are used for telecommunication applications and, conversely, data communication networks that are used for telecommunications applications.
Network technologies that are most likely to use high-end Network Processors applications in network nodes (devices) are Ethernet (enterprise and carrier class), MPLS, and IP. Therefore, most of the description in this chapter is focused on these networks, with some discussion of other network technologies required for an overview of how networks are combined, relate to each other, and evolve.
3.2 FROM TELECOM NETWORKS TO DATA NETWORKS
As noted before, one convergence approach between data networks and telecommunications networks is to adopt various techniques that enable transmis-sion of data-patterned traffi c over telecommunications networks. The idea of such convergence has also led a move toward more data-centric traffi c in traditional telecommunications networks, equipment, and services.
As of today, SONET/SDH is the technology that has the greatest chance of dominating data and telecommunications networks, mainly due to the huge installed base and investments made in these networks. Nevertheless, the supporters of this technology are seriously threatened by the alternative Ethernet-based core networks, mainly due to their low-cost, simplicity, and natural extension from the residential and enterprise networks, as described in the next subsection.
We start with a discussion of the Asynchronous Transfer Mode technology despite the fact that it is hardly in use now, since it still carries several important services, and contributed important networking concepts. We continue with next generation SONET/SDH, which, as mentioned previously, is the main telecommuni-cation platform, and end with Resilient Packet Ring (RPR), which some categorize as a data network platform. We present the general concept of Multiservice Platforms (MSPs) that are referenced in the telecom industry for coping with data services alongside telecommunications.
3.2.1 Asynchronous Transfer Mode
The Asynchronous Transfer Mode introduced a new concept of cell switching (or relaying) that defi nes an architecture, protocols, cell formats, services, and interfaces.
The idea was to allow data and video services to be transmitted over circuit-switched networks together with traditional voice services, and to integrate the three into one network technology. A fi xed cell size of 53 bytes was defi ned to carry 48 bytes of payload,1 with a 5-byte header. The fi xed, small-sized cells supported high-speed and low-cost switching equipment, but created diffi culty in interfacing with the variable-length, long packets that are common in packet-switching networks.
1Some say the size was determined by choosing the average of 25⫽ 32 and 26⫽ 64 bytes, or by compromising between long packets for data requirements and short packets for voice requirements.
3.2 From Telecom Networks to Data Networks 79
ATM was promoted by the telecom industry, and standardized by the ITU-T [221, 222] as a Broadband Integrated Services Data Network (B-ISDN) at the beginning of the 1990s. As such, ATM fi t smoothly into the SONET/SDH framework and its frames’
structures. Another important contribution to ATM standardization came from the ATM forum (now part of the MFA forum, which combines the ATM, Frame-relay, and MPLS forums). However, despite the huge investments, interest, research, and publications, it received relatively little attention from the Internet community, and although ATM was offered for data networks infrastructures, and even used for a while, it was rejected eventually by the datacom industry. As of today, ATM technol-ogy has almost vanished (except in Asymmetric Digital Subscriber Line [ADSL] infra-structure). Nevertheless, it is important to understand ATM concepts because they were used for similar approaches in data networks such as MPLS, which is described in the following.
ATM is a connection-oriented technology, like most telecom technologies, according to which a call set-up and the establishment of a connection must take place prior to data transfer. The connection-oriented approach and supporting pro-tocols allow a Quality of Service (QoS, explained in Chapter 6) to be used for each of the ATM sessions, which enables voice, video, and real-time data to be supported.
The connections are called Virtual Channels (VC), and they are bundled into Virtual Paths (VP), which are groups of VCs that share a common link with the same end-points. These two layers of connections, which are called ATM layers, are places in a Transmission Path (e.g., fi ber), which belong to the physical layer. Each cell’s header uses a VC identifi er (VCI) and a VP identifi er (VPI) to indicate to which VC and VP it belongs. In other words, VCI identifi es the particular VC link for a given Virtual Path Connection (VPC), whereas VPI identifi es a group of VC links that share the same VPC (see Figure 3.1).
