IPv6 Address PDF

IPv6 Address CHAPTER - 7 IPv6 ADDRESS 7.0 Introduction Internet Protocol Version 6 (IPv6) is the latest revision of the Internet Protocol (IP), the communications protocol that routes traffic across the Internet...

IPv6 Address CHAPTER - 7 IPv6 ADDRESS 7.0 Introduction Internet Protocol Version 6 (IPv6) is the latest revision of the Internet Protocol (IP), the communications protocol that routes traffic across the Internet. It is intended to replace IPv4, which still carries the vast majority of Internet traffic. IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4 address exhaustion. Every device on the Internet must be assigned an IP address for identification and location addressing in order to communicate with other devices. With the ever-increasing number of new devices like mobile phones, smart phones, home & industrial appliances, integrated telephony, sensor networks, distributed computing, gaming and on line business etc. are being driven by the internet increasingly. The network security and QoS making transition from IPv4 to IPv6 inevitable. 7.1 IPv6 Format IPv6 uses a 128-bit address, allowing for approximately 340 trillion addresses in contrast to the IPv4 (32-bit address) which is limited to approximately 4.3 billion addresses. IPv6 will eliminate the need for Network Address Translation (NAT); the IP security protocol suite (IPSec) has been built into the IPv6 architecture thereby making way for an intrinsic security mechanism with IPv6 implementation. An IPv6 address is represented by 8 groups of 16-bit values, each group represented as 4 hexadecimal digits and separated by colons (:) called as Hex-colon notation. e.g.: 2001:0db8:0000:0000:0000:ff00:0042:8329 An IPv6 address may be abbreviated by using one or more of the following rules:  Remove one or more leading zeroes from one or more groups of hexadecimal digits; this is usually done to either all or none of the leading zeroes. (For example, convert the group 0042 to 42)  Omit one or more consecutive sections of zeroes, using a double colon (::) to denote the omitted sections. The double colon may only be used once in any given address, as the address would be indeterminate if the double colon was used multiple times. (For example, 2001:db8::1:2 is valid, but 2001:db8::1::2 is not permitted.) Below is an example of these rules: Initial address: 2001:0db8:0000:0000:0000:ff00:0042:8329 After removing all leading zeroes: 2001:db8:0:0:0:ff00:42:8329 After doing both: 2001:db8::ff00:42:8329 Another example is the loopback address, which can be abbreviated to:: 1 by using both rules above IRISET 98 TA2 – Data Communication & Networking IPv6 Address Initial address: 0000:0000:0000:0000:0000:0000:0000:0001 After removing all leading zeroes: 0:0:0:0:0:0:0:1 After doing both::1  Dotted-quad notation During the transition of the Internet from IPv4 to the IPv6 it is typical to operate in a mixed addressing environment, and for this purpose a special notation has been introduced to express IPv4-compatible IPv6 addresses by writing the final 32 bits of an address in the familiar IPv4 dotted-quad notation. For example, the IPv4-mapped IPv6 address:: ffff:c000:0280 is usually written as ::ffff:192.0.2.128, thus expressing clearly the original IPv4 address that was mapped to IPv6. 2000::/3 1 The current IPv6 address space for unicast allocations is 1/8 of the total address space IANA Allocation to Registries (Varies) IANA makes assignments to regional registries. New allocations are /12 bits 2 previous assignments are varied. For example: 2e01:0000::/16 was assigned by IANA to RIPE NCC(the European and Middle Ease registry) in December 2005 “ISP Allocations” Regional registries make assignments to local ISPs. A typical assignment is /32 bits, 3 but more space may be assigned. For example: RIPE NCC assigned 2e01:c000::/19 to France Telecom in December 2005 “End-Site Allocations” ISPs make assignments to their customer. The amount of address space varies, but a /48 allocation is common. 