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 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 Flag CRC
Information (data)
Control Address
Flag
HDLC Control Byte for Information Frame
• 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)
Receive Sequence Poll/Final
Send sequence 0
6-8 5
2-4 1
HDLC Control Byte for Supervisory Frame
• 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 Receive Sequence Poll/Final
Control field 10
6-8 5
3-4 1-2
HDLC Control Byte for Unnumbered Frame
• Starts with bits 11
• Used to set up connections and master/slave relationships
Message Poll/Final
Message 11
6-8 5
3-4 1-2
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 pääte-
emulaattoriohjelmistoa, 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
• A simplified PPP state diagram:
– 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
Description of a PPP Session
Dead
Network Terminate
Established Authenticate
Up Opened Success/
None
Fail Fail
Down Closing
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)
