Digital Access Networks. T /301 Jouni Karvo, Timo Kiravuo

Transcription

1 Digital Access Networks T /301 Jouni Karvo, Timo Kiravuo

2 User Access to the Telecommunications Network The traditional access link is an analog copper wire to customer premises. Allows full-duplex analog speech transmission, and use of modems, typically up to 56kbit/s. ISDN, Integrated Services Digital Network xdsl (x-digital Subscriber Line) techniques use frequencies higher than the ones used for speech transmission to provide a data connection HomePNA (Home Phone Line Networking Alliance) uses frequencies higher then the xdsl techniques for short distance high-speed connections CATV (Cable TV) lines can be used to connect homes to high speed networks. Connections are asymmetric, and only available on urban areas

3 Integrated Services Digital Network (ISDN) CCITT study group 1968 for all digital telephone network All services to the same network: telephone, fax etc. videoconferencing data transfer (frame relay) Integrated access (no separate connection or equipment required) ISDN is a pipe from the user to the central office Kessler & Southwick: ISDN (McGraw-Hill, 1998)

4 ISDN cont. Narrowband ISDN: Basic rate 144kbps net using single pair Broadband ISDN: 25Mbps and up (using ATM) Already before ISDN, the telecom network was digital, ISDN is different in reaching the users digitally Digital Subscriber Line (DSL) coils, bridged taps limit the available bandwidth full duplex transmission in the same pair (echoes) four wire solution or TDM hybrid + echo canceller (the solution used in ISDN)

5 D-channel: signaling has priority, user data can use excess capacity. Uses Link Access Protocol on the D-channel (LAPD) B-channel: both circuit mode and packet mode available No standards by ITU-T for circuit mode (although clearly PCM used for voice) X.25 or Frame Relay for packet mode Either on demand or permanent connections H-channel: groups of B-channels D B H0 H11 H12 Nx64 ISDN Channels signaling and packet mode data bearer service wideband bearer wideband bearer variable bandwidth bearer 16kpbs (BRI) 64kbps (PRI) 64kbps 6B 24B = T1 30B = E1 1B... 24B

6 ISDN Services Telephone calls: delay sensitive, long holding time (e.g. call establishment 3-11s, call holding time 5min), constant narrow capacity requirement X.25 packet switching networking between users and Public Switched Packet Data Networks (PSPDNs) user host: Data Terminal Equipment (DTE) network node: Data Circuit-Terminating Equipment (DCE) Layer 1: physical, Layer 2: data link (Link Access Protocol Balanced, LAPB), Layer 3: virtual call establishment, multiplexing (Packet Layer Protocol, PLP) X.75 interworking between PSPDN:s (and between PSPDN:s and ISDNs)

7 ISDN services Frame Relay (Link Access Procedures for Frame Mode Bearer Services, LAPF) ---a link level data transfer service developed from LAPD provides similar services as its successor, the ATM. A couple of differences: ATM uses constant length frames called cells; ATM has hierarchical virtual connection identifier (VPI/VCI) Supplementary services: Number identification services, such as Calling Line Identification Presentation (CLIP), Connected Line Identification Presentation (COLP), Call offering services, such as Call Transfer (CT), Call Hold (CH), Call completion services such as Call Waiting (CW), Multiparty services such as Conference Calling (CONF), Community of Interest services such as Private Numbering Plan (PNP), Charging services such as Credit Card Calling (CRED), Advice of Charges (AOC) and Additional Information Transfer i.e. User-to-User Signaling (UUS).

8 Other Digital Subscriber Lines Due to tight cabling requirements of PDH/SDH 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.

9 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.

10 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 supports 6.144Mbps downstream and 640kbps upstream. ADSL version ITU-T G supports up to 1.563Mbps downstream and 512kbps upstream. ADSL provides transport of STM and/or ATM.

11 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.

12 HomePNA Based on Frank, E. H. and Holloway, J. "Connecting the Home With a Phone Line Network Chip Set", IEEE Micro, Mar--Apr The idea in HomePNA is to use the existing telephone cabling to provide a local area network. Uses IEEE framing and an 8-priority CSMA/CD MAC. Supports unicast, multicast and broadcast; has a look and feel of Ethernet. Logical topology: bus, physical topology free. Uses the 4Mhz - 10Mhz frequency band. HPNA 2.0 has up to 32Mbps data rate, and projected growth to 100Mbps.