4 Organizations can get their assignments, based on need (Command information has a /32 allocation) smaller organizations may get less space (for small companies a /56 is common). “Subnet assignments” Organizations make assignments to individual subnets, where the most common 5 size is /64. With 16 bits subnetting bits available, an organization can deploy as many as 65536 subnets. “Interface ID” Interfaces must have a unique identity on the subnet – often created by embedding 6 the underlying 48-bit (L2) MAC address. Theoretically then, a single subnet could support 264 active hosts – clearly far beyond the practical limt. Global IPv6 address format IRISET 99 TA2 – Data Communication & Networking IPv6 Address 7.2. IPv6 Address classification: IPv6 addresses are classified as unicast addressing, anycast addressing, and multicast addressing. A unicast address identifies a single network interface. The Internet Protocol delivers packets sent to a unicast address to that specific interface. An anycast address is assigned to a group of interfaces, usually belonging to different nodes. A packet sent to an anycast address is delivered to just one of the member interfaces, typically the nearest host, according to the routing protocol’s definition of distance. Anycast addresses cannot be identified easily, they have the same format as unicast addresses, and differ only by their presence in the network at multiple points. Almost any unicast address can be employed as an anycast address. A multicast address is also used by multiple hosts, which acquire the multicast address destination by participating in the multicast distribution protocol among the network routers. A packet that is sent to a multicast address is delivered to all interfaces that have joined the corresponding multicast group. IPv6 does not implement broadcast addressing. Broadcast's traditional role is subsumed by multicast addressing to the all-nodes link-local multicast group ff02::1. However, the use of the all-nodes group is not recommended, and most IPv6 protocols use a dedicated link-local multicast group to avoid disturbing every interface in the network. 7.3 IPv6 Address types and their formats Unicast and anycast addresses are typically composed of two logical parts: a 64-bit network prefix used for routing, and a 64-bit interface identifier used to identify a host Network interface. General unicast address format (routing prefix size varies) bits 48 (or more) 16 (or fewer) 64 field routing prefix subnet id interface identifier The network prefix (the routing prefix combined with the subnet id) is contained in the most significant 64 bits of the address. The size of the routing prefix may vary; a larger prefix size means a smaller subnet id size. The bits of the subnet identifier field are available to the network administrator to define subnets within the given network. Different ways of assigning IPv6 address to the system: 1. Static Method: address is assigned manually 2. Auto configuration :The system will find out the router using NDP protocol and using this it will automatically assigns the IPv6 address. 3. EUI-64 (Extended Unique Identifier) Method Here we provide only network part of IPv6 address and the host part will be automatically assigned from the MAC address of the system. Conversion of 48 bits MAC address to 64 bit EUI-64 format is done in the following steps as shown in the example with the following MAC address. A0-1D-48-C1-76-14 1. First 24 bits of MAC address is called as OUI (Organizational Unique Identifier and the Second 24 bits of MAC address is called as Unique ID. IRISET 100 TA2 – Data Communication & Networking IPv6 Address 2. 7th bit value of OUI should be made as opposite i.e. 0 to 1 and vice versa, taking the above example the seventh bit of A0 (10100000) will become A2 (10100010). 3. FF-FE are inserted between OUI and Unique ID 4. 48 bit MAC is now converted to EUI-64 format as A2-1D-48-FF-FE-C1-76-14 7.4 IPv6 Networks: An IPv6 network uses an address block that is a contiguous group of IPv6 addresses of a size that is a power of two. The leading set of bits of the addresses is identical for all hosts in a given network, and is called the network's address or routing prefix. Network address ranges are written in CIDR notation. A network is denoted by the first address in the block (ending in all zeroes), a slash ( /), and a decimal value equal to the size in bits of the prefix. For example, the network written as 2001:db8:1234::/48 starts at address 2001:db8:1234:0000:0000:0000:0000:0000 and ends at 2001:db8:1234:ffff:ffff:ffff:ffff:ffff. The routing prefix of an interface address may be directly indicated with the address by CIDR notation. For example, the configuration of an interface with address 2001:db8:a::123 connected to subnet 2001:db8:a::/64 is written as 2001:db8:a::123/64. 