There are two main applications of VPCs; the fi rst is the User-to-Network VPC and the second is Network-to-Network VPC. ATM headers are defi ned according to the interface used: User–Network Interface (UNI) or Network–Network Interface (NNI).
The Generic Flow Control, which appears only in the UNI header, is by default 0, and is used for protocol procedures in controlled equipment. The VPI and VCI fi elds are used for routing and switching, with some preassigned VCI values for
FIGURE 3.1 ATM connections
VP VP
VP
Transmission Path
VP VP
VP
VCs VCs
VCs VCs
VCs
VCs
specifi c uses (e.g., various signaling types). The Payload Type (PT) fi eld specifi es the type and some attributes of the cell (e.g., Operation and Maintenance [OAM], user cells with or without congestion experience). The Cell Loss Priority (CLP) fi eld indicates how cells should be handled when network conditions request that cells should be dropped (e.g., in congestion); if CLP contains a ‘1’ bit, then this cell should be dropped before cells with CLP ⫽ 0. The last fi eld of the header is the Header Error Control, which contains the calculated Cyclic Redundancy Check (CRC) of the header fi elds, for validating the header by the receiving node (see Figure 3.2).2
The usage of VPI/VCI in the header tremendously simplifi es the cell forward-ing and switchforward-ing in an ATM network. At set-up time, a path is constructed by a routing protocol,3 such that each switching node in the routed path maintains a
“routing table” that associates the incoming VPI/VCI with an outgoing port, and potentially with a new VPI/VCI path identifi er that is known to the next switching node along the path. In this way, each switching node knows how to relay cells rapidly during the session time according to their VPI/VCI content (while writing their new VPI/VCI for the next hop). This principal is used in MPLS, and is detailed in subsection 3.3.1.1.
The ATM layer by itself defi nes only the data link layer functionalities. In order to use ATM for applications and services, there is another layer above the ATM, which is called the ATM Adaptation Layer (AAL). Various types of AAL mechanisms are used to match the applications’ data frames/packets and service requirements to that of the ATM cells. There are such fi ve mechanisms, called AAL1 to AAL5, defi ned by the
2The HEC is based on x8⫹ x2⫹ x ⫹ 1 CRC (or 0x07); CRC is explained in Chapter 5.
3The common routing protocol used for ATM signaling is called ATM Private Network–Network Interface (PNNI), which is defi ned by the ATM-Forum (today MFA forum) [30].
FIGURE 3.2
ATM cell structure; for all fi elds, the fi rst bit sent is the MSB
UNI—User-Network Interface NNI—Network-Network Interface GFC—Generic Flow Control VPI—Virtual Path Identifier VCI—Virtual Channel Indentifier PT—Payload Type
P—Cell Loss Priority (CLP) HEC—Header Error Control
VPI
VPI VCI
VCI
VCI PT
HEC
Payload (48 bytes) 8 7 6 5 4 3 2 1
NNI Structure
P
GFC VPI
VPI VCI
VCI
VCI PT
HEC
Payload (48 bytes) 8 7 6 5 4 3 2 1
UNI Structure
P
3.2 From Telecom Networks to Data Networks 81
ITU-T. AAL1 is used for voice and other TDM E1/T1 circuit emulations, characterized by a Constant Bit Rate CBR and connection-oriented type of traffi c. At the other end of the spectrum, AAL5 is used for IP and LAN data type traffi c, characterized by a connectionless orientation and Variable Bit Rate (VBR) type of service. In between, AAL2 was used for real time VBR data traffi c (usually compressed TDM data such as video and voice), while AAL3 and AAL4 target data services with VBR and asyn-chronous connection and connectionless-oriented types of traffi c such as frame relay, X.25, and Switched Multimegabit Data Service. AAL3 and AAL4 were used only rarely, while AAL5 was the main platform of most data services and protocols for carrying IP and LAN over ATM (e.g., classic IP over ATM [273], LAN Emulation—
LANE [28], and Multi-Protocol Over ATM—MPOA [29]). As mentioned before, ATM is not used for high-speed networks any more, but still it can be found in the Mbps range WANs, mainly in ADSL and wireless (cellular) infrastructure.