13 HomePNA cont. The physical media used suffers from reflections and frequency dependent transfer functions (telephone lines are not designed for higher frequencies), interference from the telephone equipment, coupled AC noise, RF interference (especially on amateur radio frequencies). HPNA uses adaptive rates and modulations; when interference occurs, stations switch to a lower rate. The modulation used is modified QAM. A possible application is connecting a block of flats to the Internet; there is for example an ADSL connection to the distribution panel on the basement, and the network inside the house is implemented using HomePNA.

14 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.

15 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

16 Data Link Protocols

17 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

18 HDLC High-level Data Link Control Data link layer protocol 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

19 Flag HDLC Frame Format Address Control Information (data) CRC Flag Flag is Bit stuffing is used for other data, every 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

20 HDLC Control Byte for Information Frame 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)

21 HDLC Control Byte for Supervisory Frame 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

22 HDLC Control Byte for Unnumbered Frame Message Poll/Final Message Starts with bits 11 Used to set up connections and master/slave relationships

23 The Ethernet IEEE standard A limited distance LAN protocol and cabling standard Several physical cabling and bandwidth options Coaxial cable 10Base5 and 10Base2, mostly historical All nodes connect to the same coax Twisted pair 10BaseT, 100BaseT Nodes are connected using a hub or switch Also optical and wireless Ethernet and gigabit speeds

24 How the Ethernet Uses the Shared Media? 1-persistent CSMA/CD Carrier Sense Multiple Access / Collision Detection Each node waits for a free moment to send If two or more send simultaneously they detect the collision and each stops sending and waits a random interval All nodes listen to all traffic and pick their own as directed by the MAC address

25 Ethernet Addressing Each network card has a unique hardware address (MAC) In some hardware can be changed with software The MAC address usually has a manufacturer ID part and an unique part Duplicate MAC addresses in the same LAN happen, but extremely rarely Ethernet frames start with a 48-bit destination address followed by a 48-bit source address Hosts can send unicasts or broadcasts The broadcast is used to find who else is on the same LAN Or to find who has a certain IP address on this LAN (ARP) Or to find configuration parameters for this host (DHCP) Unicasts are used for normal traffic

26 Hubs and switches Twisted pair Ethernet hosts use a physical star configuration A hub repeats all traffic to all stations A switch learns which MAC addresses are in which physical ports and transmit only necessary traffic Security info: switches are harder to eavesdrop, but can usually be made to enter a fail-safe mode, where they act as a hub Switches provide better performance than hubs Both may be chained within certain limits

27 Extending Ethernet The physical reach of the Ethernet can be extended using repeaters, bridges and other devices However having a large (many hosts) Ethernet is very vulnerable to problems Broadcast storms can cripple the whole network (as happened to FUNET once) There is a reason for the higher level protocols like IP

28 LAN Topologies LANs can be Buses Stars Rings Other A switch with connections to local hubs, a star of stars Wireless Raises interesting problems, A can see B, B can see C, A can not see C...

29 LAN cabling Expensive, difficult to change afterwards The reason for 10BaseT was the ability to use existing telephone cabling Since the analog signals are RF, the quality of cabling and connectors is very important The physical routing of the cabling creates security considerations The physical maximum length of a single hub to workstation and overall cabling length creates installation considerations

30 Token Ring Instead of collision detection, there is a token (data) circulating in the network The node having the token at the moment has the permission to send, all others must wait for their turn The protocol must be able to handle situations like the node dieing, while it possesses the token Usually if there is no token seen within a certain time period, a new token is created IBM Token Ring network was physically a star, logically a ring

31 ATM LANs ATM provides only point to point connections IP on a LAN assumes that local network addresses can be reached directly and traffic to other addresses must be sent to the router There are several workarounds that make it possible to build an IP LAN using ATM Most common is an ATM LAN Emulation server that emulates broadcasts and other services But the 100 Mbps switched Ethernet killed the need for ATM LANs

32 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)

33 SLIP overview Simple packet framing protocol 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

34 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 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

35 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

36 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

37 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

38 Some Other Data Link Protocols ISDN Frame Relay FDDI GPRS + UMTS WLAN Bluetooth Cable modems And the mighty IP runs over everything