7.4.1 IPv6 address space: The management of IPv6 address allocation process is delegated to the Internet Assigned Numbers Authority (IANA) by the Internet Architecture Board and the Internet Engineering Steering Group. Its main function is the assignment of large address blocks to the regional Internet registries (RIRs), which have the delegated task of allocation to network service providers and other local registries. The IANA has maintained the official list of allocations of the IPv6 address space since December 1995. Only one eighth of the total address space is currently allocated for use on the Internet, 2000::/3, in order to provide efficient route aggregation, thereby reducing the size of the Internet routing tables; the rest of the IPv6 address space is reserved for future use or for special purposes. The address space is assigned to the RIRs in large blocks of /23 up to /12. The RIRs assign smaller blocks to local Internet registries that distributes them to users. These are typically in sizes from /19 to /32. The addresses are typically distributed in /48 to /56 sized blocks to the end users. Each RIR can divide each of its multiple /23 blocks into 512 /32 blocks, typically one for each ISP; an ISP can divide its /32 block into 65536 /48 blocks, typically one for each customer; customers can create 65536 /64 networks from their assigned /48 block, each having 264 addresses. In contrast, the entire IPv4 address space has only 232 (about 4.3×109) addresses. By design, only a very small fraction of the address space will actually be used. The large address space ensures that addresses are almost always available, which makes the use of network address translation (NAT) for the purposes of address conservation completely unnecessary. IRISET 101 TA2 – Data Communication & Networking IPv6 Address Special allocation: To allow for provider changes without renumbering, provider-independent address space – assigned directly to the end user by the RIRs – is taken from the special range 2001:678::/29. Internet Exchange Points (IXPs) are assigned special addresses from the range 2001:7f8::/29 for communication with their connected ISPs. Root name servers have been assigned addresses from the same range. Reserved anycast addresses: The lowest address within each subnet prefix (the interface identifier set to all zeroes) is reserved as the "subnet-router" anycast address. Applications may use this address when talking to any one of the available routers, as packets sent to this address are delivered to just one router. The 128 highest addresses within each /64 subnet prefix are reserved to be used as anycast addresses. These addresses usually have the 57 first bits of the interface identifier set to 1, followed by the 7-bit anycast ID. Prefixes for the network, including subnets, are required to have a length of 64 bits, in which case the universal/local bit must be set to 0 to indicate the address is not globally unique. The address with value 0x7e in the 7 least-significant bits is defined as a mobile IPv6 home agents anycast address. The address with value 0x7f (all bits 1) is reserved and may not be used. No more assignments from this range are made, so values 0x00 through 0x7d are reserved as well. 7.4.2 Special addresses: There are a number of addresses with special meaning in IPv6: Unique local addresses: (FC00::/7) Unique local addresses (ULAs) are intended for local communication. They are routable only within a set of cooperating sites (analogous to the private address ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 of IPv4). Global Unicast addresses: (2000::/3) Global Unicast addresses are intended for use over the global network like Internet. They are routable over the Internet (analogous to the public IP addresses in IPv4) Link local addresses: (FE80::/10) Link local addresses (LLAs) is required on every IPv6- enabled interface—in other words, applications may rely on the existence of a link-local address even when there is no IPv6 routing. These addresses are comparable to the auto-configuration addresses (analogous to the automatic private IP address (APIPA) ranges 169.254.0.0 / 16 of IPv4). Multicast addresses: (FF00:: /8) Multicast addresses are intended for multicast services like video conference applications, real-time applications and also used by routing protocols for route determination etc.