeemeissa on omat yhteyskäytäntönsä (mm. PPP), käyttäjälle ne näkyvät linkkikerroksen yhteyksinä (usein Ethernet)