(analogous to the Class ‘D’ (224.0.0.0 to 239.255.255.255 of IPv4). Loop back addresses: (::1/128) The loopback address is a unicast local host address. If an application in a host sends packets to this address, the IPv6 stack will loop these packets back on the same virtual interface (analogous to the universal loop back address 127.0.0.0/8 in IPv4). Default route : (::/0) The default unicast route address is required for routing towards a stub network like Internet (analogous to 0.0.0.0/0 in IPv4). The loopback address is a unicast local host address. If an application in a host sends packets to this address, the IPv6 stack will loop these packets back on the same virtual interface (analogous to the universal loop back address 127.0.0.0/8 in IPv4). IRISET 102 TA2 – Data Communication & Networking IPv6 Address List of Special Addresses used in IPv4 & IPv6 is appended below in the table 7.5 IPv6 Migration: IPv4 and IPv6 will continue to coexist, for some years, the true potential of the digital economy and next-generation services can only be realized once operators plan their IPv6 migration. And it is extremely important that all software and hardware aspects are clearly evaluated before launching a migration, as any gaps can have direct impact on the availability of many critical services. A variety of technologies are available to facilitate the migration to IPv6. These technologies are  Dual stack – support of both IPv4 and IPv6 on network devices  Tunneling – encapsulation of an IPv6 packet within an IPv4 packet for transmission over an IPv4 network  Translation – address (or) port translation of addresses such as via a gateway device (or) translation code in the TCP/IP code of the host (or) router 7.5.1 Dual-Stack Approach: The dual-stack approach consists of implementing both IPv4 and IPv6 protocol stacks on devices requiring access to both network-layer technologies, including routers, other infrastructure devices and end-user devices. Such devices would be configured with both IPv4 and IPv6 addresses as shown in fig.7.1 Dual-stack deployment: Deployment of dual-stacked devices sharing a common network interface implies the operation of both IPv4 and IPv6 over the same physical link. Dual-stacked routers support IPv4 and IPv6 and route IPv4 packets among native IPv4 hosts and IPv6 packets among IPv6-capable hosts. Fig.7.1: Dual-stacked Network Perspectives IRISET 103 TA2 – Data Communication & Networking IPv6 Address 7.5.2 Tunneling Approaches: A variety of tunneling technologies has been developed to support IPv4 over IPv6 as well as IPv6 over IPv4 tunneling. These technologies are generally categorized as configured (or) automatic. Configured tunnels are predefined, whereas automatic tunnels are created and torn down “on the fly.” Tunnel types: While the process of tunneling is the same for all types of tunnels, there is a variety of scenarios based on defined tunnel endpoints. Probably the most common configuration is a router-to-router tunnel, which is the typical approach for configured tunnels. Created and torn down “on the fly.” Figure 7.2 a : Router-to-Router Tunnel In the above figure, the originating IPv6 host on the left has an IPv6 address of W. A packet destined for the host on the far end of the diagram with an IPv6 address of Z is sent to a router serving the subnet. This router (with an IPv4 address of B and an IPv6 address of X) receives the IPv6 packet. Configured to tunnel packets destined for the network on which host Z resides, the router encapsulates the IPv6 packet with an IPv4 header. The router uses its IPv4 address (B) as the source IPv4 address and the tunnel endpoint router (with an IPv4 address of C) as the destination address, which is depicted beneath the IPv4 network in the center of the figure. The endpoint router decapsulates the packet, stripping off the IPv4 header and routes the original IPv6 packet to its intended destination (Z). Figure 7.2 b: Host-to-Router Tunneling Configuration Another tunneling scenario features an IPv6/IPv4 host capable of supporting both IPv4 and IPv6 protocols, tunneling a packet to a router, which in turn decapsulates the packet and routes it natively via IPv6. The tunneling mechanism is the same as in the router-to-router case, but the tunnel endpoints are different. Figure 7.2.c: Host-to-Host Tunnel Configuration IRISET 104 TA2 – Data Communication & Networking IPv6 Address The final tunneling configuration is one that spans end-to-end, from host-to-host. If the routing infrastructure has not yet been upgraded to support IPv6, this tunneling configuration enables two IPv6/IPv4 hosts to communicate via a tunnel over an IPv4 network Automatic tunneling of IPv6 packets over IPv4 networks: Tunnels are either configured or automatic. Configured tunnels are pre- defined by administrators in advance of communications, much as static routes would be pre- configured. An automatic tunnel does not require pre-configuration. Tunnels are created based on information contained in the IPv6 packet, such as the source or destination IP address. The following automatic tunneling techniques are described in this section: 1. 6 to 4 – automatic router-to-router tunneling based on a particular global address prefix and embedded IPv4 address 2. ISATAP – automatic host-to-router, router-to-host or host-to-host tunneling based on a particular IPv6 address format with inclusion of an embedded IPv4 address 3. 6 over 4 – automatic host-to-host tunneling using IPv4 multicasting 4. Tunnel Brokers – automatic tunnel setup by a server acting as a tunnel broker in assigning tunnel gateway resources on behalf of hosts requiring tunneling 5. Teredo – automatic tunneling through NAT firewalls over IPv4 networks 6. Dual-Stack Transition Mechanism – enables automatic tunneling of IPv4 packets over IPv6 networks 7.5.3 Translation Approaches: Translation techniques perform IPv4-to-IPv6 translation (and vice versa) at a particular layer of the protocol stack, typically the network, transport or application layer. Unlike tunneling, which does not alter the tunneled data packet; translation mechanisms do modify or translate IP packets commutatively between IPv4 and IPv6. Translation approaches are generally recommended in an environment with IPv6-only nodes communicating with IPv4-only nodes. Stateless IP/ICMP Translation (SIIT) algorithm: SIIT provides translation of IP packet headers between IPv4 and IPv6. SIIT resides on an IPv6 host and converts outgoing IPv6 packet headers into IPv4 headers, and incoming IPv4 headers into IPv6. To perform this task, the IPv6 host must be assigned an IPv4 address either configured on the host or obtained via a network service. When the IPv6 host desires to communicate with an IPv4 host, based on DNS resolution to an IPv4 address, the SIIT algorithm would convert the IPv6 packet header into IPv4 format. The SIIT algorithm recognizes such a case when the IPv6 address is an IPv4-mapped address, the mechanism to convert the resolved IPv4 address into an IPv4-mapped address is provided by bump-in-the-stack (BIS) or bump-in-the-API (BIA) techniques. Figure 7.3: IPv4 Mapped Address Format Based on the presence of the IPv4-mapped address format as the destination IP address, the SIIT algorithm performs header translation to yield an IPv4 packet for transmission via the data link and physical layers. The source IP address uses a different format, that of the IPv4- translated format. The IPv4-mapped address format is invalid as a source address for tunneling. Therefore, its use as the source address would disqualify communications through any intervening tunnels. The use of the IPv4-translated format bypasses this potential restriction. Figure 7.4: IPv4-Translated Address Format used within SIIT IRISET 105 TA2 – Data Communication & Networking Annexure ANNEXURE 1 LINE ENCODING & BLOCK CODING Line encoding The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding. The common types of line encoding techniques as shown in fig Annex.1.1 are Unipolar, Polar, Bipolar and Multi level. Fig.Annex.1.1 Line encoding techniques A. Unipolar Encoding Fig.Annex.1.2(a) Unipolar encoding Unipolar encoding as shown in fig. Annex.1.2(a) has 2 voltage states, with one of the states being 0 volts. Since unipolar line encoding has one of its states at 0 Volts, it is also called Return to Zero (RTZ). The average amplitude of unipolar encoded signal is non-zero. This creates a direct (DC) component (a component with zero frequency). When a signal contains a DC component, it cannot travel through media that cannot handle DC components. Unipolar line encoding works well for inside machines—where the signal path is short—but is unsuitable for long distances, due to the presence of stray capacitance in the transmission medium. On long transmission paths, the constant level shift from 0 to 5 volts, which causes the stray capacitance to charge up. There will be a “stray” capacitor effect between any two conductors as shown in fig Annex.1.2(b) that are in close proximity to each other. For example, parallel running cables or wires are very suspect to stray capacitance. Fig.Annex,1. 2(b) Stray capacitance effects IRISET 106 TA2 – Data Communication & Networking Annexure If there is sufficient capacitance on the line (and a sufficient stream of 1s) a DC voltage component will be added to the data stream. Instead of returning to 0 volts, it would only return to 2 or 3 volts, as shown in fig Annex.1.2(c). The receiving station may not recognize a digital low at voltage of 2 volts due to base lines wander. Fig. Annex.1.2(c) Base lines wander Fig.Annex. 1.2(d) Drifting of synchronization Unipolar line encoding can have synchronization problems between the transmitter and receiver’s clock oscillator. The receiver’s clock oscillator locks on to the transmitted signal’s level shifts (logic changes from 0 to 1) if there is a long series of logical 1s or 0s in a row. There is no level shift for the receiver’s oscillator to lock to. The receiver oscillator’s frequency may drift and become unsynchronized as shown in fig Annex. 1.2(d) it could lose track, of where the receiver is supposed to sample the transmitted data. B. Polar Encoding When the digital encoding is symmetrical—around 0 Volts—it is called a Polar Code. For example, the RS-232D interface uses Polar line encoding as shown in fig. Annex.1.3(a) The signal does not return to zero; it is either a +ve voltage or a -ve voltage. Polar line encoding is also called None Return to Zero (NRZ). Fig. Annex. 1.3(a) RS 232D Polar encoding Polar line encoding is the simplest pattern that eliminates most of the residual DC problem. Variation of Voltage levels for Polar encoding at Transmitter & Receiver end is shown in fig. Annex.1.3(b) IRISET 107 TA2 – Data Communication & Networking  Annexure Fig.Annex. 1.3(b) Variation of Voltage levels There is still a small residual DC problem, but Polar line encoding is a great improvement over Uni polar line encoding. Polar encoding has an added benefit in that it reduces the power required to transmit the signal by one-half. i. NRZ Non return to Zero 1 = signal on 0 = signal off (no signal) NRZ is used on low speed links, such as serial ports. Its problems are lack of clock recovery during long string of 0 or 1 bits and it has a DC component resulting in “baseline wander” during long strings of 0 or 1 bits as shown if fig Annex. 1.4 Fig.Annex. 1.4 NRZ Signal ii. NRZ I (Non return to Zero Inverted) 1 = change of signal level (on-off or off-on) 0 = no change of signal level NRZI is a differential encoding used in 4B/5B on fast Ethernet. It fixes problems in clocking during long strings of 1 bit as shown in fig Annex. 1.5. The problems are the DC component and the lack of clock recovery during long string of 0 bits. Fig Annex.1,5 NRZ I Signal iii. Manchester Line Encoding In Manchester Line Encoding, there is a transition at the middle of each bit period. The mid-bit transition serves as a clocking mechanism (and also as data): a low to high transition represents a 1 and a high to low transition represents a 0as shown in the fig Annex.1.6 (a) IRISET 108 TA2 – Data Communication & Networking Annexure Fig. Annex. 1.6(a) Manchester line encoded Signal Manchester line encoding has no DC component and there is always a transition available for synchronizing receives and transmits clocks. Manchester line encoding is also called self- clocking line encoding. It has the added benefit of requiring the least amount of bandwidth compared to the other line encoding. Widely used with 10 base T legacy Ethernets. Manchester line encoding requires 2 frequencies the base carrier and 2 x the carrier frequency. All others require a range from 0 hertz to the maximum transfer rate frequency as shown in fig Annex. 1.6(b) Fig.Annex.1.6(b) Manchester line encoded Signal with base frequency Manchester line encoding can detect errors during transmission: a transition is expected during every bit period. Therefore, the absence of a transition would indicate an error condition. C. Bipolar Line Encoding i. Alternate Mark Inversion (AMI) Bipolar line encoding has 3 voltage levels. A low or 0 is represented by a 0 Volt level and a 1 is represented by alternating polarity pulses. By alternating the polarity of the pulses for 1s, the residual DC component cancels as shown in fig Annex. 1.7 Bipolar line encoding is also called Alternate Mark Inversion (AMI). Bipolar line encoding is also called Alternate Mark Inversion (AMI). Fig. Annex. 1.7 Bipolar AMI Signal IRISET 109 TA2 – Data Communication & Networking Annexure ii. B8ZS Bipolar with 8 Zero Substitution In this type of coding, a string of 8 zeros is substituted according to the following rules: If the immediate preceding pulse is of (-) polarity, then code each group of 8 zeros as 000-+0+- If the immediate preceding pulse is of (z) polarity, then code each group of 8 zeros as 0 0 0 + - 0 - + as shown fig. Annex. 1.8. Fig. Annex 1.8 B8Z8 Signal iii. High Density Bipolar 3 (HDB3) Another coding scheme is HDB3, high density bipolar 3, used primarily in Europe for 2.048MHz (E1) carriers as shown in fig Annex 1.9. This code is similar to BNZS in that it substitutes bipolar code for 4 consecutive zeros according to the following rules: If the polarity of the immediate preceding pulse is (-) and there have been an odd (even) number of logic 1 pulses since the last substitution, each group of 4 consecutive zeros is coded as 000-(+00+). If the polarity of the immediate preceding pulse is (+) and there have been an odd (even) number of logic 1 pulses since the last substitution, each group of 4 consecutive zeros is coded as 000+(-00-). Fig. Annex.1.9 HDB3 Signal iv. Pseudoternary Pseudoternary has the same behavior as Bipolar-AMI as shown in fig Annex. 1.20, except it reverses signaling: 1 = no signal (0 voltage) 0 = alternating +V and –V Fig. Annex. 1.20 Pseudoternary Signal IRISET 110 TA2 – Data Communication & Networking Annexure D. Multi Level Line Encoding i. 2B1Q The 2B1Q (two binary, one quaternary) line encoding scheme was intended to be used by the ISDN DSL and SDSL applications. This code is a four-level line code in which two binary bits (2B) represent one quaternary symbol (1Q) as shown in fig. Annex 1.21. The 2B1Q line coding was seen as a major enhancement over the original T1 line coding, because 2B1Q encoded two bits per signal change instead of just one per change. Fig. Annex. 1.21 2B1Q Signal ii. Block Coding Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group. Block coding concept is shown below (Fig Annex. 1.22). Fig. Annex. 1.22 Block coding concept iii. 4B5B (4 Bit / 5 Bit) 4B5B uses 5 bit signals for each 4 data bit. The 5 bit sequences are chosen so that there are never more than 3 consecutive zeros in the output stream. When used with NRZI, will have at least 2 signal transitions in every 5 bits. (Refer Fig. Annex 1.23 ) Fig. Annex. 1.23 4B/5B coding IRISET 111 TA2 – Data Communication & Networking Annexure iv. 8B10B (8Bit/10Bit) 8B10B coding is used in interfaces as PCI Express, Fiber Channel and others. In these applications 8B10B transmission code provides the following functions: Improves transmission characteristics Enables bit- level clock recovery Improves error detection Separates data symbols from control symbols Derives bit and word synchronization The data bytes are encoded into 10-bit data characters resulting into 1024 possible characters. 2x256=512 is reserved for the data byte transfers. One character representative has more 1’s; the other has more 0's and is selected according to the current disparity (see below). 12 special characters are defined for special signaling. The rest of the 1024-512-12 are not allowed for transmission and indicate transmission errors or unsynchronized status once they are received at the destination. Ordered sets are flexible building blocks which may be used for in-band and or out-of-band protocol functions. 8B10B code recognizes the idea of a Running Disparity (the difference between the number of 1’s and 0’s transmitted). The sender keeps the running disparity around zero, the receiver checks the data stream according to this rules and is thus able to detect some transmission errors. Other neighboring coding schemes like 64B66B are available and are used in certain applications. IRISET 112 TA2 – Data Communication & Networking